general enquiries on this form should be made to: - defra,...
TRANSCRIPT
General enquiries on this form should be made to:Defra, Procurements and Contracts Division (Science R&D Team)Telephone No. 0207 238 5734E-mail: [email protected]
SID 5 Research Project Final Report
NoteIn line with the Freedom of Information Act 2000, Defra aims to place the results of its completed research projects in the public domain wherever possible. The SID 5 (Research Project Final Report) is designed to capture the information on the results and outputs of Defra-funded research in a format that is easily publishable through the Defra website. A SID 5 must be completed for all projects.
This form is in Word format and the boxes may be expanded or reduced, as appropriate.
ACCESS TO INFORMATIONThe information collected on this form will be stored electronically and may be sent to any part of Defra, or to individual researchers or organisations outside Defra for the purposes of reviewing the project. Defra may also disclose the information to any outside organisation acting as an agent authorised by Defra to process final research reports on its behalf. Defra intends to publish this form on its website, unless there are strong reasons not to, which fully comply with exemptions under the Environmental Information Regulations or the Freedom of Information Act 2000.Defra may be required to release information, including personal data and commercial information, on request under the Environmental Information Regulations or the Freedom of Information Act 2000. However, Defra will not permit any unwarranted breach of confidentiality or act in contravention of its obligations under the Data Protection Act 1998. Defra or its appointed agents may use the name, address or other details on your form to contact you in connection with occasional customer research aimed at improving the processes through which Defra works with its contractors.
Project identification
1. Defra Project code MF1103
2. Project title
Spatial dynamics of edible crabs in the English Channel in relation to management
3. Contractororganisation(s)
Cefas
54. Total Defra project costs £ 472332(agreed fixed price)
5. Project: start date................. 01/04/2007
SID 5 (Rev. 07/10) Page 1 of 130
end date.................. 31/03/2011
SID 5 (Rev. 07/10) Page 2 of 130
6. It is Defra’s intention to publish this form. Please confirm your agreement to do so...................................................................................YES NO (a) When preparing SID 5s contractors should bear in mind that Defra intends that they be made public. They
should be written in a clear and concise manner and represent a full account of the research project which someone not closely associated with the project can follow.Defra recognises that in a small minority of cases there may be information, such as intellectual property or commercially confidential data, used in or generated by the research project, which should not be disclosed. In these cases, such information should be detailed in a separate annex (not to be published) so that the SID 5 can be placed in the public domain. Where it is impossible to complete the Final Report without including references to any sensitive or confidential data, the information should be included and section (b) completed. NB: only in exceptional circumstances will Defra expect contractors to give a "No" answer.In all cases, reasons for withholding information must be fully in line with exemptions under the Environmental Information Regulations or the Freedom of Information Act 2000.
(b) If you have answered NO, please explain why the Final report should not be released into public domain
Executive Summary7. The executive summary must not exceed 2 sides in total of A4 and should be understandable to the intelligent
non-scientist. It should cover the main objectives, methods and findings of the research, together with any other significant events and options for new work.This project successfully applied data storage tags (DSTs) and double T-bar tags to edible crabs to obtain new data that describe and quantify crab movements, growth, mortality and behaviour in the English Channel and Celtic Sea. The results generated from M1103 represent a significant advance in our understanding of the behaviour and fishery dynamics of edible crab in UK waters.
Investigation, description and quantification of patterns of movements of adult edible crabs using data storage tags (DSTs)
As far as we are aware, this work represents the first ever mass-release of DST-tagged crabs over a wide geographical scale. Fears of tag-loss through moulting, and non-return of tags by industry proved unfounded (overall return rate 34%). The study generated new information on the rates and timing of edible crab behaviour and migration. The direct observation of behaviour relating to reproduction has provided new metrics which could not have been obtained using conventional techniques. Our results demonstrate that the tagging of crabs with DSTs is not only viable, but can generate large quantities of applied data, relevant both to management of the fishery, and with wider relevance in marine environmental management (e.g. aggregate extraction, renewables, etc…).
Aquarium Experiments:
The combined use of a waterproof 2-part epoxy resin and “superglue” provided a fast acting and durable bond for field-attachment of DSTs (only one tag loss reported to date).
Field Experiments:
Of 144 DST-tagged crabs released at 5 sites (128 female, 16 male), 49 were recovered after between 8 and 575 days at liberty (17-40% return rate). Seasonality in crab behaviour and of the fishery was apparent in the recaptures distribution (few recaptures during winter). The results show:
All Channel DST-crabs migrated west (from <20m to >50m depth in the eastern Channel, remaining at ~75m in the western Channel).
Migrating crabs did not follow specific depth contours. Trevose crabs showed no apparent migration, but moved from 65m to 55m depth at the start of
incubation. Some evidence that Western Channel crabs (located in deeper, colder water), moved into warmer
SID 5 (Rev. 07/10) Page 3 of 130
water at the start of autumn.Crabs migrated between 1 and 302km while at liberty. Low-level activity by crabs subtly distorted recorded tidal signals causing some resolvable problems in migration-route reconstruction. Depth profiles from 9 long-term DSTs showed periods of inactivity from late autumn to spring or early summer, considered indicative of spawning and brooding of eggs. These data suggest:
Incubation times of 126 to 198 days (at 8.8oC to 15.0oC and at depths of 19m to 84m). Incubation not geographically restricted, but occurring throughout the study area. All 7 Channel brooding crabs migrated west prior to spawning, and were recaptured west of their
brooding locations, suggesting lack of brooding site-fidelity in successive years. High proportion of brooding over-wintering females suggests annual spawning by crabs.
Investigation, description and quantification of patterns of movements of adult edible crabs using conventional tagging methods (corroborative support the DST programme)
Over 15,000 crabs (mainly females) were tagged and released using double T-bar tags at 11 sites in the English Channel and Celtic Sea. Nearly 2,500 crabs (16%) were recaptured, similar to recapture rates obtained from tagging programmes in the 1970s, with wide variation between locations
Patterns of movements were very similar to those observed by Bennett & Brown (1983). striking long-distance, westerly or south-westerly movements by female crabs. Male crabs moved less far than females, and exhibited mainly local, undirected movements. Female movement rates were 0.8 to 6.7km/wk (excluding outliers), whilst males were slower at
0.2-2.5km/wk. Females released late in the year generally travelled further and faster than those released during
the first half of the year. Highest rates of movements for females occurred in October and November, whilst lower rates of
movement and low recapture rates over winter. Eastern Channel crabs moved furthest and those released further west were recaptured closer to
their release site (this may reflect fisheries distribution). No evidence of systematic easterly movements in spring. No evidence of an west-east counter migration.
Population dynamics modelling of edible crabs in the Channel
A population dynamics model (monthly time steps, 2 seasons, 12 spatial units) was developed using commercial effort data and tag loss information (this project) and fitted to release and recapture data from both double T-bar and DST tagging programmes (sexes separate). This provided:
Quantitative estimates of monthly movement rates. Catchability by spatial unit. Relative overall reporting rate for double T-bar tags.
The results suggest: all surviving females released in the eastern Channel moved west and away from the release
areas over 3 to 4 years. No male recaptures beyond the nearest westerly neighbour (but more uncertainty as far fewer
males were tagged or recovered). Estimated catchability parameters reflected some key features of the fishery (e.g. lower catch
rates for females, higher catch rates for males during winter and spring).
Utilisation of returns from the tagging programmes to estimate growth and exploitation rates
Combining estimated catchability from the population dynamics model with reported commercial effort provided a time series of monthly exploitation rates, highlighting seasonality in the fisheries. High exploitation rates were suggested for:
Females in the south Devon inshore and Lands End & Scilly grounds. Males in the Trevose area. Both sexes in the Lizard & west Cornwall area.
Mortality rates were further investigated using catch curve analysis and Brownie multi-year models. Results suggest that mortality was moderate or high for females and high for males.
Insufficient data were available to estimate male growth parameters, but various discontinuous and continuous models were fitted for females. Comparison of growth rates estimated in this study and using data from the 1970s did not indicate a significant change in growth parameters. A modern generalised anniversary method produced slightly higher growth rates than traditional methods under some assumptions.
SID 5 (Rev. 07/10) Page 4 of 130
Supplementary work
Additional work resulted in a large scale screening for infection by the parasite Hematodinium sp. in the mid-Channel offshore fishery indicated typical summer infection rates (3.5%), lower than reported in some other fisheries. Generally low recapture rates (and only 2 infected males returned) from this area, mean no clear indication of either increased mortality or long term survival after Hematodinium infection could be detected.
Evaluation and application of the results in the context of sustainable management of the edible crab fishery and the interaction of the crab fishery and other coastal resource uses
Results from M1103 support the continued relevance of regional-scale management, but show that migration needs to be carefully considered. Moderate or high estimated mortality rates support the need for management actions in these fisheries. New growth data will support stock assessment work and enhanced information relating to spawning and incubation sites is relevant for spatial planning.
The large data-resource accumulated under MF1103 has already provided new insights into crab movements, mortality, growth and reproductive cycle, all with implications for fisheries management. MF1103 data represent a substantial and significant resource for future analyses: there is also scope to extend and refine the analyses presented.
Project Report to Defra8. As a guide this report should be no longer than 20 sides of A4. This report is to provide Defra with details
of the outputs of the research project for internal purposes; to meet the terms of the contract; and to allow Defra to publish details of the outputs to meet Environmental Information Regulation or Freedom of Information obligations. This short report to Defra does not preclude contractors from also seeking to publish a full, formal scientific report/paper in an appropriate scientific or other journal/publication. Indeed, Defra actively encourages such publications as part of the contract terms. The report to Defra should include: the scientific objectives as set out in the contract; the extent to which the objectives set out in the contract have been met; details of methods used and the results obtained, including statistical analysis (if appropriate); a discussion of the results and their reliability; the main implications of the findings; possible future work; and any action resulting from the research (e.g. IP, Knowledge Transfer).
1. Scientific objectives set out in the contract
The general objectives of the project are to improve understanding of spatial dynamics of crabs in the English Channel and to assess the implications of crab movements for fisheries management.
A key element of the project will be to utilise ‘state of the art’ telemetry techniques using data storage tags (DSTs). Recent studies using DSTs have significantly improved understanding of the stock structure and population dynamics of a range of finfish species, including cod, plaice and rays. It is anticipated that this approach will provide detailed fishery independent information on the spatial and temporal dynamics of crab stocks. Data storage tags have not been applied previously to edible crabs and will provide new long-term datasets on a seasonal time scale as well as ‘continuous’ records of previously unrecorded fine scale observations of activity by crabs.
Data from the DST programme will be enhanced by a parallel programme of conventional tagging using double T-bar tags. Conventional tagging will provide mark recapture data at relatively low cost, while data storage tags will complement this information by providing detailed information for relatively small numbers of crabs over seasonal time scales. Additional information regarding growth and exploitation rates for crabs, both of which are also areas where knowledge is limited and uncertain, will also be obtained from the return of conventional tags.
Established analytical techniques and models will be used to estimate parameters describing migration, movement, growth and exploitation rates. Outputs will be evaluated to assess their importance to the sustainable exploitation and management of the stock and fishery.
SID 5 (Rev. 07/10) Page 5 of 130
More specifically the scientific and technical objectives are:1) To investigate, describe and quantify patterns of movements of adult edible crabs using data storage tags
(DSTs).2) To investigate, describe and quantify patterns of movements of adult edible crabs using conventional
tagging methods (to provide corroborative support for results obtained from the DST programme).3) To utilise returns from the tagging programmes to estimate growth and exploitation rates for edible crabs in
the Channel fishery.4) To evaluate and apply the results in the context of sustainable management of the edible crab fishery and
the interaction of the crab fishery and other coastal resource uses.
2. Introduction
The fishery for edible crabs (Cancer pagurus) in the English Channel has been one of the most important commercial fisheries in England and Wales for many years, yet gaps in understanding and quantification of the biology and ecology of edible crabs remain, which limit the quality of advice on their sustainable management. Understanding when, where, and on what scale crabs move (temporally, demographically and geographically) underpins successful stock assessment and management as well as identifying key life stages, locations and time periods that may be vulnerable to fishing or other human activities.
It is over 30 years since crab tagging was last carried out in the English Channel (Bennett & Brown, 1983; Cuillandre et al., 1984; Latrouite & Le Foll, 1989). Those results showed some long distance movements by crabs from east to west in the English Channel, particularly by mature females. Return movements were not demonstrated although they have been suggested by some authors in other areas (Tully et al., 2006). Female migrations were interpreted as ontogenetic behaviour; movement by females to spawning grounds in the western Channel facilitating the return of larvae by the prevailing tidal currents to the areas of maternal origin. More recent studies of crab larvae distributions throughout the Channel have shown that larval transport rates are too low to enable larvae from major spawning grounds in the western Channel to be returned fully to eastern Channel crab fishing grounds (Eaton et al., in prep.) although they would be adequate to disperse larvae in the western Channel. Further evidence from genetic studies (Defra MF0230, Paul Shaw, Royal Holloway, University of London, pers. comm.) has indicated significant genetic variation in edible crabs at local scales, but relatively little variation on a regional scale. Further information is required to help rationalise these somewhat conflicting signals into a cohesive picture of stock identity that can support assessment and management of these valuable resources. By using a combination of conventional and novel telemetric tagging techniques this project complements previous larvae studies (Defra, MF0227, 2004) and provides a comprehensive new overview of crab populations in the English Channel. Since the investigations of the 1970s there has been a significant expansion of the fishery as well as probable changes in the environment. The project addresses the possibility of changes in the populations by making comparisons with earlier data and analyses and generates new data to investigate potential changes in growth and exploitation. Results are particularly relevant given growing concern over increases in effort in this fishery and consultations with the industry regarding potential new management measures for crabs.
3. Objective 1. Investigation, of patterns of movements of adult edible crabs using data storage tags (DSTs)
3.1. Introduction
Understanding how, where and when crabs undergo large scale migrations is the key to successful stock assessment and management and is important in identifying key life stages and periods that may be vulnerable to local fishing or other human activities. Electronic data storage tags (DSTs) are now routinely applied as a tool to describe migration in finfish. However Cefas have not previously deployed DSTs on crustaceans. The principal aim of MF1103 was to describe the movements of adult crabs on local and regional migratory scales.
3.2. Materials and Methods
3.2.1 Aquarium experiments investigating methods of attachment
Crabs were held in the Cefas aquarium, Lowestoft to investigate the performance of different adhesives for attachment of DSTs. An artificial “reef”, constructed of stacked boulders around a contained soft sediment area measuring approximately 3 m x 2 m, was designed to mimic their preferred habitat (crevices for sheltering, sediment allowing burrowing). Crabs were fed fresh fish daily in the perimeter area outside the reef and enclosure, to encourage mobility.
SID 5 (Rev. 07/10) Page 6 of 130
Test individuals were tagged either with a specially designed lozenge version of the Cefas G5 DST, or a conventional Cefas G5 DST fitted with a flat-based holding-cradle. Five of the cradled DSTs were ‘live’, and were programmed to record depth and temperature at 10 second intervals up until 31/01/08, then at 1 min intervals thereafter. Dorsal attachment was made towards the back of the carapace. The carapace was cleaned at the attachment position with an abrasive pad, and for the first eighteen crabs tagged, the attachment area was dried before degreasing with acetone. This last process was omitted for the last 10 individuals tagged. For all control crabs, and the five dummy “cradle” tags (which do not bear numbers), identification was achieved by attaching coloured cable-ties to the right-hand cheliped.
Twenty eight crabs (19 females: 9 males), were held for 2 months, between 17/12/07 and 21/02/08 to evaluate the performance of a waterproof, two-part epoxy resin (‘Mr Sticky Underwater Glue’).
On 31.01.08, 4 crabs were prepared as described above, and cradled dummy tags fixed to the carapace using Loctite superglue. As all of the tags shed initially had been from individuals treated with acetone, this procedure was omitted for the superglue-tagged crabs.
In the light of results from the first two experiments, a third combined option was used implemented to examine whether the relatively slow curing time of the epoxy resin might be alleviated by obtaining instant purchase through the superglue, thereby allowing the resin to cure fully, a possible solution where the field release of the crabs was required soon after tagging. Therefore, on 14.03.08, 10 stock crabs were tagged using epoxy resin and superglue. A small dab of superglue was applied centrally, with epoxy resin surrounding the base perimeter of the tag.
A more detailed account of the experiments is provided in Appendix I.
3.2.2 Field experiments
Between August 2008 and June 2009, 128 pot-trapped, female edible crabs (carapace width 138 – 288 mm) were tagged with Cefas G5 long-life electronic data storage tags (DSTs). The DSTs were encased in secondary, lozenge-shaped perspex casings designed specifically for M1103 (Figure 3.1). To maximise high resolution data capture, the tags were programmed to record pressure at 30 s intervals and temperature at 5 min intervals for the first year at liberty, then both at 5 min intervals thereafter.
Only recently moulted individuals with no obvious external damage were selected for tagging. Crabs were tagged first with claw tags, then DSTs glued to the posterior carapace. DST and tag numbers were checked on release and the position recorded from a handheld GPS.
The DST-tagged crabs were released at 5 locations: Eastern Channel (n = 32, August 2008); Trevose (n = 29, October 2008); South Devon (n = 30, June 2009); Channel Block C (n = 37, June 2009); and in 2010, 16 refurbished tags were reprogrammed and re-deployed in the eastern Channel on male crabs. Tags were returned through the commercial fishery following a concerted publicity campaign (Figure 3.1).
Returned DSTs were downloaded, and crab movements analysed using Tidal Location Method (TLM, based on tidal data recorded when crabs remained motionless on the seabed over a full tidal cycle, Hunter et al. 2003) and the Hidden Markov Model technique (HMM, incorporating both TLM and bathymetry, Pedersen et al., 2008).
Figure 3.1. (left) Electronic data storage tag designed for release on edible crab, Cancer pagurus, and (right) posters distributed around English Channel ports to advertise rewards associated with the electronic tagging programme
3.3. Results
SID 5 (Rev. 07/10) Page 7 of 130
3.3.1 Aquarium experiments investigating methods of attachment
Epoxy resin
Three lozenge tags were lost, 2 within 4 days of tagging, and the third 12 days after tagging. Two dummy cradle tags were also lost, the first near the beginning of the experiment (found 9/01/08), the second after 2 months (14/02/08). There were 2 mortalities: a 174 mm CW berried female died on 9/1/08 (no external damage apart from recently autotomised claws, the dummy cradle DST still securely attached, requiring force for removal); a 162 mm CW female control died on 30/12/08 (2 missing legs, no obvious pathology).
After 6 months, only 2 of 9 crabs tagged with lozenge tags still had tags attached, although these required removing using force. Of the 8 crabs tagged with dummy saddle tags or active G8 tags, 4 were dead, one of which still had a tag attached, and 4 living crabs all had tags attached. These all required force for the tags to be removed.
Superglue
Two of 4 tags became detached 20 and 27 days after tagging respectively. A further tag was lost before the end of the experiment on 25.06.08, such that only one tag was still attached at the end of the experiment. It was noted however, that this tag could only be removed from the carapace using force.
Combined use of epoxy resin and superglue
This approach resulted in a rapid strong bond immediately following lozenge tag attachment, but not with the saddle tags. Two saddle tags were lost immediately following release, the remaining 2 shortly thereafter. By contrast, all six lozenge tags stayed firmly attached until the end of the experiment and all required force to remove the tags, including one individual in which mortality had occurred.
3.3.2 Field experiments
3.3.2.1 Return rates of DSTs
With an overall total return rate of 34% (Table 3.1), return rates between release sites varied between 17% (Trevose) and 40% (South Devon). Only one crab was missing its DST on recapture (3413, Eastern Channel). A second individual was recaptured, then immediately re-released once the tag details had been noted (5077, Channel Block C (South Devon offshore)). Individual data records ranged between 8 and 575 days. From 46 returned DSTs, 4519 days of high resolution crab behaviour data were downloaded.
Table 3.1. Summary of data return for edible crab, Cancer pagurus tagged with DST
Fifty percent of all DST recaptures were made within 40 days of release (see Appendix I). The recapture rate thereafter had a seasonal distribution related to the reproductive behaviour of the crabs (see section 3.2.4 below). The 2 October 2008 releases yielded recaptures immediately following release until early December (DST 3404, 09/12/08, Eastern English Channel), with a pause then until the following June (end of the egg incubation). A similar pattern was observed the following June in the Western Channel. South Devon recaptures continued from release until mid-October, with no further recaptures until the following May.
3.3.2.2 Habitat Occupancy
Individual variability in depth occupancy was low for Trevose and Western Channel (South Devon and Channel Block C (South Devon offshore)) crabs. Both Trevose crabs that recorded data over the start of “brooding” (Nov-Dec) moved from approximately 65 m up to 55 m. Male crabs released on Sovereign Shoals also showed low depth variability, however the longest record to date is only 80 days. The highest levels of variability in depth occupancy were associated with Eastern Channel females. Much of this variability was associated with 3 crabs that recorded data for a year or more, and which exhibited long-distance migrations of between 173 and 302 km (see section 3.3.2.4).
There were also some differences in temperature between eastern and western Channel releases. Eastern Channel crabs followed a seasonal temperature cycle, however Western Channel crabs (located in deeper, colder water), did appear to move into warmer water with the onset of autumn, when temperatures would be expected to fall. This was the same time that the crabs were most active in terms of migration (Figure 3.2).
SID 5 (Rev. 07/10) Page 8 of 130
Figure 3.2. Average monthly (A) depths and (B) temperatures experienced by DST-tagged crabs, Cancer pagurus. EC = English Channel; Tr =Trevose; SD = South Devon; CBC = Channel Block C (South Devon Offshore); Sv = Sovereign Shoals
Individual records of depths and temperatures recorded (by release) are shown in Appendix 1.
3.3.2.3 Reconstruction of crab migration pathways
Examples of reconstructed movements for DST 3401 using TLM and the HMM geolocation toolbox shown in Figure 3.3.
Figure 3.3. Reconstructed movements of DST 3401, released in the eastern English Channel on 27/08/08 and recaptured 386 days later having migrated 302 km, using (left) tidal location method (●: release, x: recapture, ●: individual geolocations) and (right) HMM technique.
Note that using TLM, we were able to obtain a cluster of geolocations for DST3401, immediately following release, then during October at 0.5ºW. High levels of activity during migration meant that no further locations were then obtained until the time immediately before and during presumed egg-incubation. These placed the crab in the area where it was eventually recaptured (South Devon grounds). In this example, the HMM model successfully interpolated tidal and bathymetry data to create a most probable track (but see below).
Geolocation data suggested that for migrating crabs at least, movement between release and recapture does not depart appreciably from straight line movement, even where individual depth records clearly did not follow defined isobaths during migration. However, significant problems with geolocation were encountered due to low-level movement subtly distorting the recorded tidal signals: increasing or decreasing the amplitude of tidal range, and moving the peak (suggested time of high water). As a result, usually only a limited number of TLM geolocations were generated from a single track. In addition, the HMM model was designed for use with cod depth data. Although individual records could be “trained” in supervised mode (i.e. the input variables altered to more closely match the physiological movement abilities of a crab), many of the output tracks suggested migration rates well outside the locomotory capabilities of crabs. Further work will be required therefore in order to improve predictive geolocation, however this will involve additional modelling, and a re-programming of the HMM model, which was outwith the scope of this project.
3.3.2.4 Annual cycles and reproductive behaviour
The greatest insights resulted from 6 crabs that recorded data over a full annual cycle (DSTs 3401, 3422, 3428 (Eastern English Channel), 3398 (Trevose), 5048 and 5072 (Channel “Block C” (South Devon offshore)), and a further 3 crabs that recorded some of the egg-brooding period: DSTs 3388 (Trevose), 5058 and 5061 (South Devon). Metrics on the timing and duration of the brooding period are detailed in table 3.2.
Excepting the Trevose releases, all brooding crabs migrated west following release and prior to brooding (Figure 3.4). Unlike the other releases (see section 4), Trevose crabs did not appear to demonstrate directed migration, and mark-recapture data support the idea that there was limited dispersion of these crabs away from the point of release. Although the suggested brooding location for DST 3398 implies eastward movement towards the coast, it
SID 5 (Rev. 07/10) Page 9 of 130
Average monthly temperature (deg. C)
0
5
10
15
20
Aug
-08
Sep
-08
Oct
-08
Nov
-08
Dec
-08
Jan-
09
Feb-
09
Mar
-09
Apr
-09
May
-09
Jun-
09
Jul-0
9
Aug
-09
Sep
-09
Oct
-09
Nov
-09
Dec
-09
Jan-
10
Feb-
10
Mar
-10
Apr
-10
Tem
pera
ture
(deg
. C)
EC
Tr
SD
CBC
Sv
Average monthly depth (m)
0
25
50
75
100Au
g-08
Sep-
08
Oct
-08
Nov
-08
Dec
-08
Jan-
09
Feb-
09
Mar
-09
Apr-0
9
May
-09
Jun-
09
Jul-0
9
Aug-
09
Sep-
09
Oct
-09
Nov
-09
Dec
-09
Jan-
10
Feb-
10
Mar
-10
Apr-1
0
Dept
h (m
)
EC
Tr
SD
CBC
Sv
- 7 - 6 - 5 - 4 - 3 - 2 - 1 0 1 24 9
5 0
5 1
5 2
is likely that the imprecision associated with TLM in this area (see section 3.2.3 above) provides some degree of distortion. With little evidence of significant levels of activity, it is suggested that the true brooding locations of both these Trevose crabs was close to the release position.
Table 3.2. Brooding metrics (time period, depth and temperature at onset and completion, and location estimated using tidal location method) of the eight edible crabs, Cancer pagurus, that recorded over all or part of the egg-brooding season. “Tag fail” indicates where data recording ceased before the end of the brooding period
The results clearly demonstrate that Channel crabs are not restricted to a single, clearly defined brooding area, but that brooding occurred at various locations (and depths) throughout the Channel and Celtic Sea (Figure 3.4, Table 3.2). DST 3428, the only individual to record data over 2 brooding seasons, settled down in 2 locations separated by 2 degrees of longitude in the 2 years recorded. As all brooding crabs were also recaptured west of their brooding locations (some significantly so, e.g. 3422, Figure 3.4), our results suggest that the same brooding areas are not used in successive years.
Figure 3.4. Release, recapture and brooding locations and estimated geo-positions (based on tidal location method) of eight edible crabs Cancer pagurus, tagged with DSTs. Eastern Channel: A03422 (355 d, red); A03428 (575 d, purple); A03401 (386 d, orange); Trevose: A03388 (141 d, light pink); A03398 (253 d, pink); South Devon: A05058 (269 d, navy); A05061 (221 d, blue); Channel "Block C" (S. Devon offshore): A05048 (384 d, green); A05072 (323 d, turquoise).
Some intra-individual variability in depth during brooding suggests that some crabs are not always completely immobile during brooding. During her second season, DST 3428 stops “brooding” earlier than in the previous year (Table 3.2). The pot recapture of DST 3428 on 24/03/10 suggests that she was actively foraging. As individual carcasses are not returned, it is not possible to determine whether or not the crab was carrying eggs in both years.
3.4. Discussion
3.4.1 Aquarium experiments
Aquarium conditions in the current experiment were designed to simulate, as far as practically possible, field conditions, allowing realistic amounts of interaction between individuals, and to test the strength of the bond between carapace and tag by allowing abrasion due to burrowing in sediments and crawling in confined rocky
SID 5 (Rev. 07/10) Page 10 of 130
3428
areas. The other main problem with tag attachment using epoxy resin was the relatively long curing time of the glue, probably accentuated here by the relatively low operating temperature. Even under ideal conditions, the epoxy resin (specifically designed for use underwater) had a curing time of 2 hours.
The combined use of resin and superglue using the larger lozenge tags gave by far the best results, allowing a clear separation between the 2 glues, and sufficient surface area to spread out on the rugose surface of the carapace. Both the lozenge tag, and the combined use of superglue and resin were therefore chosen for the field release of DST-tagged crabs.
For the limited number of published field experiments where spider crabs (Friere et al., 1999, González Gurriarán et al., 2002), crabs (Ungfors et al., 2007, Curtis & McGaw, 2008) and lobsters (Smith et al., 1998) have been tagged with telemetry tags and DSTs, all have used fast-setting epoxy resins. For future study, it is worth noting that dentistry, medical and offshore industries all provide examples of situations requiring adhesives sometimes for applications similar to the experimental protocol described above.
3.4.2. Field experiment
The return rates of edible crabs tagged with DSTs greatly surpassed expectations, with a high overall return rate of 34%. At 17%, least DSTs were returned from the Trevose grounds, compared with return rates of 38% and 40% in other areas, reflecting the levels of fishing effort experienced in the different areas (see section 4). Although 6 of the only 16 male crabs released (March 2010) in the eastern Channel were recaptured, the longest data record was just eighty days. More male recaptures may still be returned, however the relative paucity of male data means that the results focus predominantly on female behaviour.
Remarkably, only one crab was returned missing its DST. The tags proved robust, and recorded 4519 of 5540 days at liberty (82%). Data download failure occurred from 2 tags (DSTs 5077 and 5098, both Channel Block C (South Devon offshore)) and sensor failure prior to recapture occurred in 6 other tags. This negatively impacted our findings on the annual cycle of behaviour for 5 crabs, all from the western Channel. DST 3401, one of the longest data records, and one of the furthest migrating individuals, was also one of the only individuals to move into water > 100m. Our tags were programmed for use ≤100m depth. The sensor actually functioned to 113m depth, and recorded “112” until the crab re-emerged above 112m. However, a second excursion >112m resulted in a terminal sensor failure - however a full cycle had already been captured. Several of our tagged crabs were briefly recaptured and re-released. It was noted that crab fishers widely believe that actual position can accurately be determined from the tag. This is not, in fact, the case.
There was relatively little within-release seasonal variability in temperature and depth experienced, although most variability was associated with longer records and hence migrating individuals. The data clearly demonstrate that migrating crabs do not follow defined depth bands. We did find some evidence that crabs released in the western Channel may have sought out warmer water during the pre-brooding migration. While temperatures recorded by eastern Channel releases from late summer through autumn showed declining temperatures from release (August) onwards, the temperatures recorded by the western releases continued to rise from August until October before a decline was observed.
Track reconstruction suggested that movement of crabs between release and recapture positions showed relatively little deviation from a straight line path. However, neither TLM nor HMM performed as well as initially expected. Although the relatively low-level movement recorded by crabs appears to provide a strong record of tidal conditions, this is, in fact, deceptive. Even low-level activity was adequate to distort the tidal information required by both track-reconstruction techniques. This is further complicated by the relatively complex tidal situation in the Channel, where 2 tidal “solutions” (i.e. the same time of high water and tidal range) can often occur within 1 degree of longitude (Huntley, 1980).
Consequently, the accuracy of TLM is greatly reduced, and a spread of “solutions” were often observed parallel to, or around the area where mark-recapture data suggested might be the more probable location. In addition, the relatively small scale of movement, compared, for example, with the fish species which have very successfully been studied using TLM (e.g. Metcalfe et al., 2008, Hunter et al., 2009, Righton et al., 2010), the significant inter-individual differences in scale of migration, and the slight depth gradients in many parts of the Channel, made it difficult to adequately discriminate between “correct” and “false” solutions. While HMM should have allowed for some correction by combining tidal data with bathymetry, we were unable to adequately constrain the HMM model to the low-levels of movement exhibited by most of our migrating crabs. As the model was originally developed to reconstruct demersal fish migrations (principally cod, Pedersen et al., 2008), it seems that the model will require some re-coding. We anticipate that this can be achieved, and will greatly enhance the “most probable tracks” generated from these data.
SID 5 (Rev. 07/10) Page 11 of 130
Previous observations on the brooding behaviour of crabs have been largely restricted to indirect observations on the occurrence and distribution of egg-bearing crabs, in landings or in scuba surveys (this study, Howard, 1982, Latrouite & Phillipe, 1993, Ungfors, 2007), and from histological examination (Lawler, unpublished data). The onset of brooding, or at least the time at which the crabs appeared to cease most activity, corresponded well with previous observations by Latrouite & Phillipe (1993) which suggested commencement of brooding from around mid-November through to early January. The earliest onset in the current study was from late October (Channel Block C). There was some indication that the westerly crabs started slightly earlier than those in the eastern Channel, which tended not to commence brooding until mid- to late November onwards.
The average duration of brooding was 177±24 days. By far the shortest duration observed was 126 days during the second brooding season recorded by DST 3428. Our results demonstrate that brooding females became largely inactive throughout incubation. After brooding they started to forage for food, most having been recaptured in baited pots.
Interestingly, DST 3428 migrated west prior to both incubation periods. The 2 brooding locations were separated by 2 degrees of longitude. This observation, and the fact that all of our other Channel brooding females were recaptured west of their brooding locations, suggests that females show no brooding-site fidelity in successive years. DST 3428 clearly did not moult - however, as no carcass was recovered during these experiments, we cannot be certain when or if a crab was carrying eggs. Although the timing of “incubation” was similar in both years, there was some evidence of higher levels of activity during the second season (lasting 126, as opposed to 188 days).
Estimated brooding locations, based on TLM, suggested that brooding is not restricted to one or two, clearly defined areas, but may occur at various locations throughout the Channel and Celtic Sea (although these are probably defined by substrate characteristics, Howard 1982). The average depth of brooding was 57±23m, but ranged from 19m in the shallower Eastern Channel grounds, to 84m in the deeper South Devon grounds. Temperature at the onset of brooding was 13 ± 1.5ºC, and 11 ± 2ºC when the females regained activity. The lowest temperatures at brooding onset were recorded at Trevose (10.7ºC), and the highest in the deepest (Channel Block C (South Devon offshore)) grounds (15 ºC). By contrast, brooding stopped earlier, and at lower temperatures in the Western Channel (although due to tag failure, we have no data for South Devon). Again, the exception was DST 3428, which was the earliest to cease brooding (24/03/10), at the lowest temperature (6.7ºC), when she was recaptured just off Portland Bill.
Both the current study and previous mark-recapture experiments carried out in the 1970s (e.g. Bennett & Brown 1983) have demonstrated long distance, predominantly westward movements by crabs in the English Channel, particularly mature females. None of our DST tagged crabs exhibited west to east migration. However, with the possible exception of the Trevose releases, all of our “brooding” crabs were also migratory. Work carried out under M1103 has allowed us to detail how these migrations proceeded in terms of behaviour traits, and the environmental conditions experienced during migration. Although further work will be required in order to refine the accuracy of our geolocated crab-tracks, at the scale of fishery management, our study provides valuable new information on the rates and scale of movements.
4. Objective 2. Investigation of patterns of movements of adult edible crabs using conventional tagging methods
A substantial conventional tagging programme, using double T-bar tags, was implemented tagging at 11 sites through the English Channel and into the Celtic Sea. The tagging programme was supported by a publicity campaign and by aquarium experiments to investigate tag retention rates (sees 4.1). A more detailed description of the tagging methodology and results is included in Appendix III.
4.1 Aquarium experiments to evaluate tag retention rates (for double T-bar tags)
Without information on the rate of tag loss, tagging programmes yield largely qualitative and descriptive results. Quantification of rates of mortality, in particular, is greatly improved if quantitative data on rates of tag loss (or conversely retention) are available. To this end we carried out a series of six small scale aquarium experiments to investigate rates of loss of double T-bar tags in edible crabs. In each experiment around 20 crabs were held in individual pens within a large aquarium raceway for a period of between 100 and 200 days. At the end of this time, surviving crabs were moved to holding tanks and held in small groups where monitoring continued in order to extend the experimental time period. A brief summary of the results (Table 4.1.1) shows that the tag loss rate was quite low, but analyses were made more difficult due to high rates of crab mortality during the experiments.
Table 4.1.1. Number and percentage of tags lost during experimental and additional observation periodsExperiment
SID 5 (Rev. 07/10) Page 12 of 130
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
0 100 200 300 400 500 600 700 800
Prop
ortio
n of
tag
s re
tain
ed
Days at liberty
of total double tag returnsof retuns to capture dateConstant instantaneous loss
1 2 3 4 5 6 TotalTags lost 5 1 4 0 2 1 13
n tagged 19 16 17 13 13 17 95
% lost 26.3 6.3 23.5 0.0 15.4 5.8 13.7
The data were analysed using the ‘survival’ package (Therneau & Lumley, 2009) in R (R Development Core Team, 2009), which has been developed to analyse this type of ‘right censored’ data. Tag retention rates for the experimental periods only and the full holding period were not statistically different between experiments, but experiment 1 performed more poorly and was significantly different from results for all experiments combined. This was thought to reflect inexperience in the tagging procedure early on in the project.
A retention curve for all experiments combined and including the time in the holding tanks (Figure 4.1.1) suggested that on average after 200 days and longer around 75% of tags will be retained and this may be as high as 80% if experiment 1 was excluded. A parametric curve for tag retention with exponential decline was fitted to estimate an instantaneous coefficient of daily rate of tag loss (Lambda=0.0010320) of around 0.031 per month, that was used in the modelling analyses.
A total of 6 tagged crabs moulted during the aquarium experiments of which 4-5 retained their double T-bar tag (1 was lost immediately after moulting), a retention rate of 67-83%, very similar to the overall retention rates observed. Unfortunately, high temperatures coincided with the summer moult season, causing high mortality and reducing the potential for observing occurrences of moulting.
Figure 4.1.1. Combined double T-bar tag retention curves for all aquarium experiments with parametric exponential curve superimposed with 95% C.I. dashed lines (left) and (right) double T-bar tag retention curves from double tagging field experiments
A programme of double tagging (using 2 tags per crab) a subset of crabs released during the field tagging programme produced quite similar results to the aquarium experiments, with retention stabilising after around 80-100 days at around 75% (Figure 4.1.1). A parametric curve for instantaneous exponential loss rate was estimated with a rate of 0.019 per month. This is comparable with the aquarium experiments, as it is an under-estimate, not accounting for the potential loss of both tags. However, the continuous loss rate model did not seem particularly appropriate given the shape of the curves and alternative models should be considered for future investigations.
The aquarium experiments also provided information regarding tag related mortality, although results relating to mortality were not considered transferable to the wild situation because conditions in the Lowestoft aquarium facility were not well suited to keeping edible crabs and survival rates were generally poor, particularly during warm weather. Mortality in tagged crabs was generally higher than in the control tags and significant overall. However, this was primarily due to experiment 1, with mortality in the remaining experiments not significantly different between tagged and control crabs. This was attributed to combined effects of poor tagging technique and high temperatures that occurred during experiment 1. High water temperatures in later experiments (4 & 6) resulted in high mortalities for both control and tagged crabs.
SID 5 (Rev. 07/10) Page 13 of 130
The aquarium experiments also provided observational data on spawning and growth and are documented in more detail in Appendix II.
4.2 Double T-bar tagging programme relating to movements
Between October 2007 and August 2009, 15,712 tagged crabs were released in 12 locations through the Channel and off the north Cornish coast and by the end of 2010 2,475 (15.8%) recaptures had been reported (Figure 4.2.1). Recapture rates varied between 6% and 32 % (compared with 1% to 43%, 17% overall, Bennett & Brown, 1983). Highest return rates were from release areas with the greatest effort or in close proximity to other potting grounds (e.g. south Devon) and from the eastern Channel releases, where the tagged crabs were at liberty for the longest time and could potentially move through many fishing areas if they moved west. Typically for this type of mark-recapture experiment, 54% of all returns were made within a month of release and 96.6% within one year. Returns after longer durations (7 in the third year and 3 in the fourth), were all from the earliest releases, primarily made in the eastern Channel. Similar statistics from Bennett & Brown (1983) were lower at 83% in the first year and “at least 84% by the end of the second year” sic. This was probably a consequence of releasing smaller batches over much longer time periods and the lower fishing effort at that time. In those studies 10nm (18km) was considered to be the lower limit of spatial resolution, but modern GPS technology used in this study (and by the fishing industry) permits very precise recording of release and recapture positions, enabling analysis of short distance movements by the crabs and the calculation of a range of parameters describing crab movements.
The overall pattern of movements (Figure 4.2.1) was very similar to that obtained by Bennett & Brown (1983) with striking long distance movements by female crabs in a westerly or south-westerly direction tending to obscure many smaller scale movements by crabs taken soon after release.
Figure 4.2.1. GIS summary of all tagging returns
Rates of movement for females from all releases were broadly similar and, excluding outlying values, ranged between 0.8 and 6.7km/wk. Female crabs released late in the year generally travelled further and faster, in a southwest to west direction, than those released during the first half of the year. Fahy et al. (2004) noted similar rates of movement for both sexes in south east Ireland, relatively high in spring, lowest in June then increasing to a maximum in September/October. The greatest rate of movement (and lowest tag return rate) was for females tagged in mid-Channel in August – average distance moved 59km at a mean rate of 6.7km/wk. This suggested that these crabs were rapidly moving out of the area, mainly to the SW into French waters, an area where there was little potting taking place or poor reporting of recaptures. The greatest time at liberty and longest individual movements were by crabs tagged in the eastern Channel, a result of being the first releases, the direction of movement of the crabs and of the geographical position of the release area relative to the main potting grounds. Low return rates from the westerly and mid-Channel releases most likely reflected restricted distribution and/or low levels of fishing effort in the direction moved by the crabs.
SID 5 (Rev. 07/10) Page 14 of 130
Far fewer males than females were tagged and results were much more limited. Rates of movements for males were generally lower than for the females, ranging between 0.2 km/week (west of Land’s End) and 2.5km/week (south Devon inshore in 2008). Rates of movement from Shingle Bank in the Eastern Channel were also moderately high at 2.4km/week. Movements were predominantly W, NW or SW in direction, but mean bearings of male crabs recaptured from the Sovereign Shoals and west of Land’s End releases were E and NE. Movements of males were generally of a lower magnitude than females, with a maximum mean distance of 8km for males, observed on the Trevose grounds, compared to 59km for the females, released on the Sovereign Shoals (outliers excluded).
Movement parameters calculated using the methods of Jones (1976) confirmed that movements by males tended to be less systematic than those by females, which were more directed, with greater movement during the latter part of the year and in directions between southwest and northwest. There was no evidence for systematic easterly movement in the early part of the year, as suggested by some fishermen or for return migrations, as reported for the Malin crab fishery (Tully et al., 2006).
Patterns of dispersal
FemalesIf short distance recoveries <18km were excluded from the analysis, as per Bennett & Brown (1983), then the overall direction of movement of females from all releases was generally between west and west of south. This was especially apparent for returns of crabs tagged in October/November in the eastern Channel and off Portland Bill, where the mean bearing of the recoveries lay in a narrow range between 247o and 277o. Long distance recoveries of tagged females from the eastern Channel came from the south Devon and mid-Channel potting grounds, but the only recoveries “en route” were single crabs from West Sussex and the Isle of Wight and two off the Dorset coast. Early summer releases off south Devon and west of The Lizard showed some short term movements inshore mainly in a northwest direction. Inshore movements by female crabs during the spring have also been noted in the south east Ireland crab fishery (Fahy et al., 2004). No evidence was found for systematic early season west to east movement of females, as suggested by some members of the fishing industry.
Returns of females from releases in mid-Channel in August 2009 indicated rapid movements, sometimes over 100km, within 2 months of release and mainly in the south-western quadrant. Just three recaptures were made in the release area, during the winter of 2009/2010. Thereafter all recaptures up to the end of 2010 were distant (>100km) and to the west and southwest, with two having reached waters west of the Brittany peninsula.
A lack of distant returns from the westernmost fisheries, on the Trevose Grounds and west of Lands End, limits conclusions that can be drawn about directed movements from these areas and connectivity between crab stocks in the Celtic Sea and the western Channel. Initial dispersal showed greatest movement to the southwest and the mean bearing of movements was also southwest, suggesting that if directed movement was occurring, then it was likely to take the crabs out into the Celtic Sea and away from local UK fisheries. With relatively low potting effort outside the release area and the difficulty of obtaining returns from foreign fishing vessels, this analysis was based mainly on the short term movements. Only 26 of 677 recaptures were >18km from the point of release and the early winter tagging on the Trevose grounds and west of Lands End were the only release batches with no recaptures beyond 12 months at liberty. A female recapture from a claw tagging programme in south east Ireland was recovered west of the Scillies (245km south) almost 2 years after release, whilst another female from this release moved 270km to the southwest (Fahy & Carroll, 2008). Most of the crabs in this programme were recaptured locally and indicated a south-westerly direction of movement, with rather than against prevailing current flows (Fahy et al., 2004).
MalesAlthough overall return rate of male crabs (16%) was similar to that of females (15%), there was greater variation in individual return rates from different releases due to differences in availability of male crabs. This varied seasonally and with market forces. Thus, in the Lands End/Trevose releases males were outnumbered 11:1 by females, this rising to 31:1 on the Devon grounds in May/June, but falling to less than 3:1 for the mid-Channel and Lizard releases. Although there were a few longer movements by males, most notably 4 males moving across Lyme Bay from Portland to the south Devon grounds, only 19 (5%) of the 354 males recaptured to date had moved >18km from their release point (compared with 13% of females). There was no obvious pattern to the movements by males with dispersal after tagging in all directions, a very similar conclusion to that of Bennett & Brown (1983).
A crab tagging programme in South Wales running concurrently with this study engendered very few returns, whilst crab-tagging proposed as part of the WaveHub programme off the north Cornwall coast did not occurr and a Cefas tagging trip planned in this project for the Lundy area was called off due to bad weather and recurring logistical difficulties. Planned collaboration between projects, with shared data from a chain of release sites between Lands End and South Wales, was therefore not possible.
SID 5 (Rev. 07/10) Page 15 of 130
Summary of main findings
Overall recapture rate for the double T-bar tagging programme was around 15%, very similar to that achieved using suture tags by Bennett & Brown (1983).
Typically >90% of recaptures were in the first year of release and most of these in the first few months, with very low numbers of recaptures occurring in subsequent years.
Crab sex and release location both strongly influenced rates and magnitude of movement. Recaptured female crabs had moved predominantly in a NW, W or SW direction. Movements of recaptured male crabs included directional components in a NW, W and SW direction, but
proportionally more local and undirected movements. Females generally moved faster and further than their male counterparts. Highest rates of movement occurred in October and November. Crabs released early on in the study at the eastern end of the Channel moved the furthest, whilst those released
at the western end were predominantly recaptured close to their release site. This may reflect an absence of major UK fisheries surrounding these release areas.
There was no evidence to support assertions by some fishers in the South Devon and Cornwall that crabs move in from the west in an easterly migration at the onset of each fishing season.
Some indication of crabs moving around coastal bays was noted, possibly inferring an inshore migration combined with westerly movements, but this could also reflect local distribution of gear.
Seasonal rates of movement support the biological knowledge relating to a sedentary period from December to May, during which time female crabs spawn and incubate eggs.
Physical characteristics of the released crabs such as shell condition and missing legs or claws influence the rate, and sometimes, the magnitude of movements.
4.3 Population dynamics model and its results with respect to crab movements
Section 4.2 described patterns of movement inferred from the tagging data and general measures relating to movement and dispersion of the tagged crab populations. This section briefly presents results obtained by fitting a population dynamics model to spatio-temporally structured tag release and recapture data from both the double T-bar and data storage tagging programmes. The aim of this approach was to synthesise a range of data and quantify movement rates and catchability parameters by taking account of seasonality in crab biology and behaviour as well as utilising commercial effort data available for the fisheries. A more detailed description of the study is provided in Appendix IV.
The modelling framework was based on Hilborn (1990), with modifications to include parameters for tag loss and reporting rates suggested by Aires-da-Silva et al. (2005). It was seasonally disaggregated, specifically to take account of the reproductive cycle of edible crabs, which profoundly affects their catchability. A 3 season model was initially considered, but was subsequently modified to a 2 season model (summer & autumn and winter & spring) in order to reduce the number of parameters to be estimated. The model included 12 nominal spatial units, not of equal size, with movement in any single monthly time step constrained to neighbouring spatial units. This was considered reasonable, given the slow rate of movement possible by edible crabs. An additional area outside the model system was also included, into which crabs could move, but not return. Data and expectations for this area were not included in the minimisaton objective function. Fishing effort data, as total numbers of pot hauls per month by ICES rectangle, were extracted from the Fishing Activity Database (FAD) and allocated to each spatial unit, where necessary (e.g. rectangles did not coincide exactly with spatial fishery unit) apportioning effort from one rectangle between spatial units.
Tagging data utilised, consisted of numbers of tag releases and returns by spatial unit and month for 27 tag groups for females, 22 of which were double T-bar and 5 of which were DSTs and 17 tag groups for males, including just one of DSTs made late on in the project (Table AIV.4). Tag groups included a number of re-releases of very small numbers of tagged crabs by fishermen. Minimisation followed Hilborn’s (1990) method, using a Poisson likelihood function, but minimised the simplified negative log likelihood function of Aires-da-Silva et al. (2005), which is computationally preferable.
Separate sex models were implemented, each requiring estimation of 117 parameters; 92 for movement by season and relevant spatial unit combinations, 24 for catchability by season and spatial unit and 1 for double T-bar tag reporting rate relative to DSTs, with the latter assumed 100% reported.
The modelling framework was implemented using MS Excel, supported by Visual Basic for Applications (VBA) functions and sub-routines. Although this was beneficial in helping to visualise relationships within the model structure, minimisation using the MS Solver add-in proved somewhat problematic and necessitated iteratively applying Solver to groups of cells using VBA to control the process. Minimisations were carried out iteratively until
SID 5 (Rev. 07/10) Page 16 of 130
changes in the objective function were minimal, but it is difficult to be certain that a stable and global minimum had been reached particularly in a multi-dimensional problem such as this with so many parameters. Nonetheless there was substantial change from the starting parameter values and results were generally sensible in most areas when considered in the context of the input data. Future work might involve including more complexity to the model (e.g. introducing a third season), but prior to this, developing the model in an application with more powerful minimisation capabilities would be preferable (e.g. AD Model Builder).
With far fewer data available for male crabs, outputs from the modelling exercises for males were much more uncertain and results therefore concentrate mainly on females.
Model results suggested that within any one month most female crabs stayed in their ‘current’ area, this being the case in 10 of the 12 areas during summer & autumn and 7 of 12 areas in winter & spring, the latter counter intuitive given knowledge of the reproductive cycle (see below). Exceptions were Lands End & Scilly and south Devon inshore during summer & autumn and Lizard & west Cornwall, south Cornwall inshore, south Cornwall offshore, south Devon Inshore and south Devon offshore during winter & spring. The summer & autumn movements (Figure 4.3.1) from Lands End & Scilly (eastwards to Lizard & west Cornwall) were considered erroneous and the first of the listed winter & spring exceptions movements is a reciprocal movement of almost exactly the same magnitude. The other exceptions tended to be between inshore and offshore areas of the south Devon and Cornwall coastlines. These may be realistic, although the offshore Devon release area was very close to the boundary with the inshore area, the south Cornwall areas did not have any direct presence of project staff and the offshore area was thought to have relatively poor effort data.
Figure 4.3.1. Expected monthly rates of movement predicted by the dynamics model for female crabs during summer and autumn
The greater number of exceptions during the winter & spring season is counter intuitive as most females would be expected to be ovigerous and therefore not likely to move far at this time. This could indicate the seasonality currently applied in the model is wrongly specified and/or insufficient to capture seasonal pattern of movement adequately, or it could reflect errors in the data, possibly due to lower catch rates during the winter & spring season.
Although within any one month the majority of the population remain stationary, monthly rates of movement for many areas were sufficient that the population of female crabs surviving after several years could have moved out completely. This scenario was clearly demonstrated by input data for tag groups of female crabs released in the eastern English Channel (e.g. the Shingle Bank & Sovereign Shoals tag group, Figure 4.3.2) where the shift in distribution of recaptures through time to areas further west (left on the matrix) was striking. The seasonality of the fishery is clearly shown, with recoveries in most years occurring between June and December, although some recoveries were also made in January, April and May during the first year after release. During the remainder of the
SID 5 (Rev. 07/10) Page 17 of 130
release season all recaptures were in the release area (or the nearby Bullock Bank, further east, although there may be some potential for the latter being mistakenly reported here). During the second season most female crabs were still taken in the release area, with small proportions in the nearest westerly neighbour (crab fishing effort is low in this area) and much further to the west in the major south Devon fishery. During the third fishing season only a small proportion were recaptured in the release area, whilst most were reported from areas further west, mainly the south Devon inshore fishery. In the fourth season after release, none were recaptured in the release area, but recaptures continued in the major fishery areas further west. This pattern was very closely replicated for the other eastern Channel tag group released on Bullock Bank and to a lesser extent for later releases made later and further west (e.g. Portland, south Devon inshore and offshore and mid Channel). The implication is that over a period of around 3 to 4 years all surviving crabs from the populations in the eastern Channel have migrated to the major fishing grounds in the western Channel. This suggests that crab populations in the eastern Channel are net exporters of adult crabs to the western Channel.
However, it is important to consider the scale of the fisheries and populations involved and the potential sources of recruitment, although direct evidence for quantification of some of these factors may be scarce. Crab populations and fisheries in the western Channel are much larger than those in the eastern Channel and it has been shown (Eaton et al., in prep.) that prevailing current flows would be insufficient for larvae hatched on the western Channel fishing grounds to drift fully to the eastern Channel fishing grounds within the planktonic larval duration. Adult crabs are numerous throughout both western and eastern Channel fishing grounds and conditions are suitable for spawning and incubation, so local recruitment may be substantial. Cefas plankton surveys in 1989 located high densities of crab larvae near the Dover Straits, which provide evidence for significant local spawning in the eastern Channel. Information from the DST programme has suggested that some crabs were incubating eggs in locations off the west Sussex coast, an area where the pot fishery is focussed on lobsters, but which has significant areas of gravel banks that could provide suitable spawning and incubation substrates for edible crabs. Crab larvae hatched in this area would be capable of, and tend to, drift back to the fishing grounds of the eastern Channel. Cefas plankton surveys in the western Channel have frequently shown high concentrations of crab larvae near the major fishing grounds, particularly off south Devon and west and north Cornwall. Small crabs are also common on the coasts of the south west and in the inshore fisheries of the western Channel. These two facts tend to suggest that crab populations in the western Channel have substantial local recruitment and given the scale of the fisheries they are unlikely to depend on immigration from the Eastern Channel, although this does occur. Some of the most striking movements detected by the double T-bar programme were from Portland to the south Devon inshore grounds, two areas that would fit easily into a theory of westerly migration to a major spawning ground and largely easterly larval drift to an inshore nursery area. Edible crabs are numerous and ubiquitous throughout the Channel and it seems likely that spawning occurs in a wide variety of locations, with some areas that are particularly suitable and form the major spawning grounds. Larval drift is primarily eastwards and compensated for by westerly movements of adults. The rate of westerly movement by adults is such that it may take several years for the population to move between major fishing grounds (e.g. from the eastern Channel to the western Channel) and during this time the female crabs may have spawned several times in intermediate locations, thereby providing sustained larval supply to natal areas. However, the DST programme indicates that some female crabs may undertake these large scale movements in a single season.
Figure 4.3.2. Observed tag recovery matrix for tag group 1 expressed as % of released number, showing recoveries by recapture area (columns) and time (rows). Red area title: area of release. Blue text: tag group. Red number: number released. Cell shading red: 10%+, orange: 1-9.9%, yellow: <1%
SID 5 (Rev. 07/10) Page 18 of 130
Input matrices of returns through time and space for males did not show the same pattern as females (Figure AIV.7). Fewer males were tagged and released and returns were not reported beyond the third season at liberty as a maximum. Most recaptures were made in the release area, with some in the nearest neighbour to the west. None were made beyond the nearest neighbour.
Systematic westerly movements for females and less marked movements for males concur with the results obtained by Bennett & Brown (1983) and suggest strongly that the movements are related to the reproductive cycle. That observed movements are always westerly and contra to the prevailing current flows suggests strongly this is related to larval dispersal rather than seeking areas suitable for spawning and incubation which could occur in any direction, depending on current location.
This study extended tagging to fishing grounds off west and north Cornwall, but in these areas substantial movements were not detected, possibly because of the more localised nature of the fisheries. It was not possible to comment with certainty on likely movements of crabs in the Celtic Sea and the interactions between the Channel and Celtic Sea crab populations in the Lands End area. A claw tagged female crab recaptured west of Scilly (245km south) almost 2 years after release, inshore off south east Ireland (Fahy & Carroll, 2008), provides some evidence for long distance crab movements in the Celtic Sea, although most of that programme’s recoveries occurred locally (Fahy et al., 2004).
Catchability parameters were estimated by spatial area and season (Table AIV.1) in separate sex models. They provide scalars for effort data, including taking account of the different spatial extent of the fishery areas. Catchability parameters ranged from 3.7E-07 to 3.5E-05 for females during summer & autumn and with one exception were lower during winter & spring when they ranged from 9.1E-08 to 5.9E-06. The exception was for the Lizard & west Cornwall area, previously highlighted as seeming to provide erroneous results. Catchability parameters estimated for male crabs were similar in magnitude, ranging from 5.9E-07 to 4.3E-05 during summer & autumn and in contrast to the females they tended to be slightly higher during winter & spring (9.9E-07 - 2.2E-05). Zero catchabilities, reflecting no tag returns, occurred in a number of areas for male crabs. Lower catchability estimates for females and higher estimates for males in winter & spring conform well to the behaviours of edible crabs and the fishery; most females being ovigerous over winter and larger vessels that remain active focussing more on male crabs. Monthly fishing effort data were multiplied by catchability parameters to provide estimates of exploitation rate over the study period (see section 5.1 and Appendix V).
The only other parameters estimated were reporting rates for double T-bar tags relative to DSTs, the latter of which were assumed to be 100% reported. Reporting rates were estimated to be 0.629 for females and 0.815 for males. Very few DSTs were applied to male crabs and these were the final release made in the eastern Channel, leaving scope only for in-year returns within the project’s duration (Table AIV.4). Further, substantially lower numbers of double T-bar tags were applied to male crabs than females. Reporting rate for females is likely to be better estimated on both counts.
Implementing a complex population dynamics model such as this requires utilisation of all the tagging data in order to preserve ratios between releases and recaptures, so assumptions need to be made where data are incomplete. There is thus significant potential for data errors and the introduction of false trends where the data errors are systematic (e.g. reporting rates differ markedly between areas). Further, all models are simplifications, with assumptions that may not be fully met and in this case a 2 (rather than 3) season model was implemented to facilitate minimisation and 3 seasons would have been preferable from a biological perspective. Results should therefore be treated with caution.
This first attempt to implement such an approach proved complex and gave mixed results, some highly plausible and informative and others seemingly erroneous. Errors could be introduced either because the modelled dynamics do not capture population behaviour effectively or because there are errors or deficiencies in the data. It is likely that both of these may have occurred and although useful results were obtained in some areas there were clear problems in others. Further modelling work could address model deficiencies, but there is little scope to reduce data deficiencies. Using MS Excel as a modelling framework was beneficial in conceptualising the model dynamics, but limited in its minimisation capabilities. Implementing the model in a more powerful minimisation framework, such as AD model builder, would be advantageous for future work, permitting more rapid and reliable comparison between scenarios for different model variations.
5. Objective 3. Utilisation of returns from the tagging programmes to estimate exploitation and growth rates for edible crabs in the Channel fishery
5.1 Using data from the tagging programmes to provide estimates of exploitation
Three approaches were taken to investigate mortality rates using data generated from the tagging programmes, each using different sources of data and thereby providing a degree of independence.
SID 5 (Rev. 07/10) Page 19 of 130
5.1.1 Catch curve analysis
This analysis followed the rationale of Bennett (1979) and used growth parameters estimated from double T-bar tag returns from this project (or Bennett’s 1974 and 1979 works) to ‘slice’ length distributions obtained from this project into age distributions and apply a catch curve analysis to these age distributions to estimate total mortality (Z). Growth analyses indicated no statistically significant difference between female moult frequency and increment parameters estimated in this study and those of Bennett (1974) and as Bennett had more data and some bias was identified in this project’s data, Bennett’s (1979) growth parameters were used initially. Estimates of Z for female crabs were relatively low, ranging from around 0.15 to 0.5 and relatively high total mortalities were estimated for male crabs with a range around 0.5 to 0.85 (Table AV.1). Total mortalities for both sexes were lower in the eastern Channel, intermediate of the Devon coast and higher in west Cornwall. Bennett (1979) estimated Zs for females that were higher in spring and lower in autumn than these results, whilst his estimates of Z for males were comparable or lower than estimates from this study. Bennett’s (1979) low autumn estimate of Z for females is unintuitive, since this is the peak fishing season and females dominate catches, but was explained as being due to changes in the distribution of larger females.
Application of a more modern generalised anniversary method that utilises more of the data (McGarvey et al., 2002) resulted in higher estimates of moult frequency for females (insufficient data on growth of males were generated by this study) and the same ‘slicing’ and catch curve methodology was applied using these growth parameters. This resulted in considerably higher estimates for Z, with modes approximately doubled, the 1st estimate in the range of 0.3 to 0.75 (Table AV.2) and the same spatial pattern, since the same length distributions were used. These estimates tended to be comparable with, or higher than, those estimated for females by Bennett (1979).
The relatively low total mortality estimates for females in this study were also somewhat counter intuitive given the high proportion of females in the catch, particularly in the western Channel and Celtic Sea. Aggregation of large females on pre-spawning grounds could influence the results in this way, as could biases in the estimation of growth parameters that tend to under-estimate the potential for growth, especially at larger sizes. In general terms these catch curve analyses suggested total mortalities were moderate for female crabs and high for males.
5.1.2 Exploitation rates estimated from the population dynamics model
The second approach to investigate mortality rates used catchability estimates from a population dynamics model incorporating the release and recapture data for both double T-bar and data storage tagging programmes, together with reported fishing effort data, one of the inputs to the model. Seasonal exploitation rates for both sexes of crabs in 12 spatial areas were estimated (Table AV.3).
Time series of exploitation rate estimates (Figure AV.2) followed a generally similar oscillating pattern to the spatio-temporal distribution of effort (Figure AV.1) providing a clear indication of the highly seasonal nature of the crab fisheries. However, there were some marked differences between sexes, with exploitation rates for females being highest during summer & autumn in the Lands End & Scilly and south Devon inshore areas. These are two of the main fishery areas, where female crabs are thought to aggregate prior to spawning and which are also noted for high concentrations of early stage larvae, indicative of spawning/incubation/hatching areas. Seasonality of the high exploitation rates was also in concordance with the peak of activity and female catches at this time. Exploitation rates for males in these areas had less seasonality and were low and moderate, respectively.
Exploitation rates were similar for males and females in the Bullock Bank, Shingle & Sovereign, Portland and Lizard & west Cornwall areas, being low to moderate in level in the first 3 of these areas and moderate to high in the latter with less seasonality apparent for males. The Trevose area was the only area where there seemed to be a major disparity in seasonality between males and females, with low to moderate exploitation rates for both sexes in summer & autumn, minimal exploitation levels for females in winter & spring, when the exploitation rate for males was periodically high. Low return rates for both sexes and tag types gave some concerns in this area (Table AIV.4).
The population dynamics model fitted relatively poorly for the Lands End & Scilly and Lizard & west Cornwall areas, as residuals were high and some results for movements appeared erroneous, so results for these areas need to be treated with more caution. Results for males in general need to be treated with caution because the numbers of male crabs tagged and recaptured were much lower than for females and results were therefore more uncertain. Seasonally high exploitation rates for males, with minimal female exploitation rates were estimated in the south Cornwall inshore, Wight & west Sussex areas and mid-Channel, the first two of which were areas where no crabs were released and project presence was low. The mid-Channel area had the lowest return rate observed, so these results need to be treated with caution. Exploitation rates in the south Devon offshore and south Cornwall offshore areas were negligible for both sexes, the latter reflecting very low reported effort as well as no project activity in this area, while the former also had very low return rates for double T-bar tags, but high return rates for DSTs (Table AIV.4).
SID 5 (Rev. 07/10) Page 20 of 130
In general terms, results from the population dynamics modelling highlighted seasonality in the fisheries and periodically high exploitation rates for female crabs in the south Devon inshore and Lands End & Scilly grounds, for males in the Trevose area and for both sexes in the Lizard & west Cornwall area. Elsewhere mortality rates were generally low to moderate, but still exhibited a high degree of seasonality. Seasonality was less marked for males in some areas.
5.1.3 Estimating mortality rates using the Brownie multi-year tagging model
Brownie models (Brownie et al., 1978; 1985) are a family of generalised models that consider survival and tag recovery rates through time. They have been widely applied to tagging programmes carried out on recreational fisheries, but can also be applied to commercial fisheries. They provide a method to estimate mortality rates on the basis of time series of tag release and return data only, thereby providing a third alternative means of investigating mortality in this study.
In this evaluation the instantaneous rates model of Hoenig et al. (1998) was applied using a function (irm_h, written by Gary A. Nelson, Massachusetts Division of Marine Fisheries) provided in the ‘fishmethods’ package, available for implementation using the R statistical modelling framework. The model was originally developed for annual data, but was applied to monthly time series of tag release and return data, with assumptions regarding the temporal distribution of tag releases and fishing effort considered at the monthly rather than annual scale. The function includes ‘seed’ values for fishing and natural mortality as well as limits on these values and was run for 102 permutations of individual and combined tag groups with 3 different sets of constraining parameters (Table AV.4). The function also includes parameters for initial tag loss, set to 0.2 based on aquarium and double tagging experiments, and reporting rate, set using the female estimate from the population dynamics model (0.629).
In all cases a variable fishing mortality rate was estimated, along with natural mortality rate that was constant for the entire period. However, the latter always tended to the minimum limit, so fishing mortality estimates may reflect total mortality. There were some problems with convergence, 88 of the permutations converged, with more than half meeting the recommended (pooled c-hat) diagnostic test for over-dispersion and considerably more if the un-pooled c-hat statistic was used.
Results for 3 ‘selected’ data sets representing the eastern Channel, south Devon and west Cornwall (Figure 5.1.1) were internally consistent, suggesting that the results were not sensitive to the mortality constraints, except for the penultimate point which tended to the upper limit of fishing mortality. As with the population dynamics modelling, the results clearly highlighted the seasonal nature of the fisheries and suggested that fishing mortality during the months of exploitation was moderate or high in the eastern Channel and high off south Devon and west Cornwall. Annual exploitation rates would theoretically be the sum of the monthly mortality rates and would therefore be very high. Results for all runs meeting the over-dispersion diagnostics showed broadly similar results (Figures AV.4-5).
SID 5 (Rev. 07/10) Page 21 of 130
Figure 5.1.1. Fishing mortality estimates for female crabs estimated using the irm_h function with 3 datasets under 4 different sets of parameter constraints. All runs: initial F=0.05 & M=0.01, Run 1: lower limit F=0.0001 & M=0.0001, upper limit F=5.0 & M=0.5 , Run 2: lower limit F=0.0001 & M=0.01, upper limit F=1.5 & M=0.5, Run 3: lower limit F=0.0001 & M=0.01, upper limit F=1.0 & M0.2, Run 4: lower limit F=0.0001 & M=0.01, upper limit F=0.5 & M=0.1 Note that of these runs, those for Bullock Bank had unacceptable over-dispersion (pooled c-hat) values (although the un-pooled c-hat was within limits), whilst the others were within recommended c-hat limits
Results obtained using this method suggested rather higher mortality rates than the catch curve analysis and outputs from the population dynamics models and in that respect were similar to results of routine length based stock assessments carried out annually by Cefas (Cefas, 2010). In common with other methods, there is a high level of uncertainty associated with the results, which suggested that mortality on crabs was seasonally high.
As with the population dynamics model approach, the Brownie model analysis required utilisation of all tag returns, so where data relating date of recapture were deficient they were assigned using best possible assumptions or pro-rated according to existing returns from the same release. However, this means that the method will be sensitive to variations in reporting quality and require some caution.
This was the first time this modelling approach had been used by the Cefas Shellfish team and it appeared to offer some potential. Further evaluation of the seeding parameters and the initial tag loss and reporting rate parameters to determine the sensitivity of results would be useful. The model has the potential to permit the estimation of varying rates of natural mortality through time, which at this monthly scale could potentially provide a means to investigate seasonality of natural mortality and whether it is linked with the moulting cycle. Such analyses would likely require the use of additional data (i.e. fishing effort) and reprogramming of the method to incorporate them. If successful such investigations could provide very valuable insights into the natural mortality rates of crabs.
5.1.4 General conclusions regarding mortality rates
The complex biology of crabs makes stock assessment very difficult because the break many dynamic pool assumptions, cannot be routinely aged, exhibit spatial structuring which may change gradually through time and have highly varying catchability. In this section 3 different methods using different sources and combinations of data were used to estimate and investigate mortality rates. Whilst none of these are definitive, the points below are consistent with generally held views of the crab fisheries and suggest that:
mortality on crabs varies seasonally and spatially , exploitation is generally moderate or high, and at times potentially very high, exploitation may be higher for females on the main pre-spawning fishing grounds, exploitation of males may continue through the winter months and vary less than for females.
SID 5 (Rev. 07/10) Page 22 of 130
Two of the analyses carried out in this section were novel and exploratory. Both required the use of all the tagging data, which poses some problems, and both have high levels of uncertainty and require further work to refine them. However, both the population dynamics, and the Brownie, modelling apply a more quantitative approach to analysis of the tagging data and yielded plausible and useful results.
Further details on the analyses carried out to estimate mortality rates are provided in Appendix V, whilst the population dynamics model is described in Appendix IV.
5.2 Growth studies using the double T-bar data
Crustaceans grow by moulting, casting off their old shell and expanding rapidly in size before the new shell hardens. One implication of this is that persistent hard (skeletal) parts that provide a means for ageing in other animals do not occur and it is therefore difficult to age crustaceans routinely. As a result, a record of growth history, such as that often readily available from fish scales or otoliths or mollusc shells, is not available for crustaceans. Growth by moulting is also discontinuous, with crustaceans growing in a series of increments, which can be modelled explicitly by estimating moult increment and moult frequency, although average population growth can also be explained using continuous models (e.g. von Bertalanffy, 1938).
Tagging programmes provide one means of obtaining information on growth and as many stock assessment techniques (e.g. length based virtual population analysis) require growth parameters, it is important to utilise any opportunities to obtain more or better information on growth. This section summarises analyses carried out on the double T-bar tag data to estimate both discontinuous and continuous growth parameters. More detail of the analyses is presented in Appendix VI.
5.2.1 Data quality and pre-screening
Crabs were measured during tagging and release and data recorded on a database. Size at the time of recapture was one item of information requested from the fishing industry, but as remote data submission (post, telephone, email or internet) was encouraged to facilitate the ease of reporting and crab carcases were not required to be returned, there was no means of checking these data. Data were initially screened to check for consistency of tag colour, crab sex and whether information on size had been reported. A dataset of around 1700 records was obtained; however, substantial measurement error was apparent as well as a small positive measurement bias in many of these data (Figure AVI.1). The positive bias of around 4mm could result from curvature of the carapace if measured with a tape measure rather than using callipers.
5.2.2 Discontinuous growth models
Mean growth increment can be modelled as a linear function of size prior to moulting, but different authors have used a variety of variations within this generalisation. These include using different measurements for size (e.g. carapace width or weight), using absolute or relative changes in size and using different distributions for describing variation about the mean (e.g. normal, gamma, beta) or different transformations of the data (e.g. logarithmic or untransformed) .
Moult frequency can also be described using a variety of models by which annual moult frequency is related to pre-moult size. These include linear models with various transformations, including the logarithmic and the logistic function as well as variations in the measurements used to express size. Data first need to be categorised as to whether or not a moult has occurred in the annual time step. Traditionally this was achieved by using the ‘anniversary’ method, whereby only tag returns in a window approximately 1 year after release were used in the analysis, but a newer generalised anniversary method has been developed that applies weighted linear regression of the proportion moulting against the number of growth seasons at liberty and permits the use of a greater proportion of the data. The methods are described in more detail in Appendix VI.
Potential break points in the moult increment data between no moulting, 1 moult and 2 moults were identified visually and, as suggested by McGarvey et al. (2002), by comparing goodness of fit criteria for a range of probability distributions fitted to the data, although neither method was conclusive. This full screening resulted in datasets where 1 moult had occurred for 54 females and 7 males. Linear models were fitted to the data for both sexes assuming normal errors for relative increments and gamma errors for absolute increments (Figures AVI.3 & AVI.4). However, with so few data available for males, results for this sex should not be considered further. Models were also fitted to data that had been adjusted for measurement bias. Student’s t tests indicated statistically non-significant differences between both the slopes and intercepts for linear models of relative increments for female data from this study compared with those of Bennett (1974) and this result was consistent for both unadjusted and bias adjusted datasets. The linear models were also very consistent with that estimated by Latrouite & Morizur (1988) for French data (Figure AVI.10).
SID 5 (Rev. 07/10) Page 23 of 130
Conclusions drawn with regards to moult increment were that data from the current study were more disperse (mainly due to measurement error) than those of Bennett (1974) and slightly positively biased. Generally larger female crabs had been sampled, however, there was no difference in the relationship between moult increment and pre-moult size estimated here and by Bennett (1974). The number of male crabs that had moulted and been recovered was insufficient to provide useful results.
The ‘anniversary’ method was applied with a window of 12 weeks either side of a year resulting in datasets where moulting had occurred of 84 records for females and 7 for males. Logistic models were fitted for both sexes (Figure AVI.6), although the relationship for males was based on too few data to be meaningful and males were omitted from further analyses. Linear and log-linear models for moult frequency were fitted against carapace width and weight, respectively, for females (Figure AVI.7), for comparison with historic work by Latrouite & Morizur (1988) and Bennett (1974). Student’s t tests indicated statistically non-significant differences between the slopes for log-linear models of moult frequency on premoult weight for data from this study compared with those of Bennett (1974), but differences between intercepts were statistically significant. The log-linear model was heavily influenced by the occurrence of a single moult record (proportion moulting = 0.5) for the largest size class which had high leverage, but appeared to be a valid record. Removal of this point impacted the curve strongly and resulted in a relationship that did not differ in slope or intercept when compared against Bennett’s relationship using a Student’s t test. Given this and the high levels of uncertainty associated with the data, particularly at large sizes, it was not possible to detect, with any certainty, changes in moult frequency compared with Bennett’s (1974) data.
The overall conclusion was that there did not appear to be any significant change in growth rates since the 1970s. However, any change in growth due to temperature would be more likely to manifest itself through moult frequency than moult increment and evidence for some difference in this aspect of growth was inconclusive.
A generalised anniversary method (McGarvey et al., 2002) was also applied to the data for females only, with a number of scenarios for the growing season (for which records are excluded). Results were sensitive to choice of growth season, but in some cases tended to suggest that moulting continued at large sizes (180-199mm C.W.), albeit relatively infrequently (Table AVI.3). Results for the baseline moult season scenario of this approach were generally similar to those obtained by other means and authors, suggesting slightly higher moulting rates up to 170mm (size class mid-point), but zero thereafter. These were broadly in-line with expectations, particularly given that, if anything, they are likely to be underestimates of moult rate. However, above 180mm, estimated zero moult frequencies in the baseline case and for many of the other scenarios were lower than expected. Under some other scenarios for moult season and assuming a log-linear relationship between moult frequency and weight, growth continued at sizes well above 200mm (Figure AVI.12).
In conclusion, the generalised anniversary method was found to be a useful addition, using more data, but was sensitive to choice of moult season. Under some scenarios for moult season, growth continued at sizes above 200mm, which is in accordance with expectations, given quite frequent observations of large crabs in the catch. Future work to refine these analyses, particularly if better information regarding the timing of the moult season can be obtained, could provide useful new information on growth rates.
Many different growth models have been used for crabs, with varying goodness of fit. Within the range of the observed data they are usually fairly similar in behaviour, but when extrapolated they behave very differently. Crab growth would be expected to change throughout the lifetime of an individual, particularly with the onset of sexual maturity, but, it is nonetheless interesting to observe the behaviour of the models when extrapolated (Figure AVI.11), and the extent to which they are plausible, given knowledge that during the first year after settlement crabs will moult several times.
5.2.3 Continuous growth models
Discontinuous growth data available from the tagging programme did not conform straightforwardly to the data requirements of the graphical and regression methods used to estimate von Bertalanffy growth curves. Data selected for the anniversary method approach to estimating moult frequency were used with average growth increments (including non-moulting crabs) for all records in 5mm size classes to provide an average annual growth increment for females only.
Graphical methods, including those derived by Gulland and Holt (1959) and Ford-Walford (Ford, 1933; Walford, 1946) were used to derive parameters for the von Bertalanffy (1938) growth model. Chapman’s (1961) plot was also considered, but with the averaged data used in this study, was directly equivalent to the Ford-Walford method.
Results were very similar to those obtained by statistically fitting data to models by other authors (Figure AVI.14), but slightly lower than the curve of Addison & Bennett (1992), which was fitted by fixing L∞ (at 240mm) and then estimating K based on a range of lines drawn by eye. Although conforming well with other estimates, it is likely that
SID 5 (Rev. 07/10) Page 24 of 130
these parameters under estimate growth, particularly at larger sizes, because of biases associated with tagging data and growth by moult and given the frequency of observations of large crabs in the catch.
5.2.4 Conclusions regarding growth of crabs
Insufficient male crabs that had moulted were returned to provide meaningful data regarding growth and measurements provided by the industry were highly variable in quality, with wide measurement errors and a small positive bias in many cases. Female crabs sampled in this study were generally larger than those sampled by Bennett during the 1970s, but no significant difference was found for moult increment models derived from these two datasets. Modelled moult frequency from this study was sensitive to a single data point for the largest size class, and evidence for significant difference between datasets was considered inconclusive.
A generalised anniversary method was useful in utilising more data, but was sensitive to assumptions regarding the timing of moulting. Under some scenarios for moult season, growth continued at larger sizes than when using the traditional anniversary method.
Bennett obtained better growth data (particularly for moult increment) during his programme during the 1970s, in part because it was carried out over a smaller spatial scale, for a longer duration, had scientists ‘on the ground’ for more of the time and greater access to recaptured crabs. In the MF1103 programme, electronic communications technology were used to simplify the submission of recapture details, permitting reporting remotely (by telephone hotline, email and internet website) with some quality checks, but not requiring the return of the captured crab. If growth data are to be a key feature of future programmes then efforts should be made to submit the recaptured crabs, or their carapace, to ensure that measurements of carapace width are carried out consistently and accurately.
Von Bertalanffy growth models for females derived from data obtained from the double T-bar tagging programme were very similar to models obtained by other authors. However, given biases inherent in obtaining growth data from tagging programmes on crabs, these are likely to underestimate the potential for growth, particularly at large sizes.
6. Other studies carried out as part of the project
6.1 Combining a screening programme for the crab pathogen Hematodinium spp . with the tagging programme
Bitter (or pink) crab disease is a condition caused by the presence of a dinoflagellate parasite, Hematodinium sp. The disease causes significant problems in snow crabs in Canadian waters, through direct mortality of crabs and because it causes a bitter taste, reducing the quality (ICES, 2008). Cefas scientists carried out a substantial crab haemolymph (blood) sampling programme in combination with the tagging programme, whilst working onboard a mid-Channel crab vessel. This increased coverage of areas sampled for Hematodinium and by linking the haemolymph sampling and tagging programmes, provided the potential to obtain information on the fate of infected crabs.
Haemolymph samples, were drawn from approximately 2000 Cancer pagurus (of which around 1000 were tagged and released) and fixed in 10% neutral buffered formalin during summer 2009. At the Cefas Weymouth laboratory samples from the tagged crabs were re-suspended, smeared onto histological slides, air dried and stained with haematoxylin and eosin (H&E) and presence of the parasite Hematodinium sp. assessed according to the diagnostic criteria laid down by Stentiford et al. (2002). Infected samples were diagnosed by presence of uni- and multi-nucleate parasite plasmodia which replace varying proportions of the host haemocytes (blood cells) depending on the severity of infection. Severely infected crabs show almost complete replacement of host cells by Hematodinium.
Hematodinium infection rate for the 636 female and 299 male crabs analysed was around 3.5%, typical for summer prevalence, but considerably lower than has been found in some inshore areas in winter e.g. Portland >20% (pers. comm. G Stentiford) and also lower than reported, on occasion, for Irish crab fisheries (ICES, 2007). Recaptures have been reported for 58 tagged crabs with valid screening, a return rate of 6.3% compared with an overall return rate of 6.6% for this release batch. Two of these were infected with Hematodinium, an infection rate of 3.4%. These crabs were both males, recaptured after 17 and 95 days at liberty and at the time of tagging and release, had infection severities of light and medium, respectively.
The number of returns (only 2 infected) was insufficient to support a meaningful statistical investigation and infection rate for recaptured crabs was very similar to that for the population when released, therefore not providing a clear indication of increased mortality. Conversely, no infected crabs have been reported recaptured beyond 95 days at liberty, so there is also no evidence to support longer term survival after Hematodinium infection. Also, no
SID 5 (Rev. 07/10) Page 25 of 130
females infected with Hematodinium have been recaptured. Unfortunately, the return rate for this experiment has been one of the lowest observed during the programme, which has meant relatively few crabs, of those screened, have been reported recaptured, no matter whether or not they were infected with Hematodinium. Nonetheless, the project provided the means to undertake a substantial screening programme which provided important results regarding Hematodinium infection rates in edible crabs.
7. Summary, conclusions and implications for management
The project has generally been very successful. The double T-bar and data storage tagging methods were both new methods applied to crabs by Cefas, but both worked well and milestones for fieldwork were met and often exceeded. The project generated very large quantities of data and within a tight time-frame, a wide range of analyses have been performed to develop interesting and important results reported herein and meeting the remaining milestones. Some of the analyses remain preliminary and will be refined during future work, under both R & D and MOU contracts to ‘firm up’ and apply the results. The extensive data are also highly likely to be further analysed in other projects.
7.1 Summary of work carried out and main findings
Aquarium experiments utilising an artificial reef to simulate realistic field conditions were carried out to investigate methods of attachment for DSTs. Neither of two adhesives (waterproof 2-part epoxy resin and cyanoacrylate superglue) was fully effective in isolation, but used together they provided an effective solution, subsequently contributing to very high return rates achieved field deployed DSTs.
Six aquarium experiments to determine rates of tag loss were also successful in providing an indication of potential tag loss rates and the form of the relationship between tag retention and time. However, the experiments suffered from high crab mortality, particularly during periods of warm summer weather (coinciding with the moult season), which complicated analysis and limited the opportunity to obtain information on retention through the moulting process. Double tagging experiments (using two tags per crab) carried out as part of the double T-bar tagging field programme provided results that were similar in terms of both form and level of double T-bar loss rate. Results from these experiments were used to inform and parameterise later modelling exercises.
The data storage tagging programme was highly successful with high return rates (17-40%) generating large amounts of high quality recorded data for periods at liberty ranging between 8 and 575 days. By re-using tags returned soon after release, targets for the numbers of DSTs released were substantially exceeded, with a total of 144 releases at 5 different sites (128 female, 16 male). In total, 49 DST tagged crabs were recovered including one that had lost the DST, but retained a claw tag. Fifty percent of DST recaptures occurred during the first 40 days after release and 80% within 120 days, but thereafter the seasonality of crab behaviour and fisheries became apparent with low occurrences of recaptures over the winter and more in the subsequent summer and autumn fishing seasons, with times at liberty between 280 and 400 days.
Analysis of average monthly depth profiles showed an increasing trend from <20m to >50m for female crabs released in the eastern Channel and making their way west and a slightly decreasing trend from c. 70m to >50m for crabs released on the Trevose Grounds, while those released of south Devon (inshore and offshore) remained at around 75m. A similar analysis for temperature suggested crabs experienced generally similar temperature profiles, fluctuating seasonally between around 7-9oC as a minimum and 15-17oC as a maximum, with the more extreme values occurring in the Eastern Channel. Analyses of depth and temperature profiles for individual tags on a daily scale gave similar results. Depth profiles for actively moving crabs indicated that they moved through a considerable depth range, rather than following fixed depth contours, with some crabs on occasion moving down to depths below the maximum for the DST pressure sensor (112m).
Migration routes were reconstructed using the tidal location method (TLM) and hidden Markov method (HMM), both previously successfully applied to bottom dwelling fish. However, significant problems were encountered with both methods, due to low level movements by crabs which subtly distorted recorded tidal signals in terms of both amplitude and timing. Further work, potentially including reprogramming the HMM method to accommodate locomotory features of crabs, will be needed to fully resolve these problems, which was outside the scope of this study. Nonetheless, predicted tracks could be produced for many of the recaptured crabs and these confirmed the key features of westerly movement by female crabs with little or no systematic easterly movement.
Data from 9 DSTs that had recorded over protracted time periods, including winter, provided new insights in to spawning and incubation behaviour for female crabs. Depth profiles for these crabs showed periods of inactivity commencing in late autumn and ending in spring or early summer that were considered to be indicative of spawning and subsequent brooding of eggs. The timing of these inactive periods fits very well with existing knowledge regarding the timing of spawning in edible crabs and suggested incubation times of between 126 and 198 days at temperatures ranging between 8.8oC and 15.0oC and at depth ranges of 19m to 84m. These results suggest that spawning and brooding took place at a variety of sites through the Channel and in the Celtic Sea,
SID 5 (Rev. 07/10) Page 26 of 130
including off the Sussex, Dorset, Devon and Cornish coasts. The 2 Trevose released recaptures did not demonstrate clear directed movement, but the remaining 7 crabs showing this spawning/brooding behaviour all moved west following release and prior to spawning. Further, these crabs were all recaptured west of their estimated spawning/brooding locations suggesting they continue to move in this direction and do not use the same locations in successive years. One crab was at liberty over 2 winter periods and it did show 2 periods of inactivity, both starting with similar timing, although the second was of much shorter duration and the crab slightly more active during this period. It remains unclear whether or not this second period was representative of spawning and incubation. The proportion of over-wintering females showing this behaviour suggests that most female crabs are spawning annually, supporting other information, such as high proportions of late maturity stage females during autumn and the substantial decline in catch rates of females over winter. With hindsight, obtaining some of the crab carcasses from the DST programme might have been useful to provide information on the physiological condition of recaptured crabs, potentially increasing certainty that the periods of inactivity did correspond to an ovigerous state. This should be considered in any future programmes.
The double T-bar tagging programme also fully met its milestones relating to the field programme and analyses and generated a large volume of data that will be further analysed in the future. Over 15,000 crabs (mainly females) were tagged and released in 11 locations through the English Channel and Celtic Sea. Just under 2,500 crabs were reported as recaptured, an overall recapture rate of 16%, very similar to that obtained in a programme carried out during the 1970s. Return rates varied widely between locations and were substantially lower than those for DSTs. In common with the DSTs return rates were very high immediately after release with 56% of double T-bar tag recoveries made during the first month and 97% during the first year. The overall pattern of movements (Figure 4.3.1) was very similar to that obtained by Bennett & Brown (1983) with striking long distance movements by female crabs in a westerly or south-westerly direction. Movements of recaptured male crabs included directional components in a NW, W and SW direction, but proportionally more local and undirected movements.
Rates of movement for females were broadly similar from all releases and, excluding outlying values, ranged between 0.8 and 6.7km/wk. Female crabs released late in the year generally travelled further and faster, in a southwest to west direction, than those released during the first half of the year. Rates of movement for males were generally lower (0.2-2.5km/wk), although far fewer males were released or recaptured. Distances moved by females were also generally further than those by males, with a maximum mean distance of 59km for females released on Sovereign Shoals in the eastern Channel compared to a maximum mean distance of 8k for males from the Trevose release. Analyses of the rates of movements by recapture month suggested that highest rates of movements for females occurred in October and November, whilst lower rates of movement and low recapture rates over winter concur with biological knowledge of a sedentary period while females are ovigerous, that was also very apparent in the DST depth data. Statistical modelling suggested that physical characteristics of released crabs, such as shell condition or missing limbs also influenced the rate and sometimes magnitude of movements. Crabs released in the eastern Channel moved furthest and those released further west were predominantly recaptured close to their release site. However, this result is heavily influenced by the relative positions of the major fisheries and the smaller movements in the west may reflect an absence of major UK fisheries surrounding these release areas. Although occasional easterly movements were reported, uncertainty for some was high and there was no evidence for systematic easterly movements as have been suggested by some members of the industry for crabs in spring. There was some evidence for crabs moving around coastal bays, although this could also reflect local distribution of fishing gear. Studies off the south east coast of Ireland have suggested seasonal inshore offshore movements by female crabs (Fahy et al., 2004).
A population dynamics model incorporating monthly time steps, 2 seasons and 12 spatial units and utilising commercially reported effort data was developed and fitted to release and recapture data from both double T-bar and DST tagging programmes. The model also used tag loss rate information generated from aquarium experiments and a double tagging programme and was fitted separately for each sex of crab. With 117 parameters to estimate and a large ‘boundary’ area for which data were incomplete, minimisation was not straightforward. Results were generally plausible when considered carefully in the light of the input data, although for some spatial units they appeared erroneous. Nonetheless, the modelling exercise was very insightful and provided quantitative estimates for a number of parameters (seasonal monthly movement rates and catchability for each spatial unit and an overall reporting rate for double t-bar tags relative to DSTs). Input matrices of monthly recaptures by spatial unit through time were most illuminating, particularly for the earliest release groups, made in the eastern Channel. They showed recaptures of females during the release fishing season taken locally, then in the subsequent season mainly taken locally or in the nearest westerly neighbour and occasionally further west. In the third season they were mainly recaptured further west with a few locally and in the 4th season all were recaptured further west. This implied that over 3 to 4 years all surviving females had moved away from the area of release and where evidenced to fishing grounds further west. Conversely the input matrices for males showed no recaptures beyond the nearest westerly neighbour, although far fewer males were tagged or recovered, times at liberty were not as protracted and results were more uncertain.
Catchability parameters estimated by the model were relatively consistent and reflected features of the fishery such as lower catch rates for females and higher catch rates for males during winter and spring. They were combined
SID 5 (Rev. 07/10) Page 27 of 130
with reported commercial effort to provide a time series of monthly exploitation rates by spatial unit that were used in analyses investigating mortality rates. Results highlighted seasonality in the fisheries and periodically high exploitation rates for females crabs in the south Devon inshore and Lands End & Scilly grounds, for males in the Trevose area and for both sexes in the Lizard & west Cornwall area. Elsewhere mortality rates were generally low to moderate, but still exhibited a high degree of seasonality. Seasonality was less marked for males in some areas.
Mortality rates were also investigated by ‘slicing’ length distributions obtained during the double T-bar tagging programme into pseudo-ages and applying catch curve analysis to estimate total mortality. Growth parameters estimated from Bennett’s (1973) data and this study were used in the age slicing. Estimated mortality rates were low to moderate for females and high for males when using historical growth parameters, but when using new growth parameters for moult frequency for females, derived using a generalised anniversary method, they were estimated to be moderate or high. Estimated total mortalities for both sexes were lower in the eastern Channel, intermediate of the Devon coast and higher in west Cornwall.
A 3rd investigation into mortality used Brownie multi-year models applied to tag release and recovery data using a monthly scale. Results suggested rather higher mortality rates than the catch curve analysis and outputs from the population dynamics models and in that respect were similar to results from routine length based stock assessments carried out annually by Cefas (Cefas, 2010). In common with other methods, there is a high level of uncertainty associated with the results.
The main conclusions from the mortality studies were that: mortality on crabs varies seasonally and spatially , exploitation is generally moderate or high, and at times potentially very high, exploitation may be higher for females on the main pre-spawning fishing grounds, exploitation of males may continue through the winter months and vary less than for females.
Analyses were also carried out to estimate growth parameters from double T-bar tagging data obtained in this study and compare it with historic results. Insufficient data were available to estimate meaningful parameters for males, but moult increment parameters estimated for females were not significantly different from estimates based on Bennett’s (1973) data although crabs sampled in this study were generally larger than those tagged historically. Variance of moult increment estimated in this study was larger, but this was due to a high level of measurement error in the data, which were also slightly positively biased. Estimated moult frequency parameters were sensitive to a single data point in the largest size class and it was considered that insufficient evidence was available to indicate a significant change in growth over the last 30-40 years. Application of a modern generalised anniversary method resulted in slightly higher rates of moult frequency and under some scenarios growth continued at larger sizes than when using the conventional approach. However, the method was sensitive to assumptions about the growing season. Traditional graphical methods were used to estimate parameters for the von Bertalanffy continuous growth model. The curves estimated were very similar to curves estimated by a number of other authors, but may still under-estimate the growth potential of crabs due to biases inherent in obtaining growth data for crabs from tagging programmes.
The project provided the means to carry out a major haemolymph (blood) screening exercise to evaluate infection rates by the parasite Hematodinium sp. in the mid-Channel offshore fishery and linking this with the tagging programme. Infection rate (3.5%) was typical of summer infection rates, but lower than has been found elsewhere. Only two returns of infected tagged crabs, both males, were made after 17 and 95 days at liberty. Recapture rates for screened crabs (6.3%) were similar to the overall recapture rate for the area (6.6%), therefore not providing a clear indication of increased mortality. Conversely, no infected crabs have been reported recaptured beyond 95 days at liberty, so there is also no evidence to support longer term survival after Hematodinium infection. No recaptures of females infected with Hematodinium have been reported.
The large amounts of data collected by the project will provide a substantial resource for future analyses and as many of the analyses completed were new for Cefas staff or edible crabs there is considerable scope to extend and refine them in future.
Future work will be required to fully analyse DST data gathered under this project, which will provide further insights into edible crab behaviour including patterns of activity in relation to diurnal, tidal and reproductive cycles. Further work will also be required in order to refine the level of accuracy associated with the geo-located crab-tracks produced as a result of this study. Success in this regard would add value to the results obtained from the DST programme and reinforce the success of the methodology in collecting data to analyse crab movements. This would likely lead to further application of DSTs to elucidate crab behaviour in other areas.
The population dynamics modelling provided some promising results, but minimisation was time consuming and the model implemented was simpler than preferred. Future work to implement the model in a more powerful minimisation environment would be advantageous and could permit the fitting of more complex models and more efficient minimisation and comparison of model scenarios.
SID 5 (Rev. 07/10) Page 28 of 130
The Brownie modelling exercise to estimate mortality rates was also new to the Cefas staff involved and appeared to provide useful results. The results would be strengthened by carrying out further scenarios to investigate the sensitivity to input parameters not already investigated (e.g. initial tag loss rate). This method offers the potential to estimate variable natural mortality rates, which if investigated on a short time scale (e.g. monthly), could be highly informative in identifying possible links with the moult cycle. However, estimating both natural and fishing mortality is likely to require additional data such as fishing effort and the method would need re-programming to accommodate this. There is considerable scope to further utilise data from the project, but the analyses carried out so far have already provided a substantial body of new evidence on edible crab stock identity, movements, reproductive cycle, mortality rates and growth that will enhance stock assessment and management of the resource.
7.2 Implications for stock assessment and management
Stock identity
Results from the project confirm clearly confirm linkages between crab stocks in the eastern and western Channel, with the former appearing to be a net exporter of adults to the latter. These results concur with historic work carried out in the 1970s in this aspect and also in showing movements of some crabs from the western Channel to French fishing grounds around the Brittany coast. Although the scale of these returns is lower this demonstrates some export of crabs from the western Channel to more southerly grounds. Despite substantial tagging efforts around the Cornish coast, this study has not demonstrated linkages between the Celtic Sea, and western Channel or Western Approaches, although a long term claw tag recovery made near the Scillies (Fahy & Carroll, 2008) provides some evidence in this respect. Linkages demonstrated consist of adult movements principally by females and without return migrations.
The current stock assessment approach considers this stock assemblage as 3 units eastern Channel, western Channel and Western Approaches and Celtic Sea. Management measures consist primarily of minimum landing size which varies over a wide scale, with some local Inshore Fisheries and Conservation Authority variations. The scale and rates of movements from eastern to western Channel by female crabs appear to be individually variable, with some moving from one unit to another in 1 year while others may take 3 to 4 years and males appear not to make long distance movements. These rates suggest that current stock assessment units and management units should still be appropriate, but consideration should be given to the potential for emigration from both the eastern and western Channel.
Reproductive cycle
Periods of inactivity apparent over winter in DST data and coincided closely with knowledge and other evidence on timing of spawning and incubating eggs were interpreted as indicative of such activity. These data suggest multiple spawning and incubation sites for crabs throughout the Channel and Celtic Sea. Westerly movement by individual female crabs both before and after winter inactivity suggests that these crabs may not use the same spawning area in successive years. Crabs are very widespread and ubiquitous in the Channel and although there may be important major spawning grounds indicated by high catch rates of pre-spawning females and high plankton concentrations, data generated in this project suggest that spawning and incubation sites may be widespread.
High proportions of recaptured females at liberty overwinter with DST data showing probable spawning behaviour suggests most females are spawning annually; consistent with information from maturity studies indicating high proportions of late ripeness stage females in autumn and the strong decline in catch rates of female crabs in late autumn/early winter. Spawning frequency is a key assumption for stock assessment and projection of spawning potential and essential for consideration of sustainability. Evidence supporting annual spawning despite large crabs not moulting (and therefore having matting opportunity) annually is therefore important.
Mortality
A number of approaches were used to provide information on mortality, all with high uncertainty, but tending to indicate that fishing mortality can be seasonally very variable and is moderate or high on females and high on males. Industry is supportive of additional management to constrain exploitation of crabs and the additional evidence generated does not contradict the need to improve management of crab fisheries.
One method applied has the potential to investigate seasonalities in natural mortality, although this would potentially require substantial extra modelling work. This should be investigated further as natural mortality is very poorly quantified but highly influential in stock assessments and consideration of stock sustainability.
SID 5 (Rev. 07/10) Page 29 of 130
Growth
Growth rates do not appear to have changed since the 1970s despite significant changes in the environment. Evidence indicating no change in moult increment was stronger than that for moult frequency which was sensitive to a single data point. Continuous growth curves estimated using data from this project were very consistent with those of other authors, but may under-estimate growth due to biases associated with tagging data and growth by moult. A new generalised anniversary method that utilised more data gave slightly higher growth potential at large sizes under some scenarios for the moult season. Insufficient data were available to estimate meaningful growth parameters for males.
Growth parameters are key inputs to length based stock assessments which tend to be sensitive to them. Higher growth rates indicate a more productive stock which has implications for stock assessment and management. Data corroborating existing growth parameters supports the basis for stock assessment. However, biases inherent in estimating growth for moulting animals using tagging data suggest that growth may be underestimated. A modern generalised anniversary method was able to utilise more data than the traditional method and suggested slightly higher potential for growth under some assumptions for the moulting season.
8. References
Addison, J.T & Bennett, D.B., 1992. Assessment of minimum landing sizes of the edible crab, Cancer pagurus L., on the east coast of England. Fish. Res., 13:67-88.
Aires-da-Silva, A., Taylor, I., Punt, A.E., Gallucci, V.F., Kohler, N.E., Turner P.A., Briggs R. & Hoey, J.J., 2005. A framework for estimating movement and fishing mortality rates of the blue shark, Prionace glauca, in the North Atlantic from tag recapture. Col. Vol. Sci. Pap. ICCAT, 58:1073-1086.
Bell, M.C., Eaton, D.R., Bannister, R.C.A. & Addison, J.T., 2003. A mark recapture approach to estimating population density from continuous trapping data: application to edible crabs, Cancer pagurus, on the east coast of England. Fish. Res. 65: 361-378.
Bennett, D.B. 1974 Growth of the edible crab (Cancer pagurus L.) off southwest England. J. Mar Biol. Ass. U.K. 54:803-23.
Bennett, D.B. & Brown, C.G., 1983. Crab (Cancer pagurus) migrations in the English Channel. Journal of the Marine Biological Association of the United Kingdom, 63: 371–398.
Bergh, M.O., Johnston, S.J., 1992. A size-structured model for renewable resource management with applications to resources of rock lobster in the South-East Atlantic. S.Afr. J. Mar. Sci. 12, 1005-1016.
Bertalanffy, L. von, 1938.A quantitative theory of organic growth (inquiries on growth laws. II) Hum. Biol. 10:181-213.
Brownie, C., Anderson, D.R., Burnham, K.P., and Robson, D.S., 1978. Statistical inference from band recovery data - a handbook. U.S. Fish Wildl. Serv. Resour. Publ. No. 131.
Brownie, C., Anderson, D.R., Burnham, K.P., and Robson, D.S., 1985. Statistical inference from band recovery data: a handbook. 2nd ed. U.S. Fish Wildl. Serv. Resour. Publ. No. 156.
Burnham, K. P. and D. R. Anderson. 2002. Model selection and multimodel inference : A Practical Information-Theoretic Approach, 2nd edition. Springer-Verlag, New York, NY. 488p.
Cefas, 2010. Edible crab stock and fishery status reports, 2010. Cefas/Defra internal report. 51pp.
Chapman, D.G. 1961. Statistical problems in the dynamics of exploited fish populations. In: Proceedings of the Fourth Berkeley Symposium, 1960. Vol. Cont.Biol. and Probl.Med., 4:153-168.
Cuillandre, J-P., Latrouite, D. & Le Foll, A. 1984. Le tourteau: biologie et exploitation. La Pêche Maritime: No 1278: 502–520, September 1984.
Curtis, D.L. & McGaw, I.J., 2008. A year in the life of the Dungeness crab: methodology for determining microhabitat conditions experienced by large decapod crustaceans in estuaries. Journal of Zoology 274: 375-385.
Eaton, D.R., Brown, J., Addison, J. T., Milligan, S. P. & Fernand, L. 2003. Larvae surveys
SID 5 (Rev. 07/10) Page 30 of 130
of edible crab (Cancer pagurus) off the east coast of England: implications for stock structure. Fisheries Research, 65: 191-199.
Eaton, D., Fernand, L., Milligan, S. & Addison, J.T., in prep. Evidence from larvae surveys of a metapopulation structure for the Edible Crab (Cancer pagurus L) in the English Channel.
Fahy, E. & Carroll, J., 2008. Two records of long migrations by brown or edible crab (Cancer pagurus L.) from the Irish inshore of the Celtic Sea. Irish Naturalists’ Journal 29:119-121.
Fahy, E., Hickey, J., Perella, N., Hervas, A., Carroll, J. & Andray, C., 2004. Bionomics of brown crab, Cancer pagurus in the south-east Ireland inshore fishery. Irish Fisheries Investigation series No. 12. Marine Institute, Galway.
Ford, E., 1933. An account of the herring investigations conducted at Plymouth during the years from 1924 to 1933. J.Mar.Biol.Assoc.U.K., 19:305-384.
Freilich, J. E., 1989. A method for tagging individual benthic macroinvertebrates. Journal of the North American Benthological Society, 8(4), 351-354. Retrieved December 20, 2007, from ASFA 1: Biological Sciences and Living Resources database.
Freire, J., Gonzalez-Gurriaran, E. &, Bernardez, C., 1999. Migratory patterns in the spider crab Maja squinado using telemetry and electronic tags. ICES CM 1999/AA: 14, 11 pp.
Gonzalez-Gurriaran, E. & Freire, J., 1994. Movement patterns and habitat utilization in the spider crab Maja squinado (Herbst) (Decapoda, Majidae) measured by ultrasonic telemetry. Journal of experimental marine biology and ecology 184: 269-291.
Gonzales-Gurriaran, E., Friere, J. & Bernardez, C., 2002. Migratory patterns of female spider crabs Maja squinado detected using electronic tags and telemetry. J. Crust. Biol. 22:91-97.
Gulland, J.A. 1955. On the estimation of population parameters from marked members. Biometrika, 42: 269–270.
Gulland, J.A. & S.J. Holt, 1959. Estimation of growth parameters for data at unequal time intervals. J.Cons.CIEM, 25(1): 47-49.
Hancock, D.A. & Edwards, E., 1967. Estimation of annual growth in the edible crab (Cancer pagurus L.) J. Cons. Int. Explor. Mer. 31:246-264.
Heald, D., 1978. A successful marking method for the saucer scallop Amusium balloti (Bernardi). Aust.J.Mar.Freshwater Res., 29(6), 845-851, Retrieved December 20, 2007, from ASFA 1: Biological Sciences and Living Resources database.
Hilborn, R., 1990. Determination of fish movement patterns from tag recoveries using maximum likelihood estimators. Can J.Fish. Aquat. Sci. 47:635-643.
Hoenig, J. M, N. J. Barrowman, W. S. Hearn, & K. H. Pollock. 1998. Multiyear tagging studies incorporating fishing effort data. Canadian Journal of Fisheries and Aquatic Sciences 55: 1466-1476.
Howard, A.E., 1982. The distribution and behaviour of ovigerous edible crabs (Cancer pagurus), and consequent sampling bias. Journal du Conseil 40: 259-261.
Hunter, E., Aldridge, J.N., Metcalfe, J.D. & Arnold, G.P. 2003. Geolocation of free-ranging fish on the European continental shelf as determined from environmental variables. I. Tidal location method. Mar Biol 142: 601-609
Hunter, E., Cotton, R.J., Metcalfe, J.D. & Reynolds, J.D., 2009. Large-scale variation in seasonal swimming patterns by plaice in the North Sea. Marine Ecology Progress Series 392: 167-178.
Huntley, D.A., 1980. Tides of the north-west European Continental Shelf. In: Banner, F.T., Collins, M.B. & Massie, K.S. (eds) The North-west European Shelf seas: the sea bed and the sea in motion. II. Physical and chemical oceanography and physical resources. Elsevier, Amsterdam
ICES, 2007. Report of the Working Group on the Biology and Life History of Crabs (WGCRAB). ICES CM 2007/LRC:09.
SID 5 (Rev. 07/10) Page 31 of 130
ICES, 2008. Report of the Working Group on the Biology and Life History of Crabs (WGCRAB). ICES CM 2008/LRC:11.
Jepsen, N., Schreck, C., Clements, S. & Thorstad, E.B., 2005. A brief discussion on the 2% tag/bodymass rule of thumb. In: Spedicato, M.T., Lembo, G. &, Marmulla, G, (eds) Aquatic telemetry: advances and applications. Proceedings of the Fifth Conference on Fish Telemetry held in Europe. Ustica, Italy, 9-13 June 2003. Rome, FAO/COISPA. p 255-259.
Jones, R., 1976. The use of marking data in fish population analysis. FAO Fish.Tech.Pap., (153): 42pp.
Jones, R., 1984. Assessing the effects of changes in exploitation pattern using length composition data (with notes on VPA and cohort analysis). FAO Fish.Tech.Pap., (256): 118pp.
Kaplan, E.L. & Meier, P., 1958. Nonparametric estimation from incomplete observations. Journal of the American Statistical Association, 53: 457-481.
Latrouite, D. & LeFoll, D., 1989. Données sur les migrations des crabes tourteau Cancer pagurus et araignées de mer Maja squinado. Océanis, 15(2): 133-142.
Latrouite, D. & Morizur, Y., 1988. Analyse quantitative et qualititative des captures du tourteauCancer pagurus par la flotille de Manche Ouest. CIEM 1988/K:33.
Latrouite, D. &Philippe, N., 1993. Observations sur la maturité sexuelle et la ponte du torteau (Cancer pagurus) en Manche. ICES Shellfish Committee. CMC/K :23
Lemarie, D. P., Smith, D. R., Villella, R. F., & Weller, D. A., 1996. Evaluation of tag types and adhesives for marking freshwater mussels. Journal of Shellfish Research, 15(2), 528. Retrieved December 20, 2007, from ASFA 1: Biological Sciences and Living Resources database.
Mantel, N., 1966. Evaluation of survival data and two new rank order statistics arising in its consideration. Cancer Chemotherapy Reports, 50: 163-170.
McAllister, M., Babcock, E., Apostolaki, P. & Pikitch, E., 2004. Outlining a statistical modelling framework to support the development of a new area-based management regime for Atlantic bluefin tuna. Col. Vol. Sci. Pap. ICCAT, 56: 1134-1143.
McGarvey, R. & Feenstra, J.E., 2002. Estimating rates of fish movement from tag recoveries: conditioning by recapture. Can. J. Fish. Aquat. Sci. 59: 1054–1064.
McGarvey, R., Levings, A.H. & Matthews, J.M., 2002. Moulting growth of the Australian giant crab Pseudocarcinus gigas. Mar. Freshw. Res. 53:869-881.
McShane, P., 1989. In Hancock, D. A. (Ed.), Tagging abalone. CANBERRA, A.C.T. (AUSTRALIA): AUSTRALIAN GOVERNMENT PUBLISHING SERVICE. Retrieved December 20, 2007, from ASFA 1: Biological Sciences and Living Resources database.
Metcalfe, J.D., Righton, D.A., Hunter, E., Neville, S. & Mills, D. 2008. New technologies for the advancement of fisheries science. In: Payne. A., Cotter, J. & Potter E.C.E. (eds) Advances in Fisheries Science: 50 years on from Beverton and Holt. Blackwell Publishing, Oxford, p 255-279
Michielsens, C.G.J., McAllister, M.K., Kuikka, S., Pakarinen, T., Karlsson, L., Romakkaniemi, A., Perä, I. & Mäntyniemi, S., 2006. A Bayesian state–space mark–recapture model to estimate exploitation rates in mixed-stock fisheries. Can. J. Fish. Aquat. Sci. 63: 321–334.
Pedersen, M.W., Righton, D., Thygesen, U.H., Andersen, K.H. & Madsen, H., 2008. Geolocation of North Sea cod (Gadus morhua) using hidden Markov models and behavioural switching. Can. J. Fish. Aquat. Sci. 65: 2367–2377.
Permanent labels.(1997). Fish.Product News, 12(3), 3 p. Retrieved December 20, 2007, from ASFA 1: Biological Sciences and Living Resources database.
Polacheck, T., Eveson, J.P., Laslett, G.M., Pollock, K.H. & Hearn, W.S., 2006. Integrating catch-at-age and multiyear tagging data: a combined Brownie and Petersen estimation approach in a fishery context. Can J. Fish. Aquat. Sci. 63:534-548.
SID 5 (Rev. 07/10) Page 32 of 130
Pollock, K.H., Hoenig, J.M., & Jones, C.M. 1991. Estimation of fishing and natural mortality when a tagging study is combined with a creel survey or port sampling. Am. Fish. Soc. Symp. 12:423.434.
R Development Core Team (2009). R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. http://www.R-project.org.
Righton, D.A., Andersen, K.H., Neat, F., Thorsteinsson, V., Steingrund, P., Svedäng, H., Michalsen, K., Hinrichsen, H.H., Bendall, V., Neuenfeldt, S., Wright, P., Jonsson, P., Huse, G., van der Kooij, J., Mosegaard, H., Hüssy, K. & Metcalfe, J., 2010. Thermal niche of Atlantic cod Gadus morhua: limits, tolerance and optima. Marine Ecology Progress Series. 420: 1-13.
Robson, D.S., & Youngs, W.D. 1971. Statistical analysis of reported tag recaptures in the harvest from an exploited population. Biometrics Unit Mimeo Ser. Pap. No. BU-369-M. Cornell University, Ithaca, N.Y.
Skellam, J.G., 1951. Random dispersal in theoretical population. Biometrika, 38: 196-218.
Schweigert, J.F. & Schwarz, C.J., 1993. Estimating migration rates for Pacific herring (Clupea pallasi) using tag recovery data. Can. J. Fish. Aquat. Sci. 50: 1530-1540.
Schwarz, C. J. & Arnason, A.N., 1990. Use of tag-recovery information in migration and movement studies. Proceedings of the American Fisheries Society workshop on tagging.
Seber, G.A.F. 1970. Estimating time-specific survival and reporting rates for adult birds from band returns. Biometrika, 57: 313.318.
Sheehy, M.R.J. & Prior, A.E., 2008. Progress on an old question for stock assessment of the edible crab Cancer pagurus. Mar. Ecol. Prog. Ser. 353:191-202.
Smith, I.P., Collins, K.J. & Jensen, A.C., 1998. Movement and activity patterns of the European lobster, Homaras gammarus, revealed by electromagnetic telemetry. Marine Biology 132: 611-623
Sparre, P.& Venema, S.C., 1998. Introduction to tropical fish stock assessment. Part 1. Manual.FAO Fisheries Technical Paper. No. 306.1, Rev. 2.Rome, FAO. 407p.
Stentiford, G.D., Green, M., Bateman, K., Small, H.J., Neil, D.M. & Feist, S.W., 2002. Infection by a Hematodinium-like parasitic dinoflagellate causes Pink Crab Disease (PCD) in the edible crab Cancer pagurus. Journal of Invertebrate Pathology 79: 179-191.
Stevenson, C., 1954. The movement of herring in British Columbia waters as determined by tagging with a description of tagging and tag recovery methods. ICES Special Scientific Meeting on Herring Tagging and Results. Document No. 55. 39 pp.
Sullivan, P.J., Lai, H.L. & Gallucci, V.F., 1990. A catch-at-length analysis that incorporates a stochastic model of growth. Can. J. Fish. Aquat. Sci. 47:184-198.
Tallack, S.M.L., 2002, The Biology and exploitation of three crab species in the Shetland Islands, Scotland: Cancer pagurus, Necora puber & Carcinus maenas. Phd Thesis (NAFC:UHI Millenium institute, Shetland).
Therneau, T. & Lumley, T., 2009. survival: Survival analysis, including penalised likelihood. R package version 2.35-4. http://CRAN.R-project.org/package=survival
Trendall, J., 1989. In Hancock D. A. (Ed.), Influence of tag type on recapture rates in the tropical rock lobster, Panulirus ornatus. CANBERRA, A.C.T. (AUSTRALIA): AUSTRALIAN GOVERNMENT PUBLISHING SERVICE. Retrieved December 20, 2007, from ASFA 1: Biological Sciences and Living Resources database.
Tully, O., Robinson, M., O'Keefe, E., Cosgrove, R., Doyle O. & Lehane, B., 2006. The Brown Crab (Cancer pagurus L.) Fishery: Analysis of the resource in 2004 - 2005. Fisheries Resource Series, Bord Iascaigh Mhara (Irish Sea Fisheries Board), Dun Laoghaire, Ireland Vol. 4, 48pp.
Ungfors, A., 2007. Sexual maturity of the edible crab (Cancer pagurus) in the Skaggerak and Kattegat, based on reproductive and morphometric characters. ICES Journal of Marine Science 64: 318-327.
Ungfors, A., Hallbäck, H. & Nilsson, P.G., 2007. Movement of adult edible crab (Cancer pagurus L.) at the Swedish West Coast by mark-recapture and acoustic tracking. Fisheries Research 84: 345-357.
SID 5 (Rev. 07/10) Page 33 of 130
Venables, W.N. & Ripley, B.D., 2002. Modern Applied Statistics with S. 4th edition. Springer, New York. 495 pp.
Walford, L.A., 1946. A new graphic method of describing the growth of animals. Biol. Bull. Mar. Biol. Lab. Woods Hole, 90:141-147.Willoughby, L.G. & Hurley, M.A., 1987. ‘Echo’ moulting used to estimate moulting periodicity of mayflies (Ephemeroptera) and stoneflies (Plecoptera), in nature. Aquatic Insects 9:221-227.
Zheng, J., Murphy, M.C. & Kruse, G.H. 1995. A length-based population model and stock-recruitment relationships for red king crab, Paralithodes camtschaticus, in Bristol Bay, Alaska. Can. J. Fish. Aquat. Sci. 52:1229-1246.
Zheng, J., Murphy, M.C. & Kruse, G.H. 1996. A catch-length analysis for crab populations. Fish. Bull. 94:576-588.
Zheng, J., Murphy, M.C. & Kruse, G.H., 1997. Analysis of harvest strategies for red king crab, Paralithodes camtschaticus, in Bristol Bay, Alaska. Can. J. Fish. Aquat. Sci. 54:1121-1134.
Zheng, J., Kruse, G.H. & Murphy, M.C., 1998. A length based approach to estimate population abundance of Tanner crab, Chionocetes bairdi, in Bristol Bay, Alaska. In: Jamieson, G.S., Campbell, A. (Eds.), Proceedings of the North Pacific Symposium on Invertebrate Stock Assessment and Management. Can. Spec. Publ. Fish. Aquat. Sci. 125, 97-105.
Zheng, J. & Kruse, G.H. 2002. Retrospective length based analysis of Bristol Bay red king crabs: model evaluation and management implications. In: Paul, A.J., Dawe, E.G., Elner, R., Jamieson, G.S., Kruse, G.H., Otto, R.S., Sainte-Marie, B., Shirley, T.C. & Woodby, D. (eds.). Crabs in cold water regions: Biology, management and economics. University of Alaska Sea Grant, AK-SG-02-01, Fairbanks. 876pp.
Appendix I. Using data storage tags to investigate, describe and quantify patterns of movements of adult edible crabs
AI.1. Introduction
Electronic data storage tags (DSTs) are now routinely applied in research on finfish migration, but Cefas have not previously deployed DSTs on crustaceans. The principal aim of MF1103 was to tag edible crabs with DSTs to provide information on crab movements between the point of release and recapture.
Obtaining long-term spatial information on Decapod crustaceans, often exploited in extensive and commercially valuable fisheries, presents a unique set of challenges. Most crabs are believed to be fully benthic rather than demersal in habit, i.e. they will remain in contact with the bottom at all times rather than periodically rising into the water column, as may be the case for many demersal fish species (e.g. Hunter et al., 2009). Furthermore, crabs are adapted to a reptant lifestyle and it is important, therefore, that the attachment of tags or other bio-logging devices, do not impede the crabs’ natural behaviours, such as burrowing in sediment and entry to rock crevices. However, perhaps the main barrier to the routine use of archival tagging in crustacean studies historically has been that the unit cost of the tags has been relatively high in relation to the perceived problems of tag loss due to moulting (i.e. periodic shedding of the exoskeleton as part of the growth process).
There are, however, aspects of the lifestyle and morphometrics of crabs that offer some potential advantages for archival tagging. For example, weight considerations will be less important than for finfish and this may permit the use of larger tags and hence an increase in the power supply and concomitant increase in data storage capacity. Furthermore, archival tag costs have reduced considerably in recent years. By targeting tagging larger animals (which moult less frequently) early in their inter-moult period, the potential to retrieve large quantities of high quality behaviour data in large-scale tag releases now potentially outweighs the risks associated with tag loss. As a result, these factors make the cost of large-scale tagging experiments scientifically and financially more viable. Indeed, the potential utility of this method has already been demonstrated in pilot studies of the movements of spider crabs (Gonzales-Gurriaran et al., 2002), and in determining habitat choice behaviour in the Dungeness crab, Cancer magister (Curtis & McGaw 2008).The fishery for edible crabs (Cancer pagurus) is one of the most important commercial fisheries in England and Wales, yet there remain several important gaps in our understanding of their biology and ecology. Previous mark-recapture experiments carried out in the 1970s in the English Channel (Bennett & Brown 1983; Cuillandre et al., 1984; Latrouite & LeFoll, 1989) indicated long distance movements by crabs, particularly mature females. Observed movements in the English Channel were generally from east to west and return movements by adults were not demonstrated. These female migrations to the western Channel spawning grounds were interpreted as facilitating the return of hatched larvae to the areas of maternal origin in the prevailing tidal currents. There have, subsequently been, however, significant changes in the environment and the fishery. More recently, surveys of larvae made in conjunction with hydrodynamic modelling (Eaton et al., 2003.) have suggested that larval transport rates were insufficient for larvae to return to the original location of the females. Further evidence from genetic
SID 5 (Rev. 07/10) Page 34 of 130
studies (Defra MF0230, Paul Shaw, Royal Holloway, University of London, pers. comm.) indicates significant genetic variation in edible crabs at relatively local scales. It is difficult therefore, to rationalise the apparently conflicting signals from these studies into a cohesive picture of stock identity which poses significant problems for assessment and management of the stock.
The large-scale application of DSTs to edible crabs therefore offers new opportunities to gain valuable insights in to aspects of their behaviour, habitat occupancy and migration patterns. Specifically, we aimed to describe the movements of adult crabs both at a local and a regional migratory scale. Understanding how, where and when crabs undergo large scale migrations is key to successful stock assessment and management and is important in identifying key life stages and periods that may be vulnerable to local fishing or other human activities. Detailed information of this type may have particular relevance to marine spatial planning as well as providing information for inclusion into population dynamics models addressing management of crabs on a wider scale.
AI.2. Materials and Methods
AI.2.1 Aquarium experiments
AI.2.1.1 Attachment using epoxy resinIn total, twenty-eight crabs (19 females: 9 males), freshly collected from the North Norfolk coast, were held in the Cefas aquarium over a period of 2 months, between 17/12/07 and 21/02/08. Temperature in the aquarium tanks ranged between 9ºC on 21/01/08 and 6.5ºC on 18/02/08.
Test individuals were tagged either with a specially designed lozenge version of the Cefas G5 DST (Fig.AI.1(left photo), n=10, 5 male: 5 female), or a conventional Cefas G5 DST fitted with a flat-based holding-cradle (Fig.AI.1(right photo), n=10, all females). All of the lozenge DSTs and 5 of the cradled DSTs were dummy tags. The other 5 cradled DSTs were live, and were programmed to record depth and temperature at 10 second intervals up until 31/01/08, and at 1 min intervals thereafter.
Figure AI.1. Edible crab, Cancer pagurus with attached dummy lozenge tag (left) and (right) conventional data storage tag with holding-cradle
The crabs were allowed to acclimate in the aquarium tanks over a period of 2-3 days prior to tagging. A wide range of epoxy-resin adhesives were available, however tagging was carried out using a waterproof, two-part epoxy resin (‘Mr Sticky Underwater Glue’), chosen as it offers strong cohesive properties with resistance to salt water and continues to cure under water. Crabs were not removed from the holding tanks until immediately prior to tagging.
The tags were attached dorsally, towards the back of the carapace (Fig.A.I.1), but care was taken not to obstruct the epimeral line. For all individuals, the carapace at the attachment position was cleaned with an abrasive pad and for the first eighteen crabs tagged; the attachment area was dried before degreasing with acetone. This last process was omitted for the last 10 individuals tagged.For all control crabs, and the five dummy “cradle” tags (which do not bear numbers), individual identification was achieved through the attachment of coloured cable-ties to the right-hand cheliped.
After attachment of the dummy tags, the crabs were held in individual plastic containers for a maximum period of 1 hour, before being returned to individual enclosures in the aquarium, where the crabs were then held overnight. The following day, the crabs were moved to a specially constructed ‘reef’, designed to mimic their preferred habitat. Constructed of stacked boulders around a contained soft sediment area measuring approximately 3 m x 2 m, this provided the tagged crabs with crevices for sheltering, but also provided habitat in which any berried females would
SID 5 (Rev. 07/10) Page 35 of 130
be able to burrow. Feeding with fresh fish was carried out daily, during the morning, in the perimeter area outside the reef and enclosure, to encourage the crabs to move around.
AI.2.1.2 Attachment using superglueOn 31.01.08, four crabs that had lost their lozenge tags were retrieved from the reef, and were prepared as described above, for the attachment of dummy “cradle” tags, fixed to the carapace using Loctite superglue. As all of the tags shed initially had been from individuals treated with acetone, this procedure was omitted for the superglue-tagged crabs.
AI.2.1.3 Combined use of epoxy resin and superglueOn 14.03.08 10 stock crabs were tagged using a combination of epoxy resin and superglue. A small dab of superglue was applied centrally, with epoxy resin surrounding the base perimeter of the tag. We wanted to examine whether the relatively slow curing time of the resin might be alleviated by obtaining instant purchase through the superglue, thereby allowing the resin to cure fully, and thus obtaining a lasting bond between the tag and the crab. It was felt that this might provide a solution where the field release of the crabs was required soon after tagging. Consequently, these tagged crabs were returned to the reef in the 15 min following attachment of the tags.
AI.2.2 Field experiments
Between August 2008 and June 2009, a total of 128, pot-trapped, female edible crabs Cancer pagurus L. (carapace width 138 – 288 mm) were tagged with Cefas G5 long-life (2 MB memory capacity) electronic data storage tags (DSTs), with 2 MB memory capacity, configured with a 10 bar pressure sensor (reliable down to ~100 m depth). The DSTs were encased in secondary, lozenge-shaped perspex casings designed specifically for M1103 (Fig.2A).
To maximise high resolution data describing the horizontal and vertical activity patterns of crabs, the tags were programmed to record pressure at 30 s intervals and temperature at 5 min intervals for the first year at liberty, then both at 5 min intervals thereafter.
Only recently moulted, “new shell” individuals with no obvious external damage were selected for tagging. Individual crabs were tagged first with claw tags. DSTs were then glued to the posterior carapace (combined use of epoxy resin and superglue, see section 3.1.3 below). DST and tag numbers were checked on release and the position recorded from a handheld GPS.
The tagged crabs were released at 5 locations in UK waters: Eastern Channel (n = 32, August 2008); Trevose (n = 29, October 2008); South Devon (n = 30, June 2009); South Devon offshore (n = 37, June 2009); and in 2010, it was possible to refurbish sixteen tags that were reprogrammed and re-deployed on Sovereign Shoals in the eastern Channel. Note that apart from the final release of sixteen male crabs on Sovereign Shoals, all other tagged crabs were females. Tags were returned through the commercial fishery following a concerted publicity campaign (Fig.2B), and the offer of £50 for each returned tag.
Following the return of individual DSTs, data were downloaded, and the movements of crabs analysed using Tidal Location Method or “TLM” (based on tidal data recorded when crabs remained motionless on the seabed over a full tidal cycle, Hunter et al. 2003) and using the Hidden Markov Model or “HMM” technique (incorporating both TLM and bathymetry, Petersen et al. 2008).
SID 5 (Rev. 07/10) Page 36 of 130
Figure AI.2. (A) Electronic data storage tag designed for release on edible crab, Cancer pagurus, and (B) posters distributed around English Channel ports to advertise rewards associated with the electronic tagging programme
AI.3. Results
AI.3.1 Aquarium experiments
AI.3.1.1 Attachment using epoxy resinShort-term attachment
Three lozenge tags were lost during the official 2 month experimental period. Two of these were found within 4 days of tagging, and the third twelve days after tagging. Furthermore, two dummy “cradle” tags were also lost, the first towards the beginning of the experiment (found on 9/01/08), the second after nearly 2 months (14/02/08). Note that in all cases, the tags were still firmly attached to the glue, and it was, therefore, the glue plus the tag that had become detached from the carapace. It was noted that for those tags that became detached rapidly following the start of the experiment, the detached glue pad felt rubbery and friable.
There were also two mortalities associated with the experiments. Of the experimental females, a 174 mm CW Berried female died on 9/1/08. There was no sign of external damage apart from recently autotomised claws, and the dummy cradle DST was securely attached, and required force to remove it. In addition, one of the control female crabs measuring 162 mm CW died on 30/12/08. This individual had 2 missing legs (from an old wound). The shell was clean and thin, with a pale orange gonad developing inside, and no obvious pathology.
Long-term attachment
After 6 months, discounting the 4 individuals that were re-deployed for the superglue trials, 17 crabs were still alive on 25.06.08, including 6 of 7 untagged control crabs. Of 9 crabs tagged with lozenge tags, only 2 still had tags attached, although these required removing using force. Of the 8 crabs tagged with dummy “cradle” tags or active G8 tags, 4 were dead, one of which still had a tag attached, and 4 living crabs all had tags attached. These all required force for the tags to be removed. Only one crab was unaccounted for on 25.06.08.
AI.3.1.2 Attachment using superglueAlthough superglue allowed a rapid strong bond between the carapace and tag base, 2 of 4 tags became detached 20 and 27 days after tagging respectively. A further tag was subsequently lost before the end of the experiment on 25.06.08, such that only one tag was still attached at the end of the experiment. It was noted however, that this tag could only be removed from the carapace using force.
AI.3.1.3 Combined use of epoxy resin and superglueThis approach appeared to work well with the lozenge tags, resulting in rapid strong bonding immediately following tag attachment. However this was not the case with the “cradle” tags, which did not achieve the rapid strong bond characteristic of the superglue, and which were therefore still loose when the crabs were released on the reef. Two
SID 5 (Rev. 07/10) Page 37 of 130
Crab recaptures with time
0
5
10
15
20
25
40 80 120
160
200
240
280
320
360
400
440
480
520
560
600
640
Days after release
Coun
t
0
20
40
60
80
100
Perc
ent
of these tags were lost as soon as the crabs were released, the remaining 2 shortly thereafter. By contrast, all six lozenge tags stayed firmly attached until the end of the experiment on 25.06.08, and all required force in order to remove the tags from the carapace, including one individual in which mortality had occurred.
AI.3.2 Field experiments
AI.3.2.1 Return rates of DSTs
Table AI.1. Summary of data return for edible crab, Cancer pagurus tagged with DSTs
Recapture rates were high, with an overall total return rate of 34% (Table AI.1). Return rates between release sites varied between 17% (Trevose) and 40% (South Devon), with only a single individual being reported as missing its DST on recapture (3413, Eastern Channel). A second individual was recaptured, then immediately re-released once the tag details had been noted (5077, Channel Block C; South Devon offshore). Individual data records ranged between 8 and 575 days and from 46 returned DSTs, 4519 days of high resolution crab behaviour data were downloaded.
Figure AI.3. Number of edible crabs, Cancer pagurus tagged with electronic data storage tags recaptured per 40 day time interval following release, and cumulative percentage recapture by time
Fifty percent of all DST recaptures were made during the first 40 days after release (Fig.3). However, the recapture rate
thereafter did not follow a regular diminution pattern, but had a seasonal distribution related to the reproductive behaviour of the crabs (see section 3.2.4 below). The two October releases in 2008 yielded recaptures immediately following release up until early December (DST 3404, 09/12/08, Eastern English Channel). No more recaptures were then made until the following June, corresponding to the end of the egg-brooding period. A similar pattern was observed the following June in the Western Channel. Recaptures from the South Devon grounds continued from release up until mid-October, following which no further recaptures were taken until the following May.
AI.3.2.2 Habitat Occupancy
SID 5 (Rev. 07/10) Page 38 of 130
Figure AI.4. Average monthly (left) depths and (right) temperatures experienced by DST-tagged crabs, Cancer pagurus. EC = English Channel; Tr =Trevose; SD = South Devon; CBC = Channel Block C (S. Devon offshore); Sv = Sovereign Shoals.
Individual Records of depths and temperatures recorded (by release) are shown in figure. 5.
SID 5 (Rev. 07/10) Page 39 of 130
Average monthly temperature (deg. C)
0
5
10
15
20
Aug
-08
Sep
-08
Oct
-08
Nov
-08
Dec
-08
Jan-
09
Feb-
09
Mar
-09
Apr
-09
May
-09
Jun-
09
Jul-0
9
Aug
-09
Sep
-09
Oct
-09
Nov
-09
Dec
-09
Jan-
10
Feb-
10
Mar
-10
Apr
-10
Tem
pera
ture
(deg
. C)
EC
Tr
SD
CBC
Sv
Average monthly depth (m)
0
25
50
75
100
Aug
-08
Sep
-08
Oct
-08
Nov
-08
Dec
-08
Jan-
09
Feb-
09
Mar
-09
Apr
-09
May
-09
Jun-
09
Jul-0
9
Aug
-09
Sep
-09
Oct
-09
Nov
-09
Dec
-09
Jan-
10
Feb-
10
Mar
-10
Apr
-10
Dep
th (m
)
EC
Tr
SD
CBC
Sv
Figure AI.5. Average daily a) depth and b) temperature records of DST tagged edible crabs, Cancer pagurus released in the (A) eastern English Channel, (B) Trevose, (C) South Devon and (D) Channel “Block C”(South Devon offshore).
AI.3.2.3 Reconstruction of crab migration pathways
The migration routes taken by DST-tagged crabs were reconstructed both using tidal location method (TLM, Hunter et al., 2003) and the HMM geolocation toolbox (HMM, Pederson et al., 2008), both of which have successfully been applied to reconstruct the movements of sea-bed dwelling fishes. Both techniques use tidal information recorded when the tagged animal remains motionless on the seabed throughout a full tidal cycle, while HMM also uses bathymetry to help discriminate location. Examples of reconstructed movements of geolocated crabs using each technique are shown below (Figure AI.6).
SID 5 (Rev. 07/10) Page 40 of 130
Channel Block C
0
30
60
90
120
May
-09
Jun-
09
Jul-0
9
Aug-
09
Sep-
09
Oct
-09
Nov
-09
Dec
-09
Jan-
10
Feb-
10
Mar
-10
Apr-
10
May
-10
Jun-
10
Jul-1
0
Aug-
10
Dep
th (m
)
50955064504250665060507050675044504850515072
Channel Block C
5
8
11
14
17
May
-09
Jun-
09
Jul-0
9
Aug-
09
Sep-
09
Oct
-09
Nov
-09
Dec
-09
Jan-
10
Feb-
10
Mar
-10
Apr-
10
May
-10
Jun-
10
Jul-1
0
Aug-
10
Tem
pera
ture
(Deg
. C)
50955064504250665060507050675044504850515072
South Devon
0
30
60
90
120
May
-09
Jun-
09
Jul-0
9
Aug-
09
Sep-
09
Oct
-09
Nov
-09
Dec
-09
Jan-
10
Feb-
10
Mar
-10
Apr-
10
Dep
th (m
)
510351045105510651015057504650945082506150925058
South Devon
5
8
11
14
17
May
-09
Jun-
09
Jul-0
9
Aug-
09
Sep-
09
Oct
-09
Nov
-09
Dec
-09
Jan-
10
Feb-
10
Mar
-10
Apr-
10
Tem
pera
ture
(deg
. C)
510351045105510651015057504650945082506150925058
Eastern English Channel
0
30
60
90
120
Jul-0
8
Jul-0
8
Aug-
08
Sep-
08
Oct
-08
Nov
-08
Dec
-08
Jan-
09
Feb-
09
Mar
-09
Apr-
09
May
-09
Jun-
09
Jul-0
9
Aug-
09
Sep-
09
Oct
-09
Nov
-09
Dec
-09
Jan-
10
Feb-
10
Mar
-10
Apr-
10
May
-10
Dep
th (m
)
34333426342534223418341534053404342134013428
Eastern English Channel
5
8
11
14
17
Jul-0
8
Aug-
08
Sep-
08
Oct
-08
Nov
-08
Dec
-08
Jan-
09
Feb-
09
Mar
-09
Apr-
09
May
-09
Jun-
09
Jul-0
9
Aug-
09
Sep-
09
Oct
-09
Nov
-09
Dec
-09
Jan-
10
Feb-
10
Mar
-10
Apr-
10
May
-10
Dep
th (m
)
34333426342534223418341534053404342134013428
Trevose
0
30
60
90
120
Sep-
08
Oct
-08
Nov
-08
Dec
-08
Jan-
09
Feb-
09
Mar
-09
Apr-
09
May
-09
Jun-
09
Jul-0
9
Dep
th (m
)
33983342334333883434
Trevose
5
8
11
14
17
Sep-
08
Oct
-08
Nov
-08
Dec
-08
Jan-
09
Feb-
09
Mar
-09
Apr-
09
May
-09
Jun-
09
Jul-0
9
Tem
pera
ture
(deg
. C)
33983342334333883434
Figure AI.6. Reconstructed movements of DST 3401 (above), released in the eastern English Channel on 27/08/08 and recaptured 386 days later having migrated 302km, and DST 3422 (below), released in the eastern English Channel on 27/08/08 and recaptured 355 days later having migrated 173km using (left) tidal location method (● = release, x = recapture, ● = individual geolocations) and (right) HMM technique
Note that using TLM, we were able to obtain a cluster of geolocations for DST3401, immediately following release, then at 0.5ºW during October (Figure AI.6). High levels of activity during migration meant that no further locations were then obtained until the time immediately before and during presumed egg-incubation. These placed the crab around the area where it was eventually recaptured on the south Devon grounds. In this example, the HMM model has been able to interpolate data, creating a most probable track using both tidal data and bathymetry (but see below).
For DST 3422, TLM intermittently charts westward progress along the English coast. A seven month pause during brooding was identified at approximately 0.5ºW from December until June, before westward migration resumed. Only one geolocation was obtained after July 1st, when the crab was located to the SE of Isle of Wight, suggesting that 2 weeks later the crab had reached the western end of Poole Bay, where the crab was recaptured in August. In this case, distorted tidal data resulted in an HMM reconstruction that suggested that the crab moved in an arc which takes the crab south towards the French coast and back again. This seemed inconsistent both with the recorded depth data, and the possible rates of movement suggested by our tagging data overall. However, in spite of trying to constrain rates of movement within HMM, we were unable to generate a more realistic model.
Geolocation data suggest that for migrating crabs at least, movement between release and recapture does not depart appreciably from straight line movement, even where individual depth records clearly demonstrate that crabs did not follow defined isobaths during migration.
However, significant problems with geolocation were encountered. This was due to low level movement by crabs which subtly distorted the recorded tidal signals: increasing or decreasing the amplitude of tidal range, and moving the peak (suggested time of high water). As a result, it was frequently the case that only a limited number of TLM geolocations could be generated from a single track. In addition, the HMM model was designed for use with cod depth data. Although individual records could be “trained” in supervised mode (i.e. the input variables altered to more closely match the physiological movement abilities of a crab), many of the output tracks suggested degrees of migration that were outside the locomotory capabilities of individuals. Further work will be required therefore in order to improve predictive geolocation using the crab data, however this will involve additional modelling, and a re-programming of the HMM model, which was outwith the scope of this project.
3.2.4 Annual cycles and reproductive behaviour
SID 5 (Rev. 07/10) Page 41 of 130
- 7 - 6 - 5 - 4 - 3 - 2 - 1 0 1 24 9
5 0
5 1
5 2
- 7 - 6 - 5 - 4 - 3 - 2 - 1 0 1 24 9
5 0
5 1
5 2
Table AI.2. Brooding metrics (time period, depth and temperature at onset and completion, and location estimated using tidal location method) of the eight edible crabs, Cancer pagurus, that recorded over all or part of the egg-brooding season. “Tag fail” indicates where data recording ceased before the end of the brooding period
Note. Channel Block C is equivalent to South Devon offshore
The greatest insights and new observations into crab behaviour in the current study resulted from six individuals that recorded data over a full annual cycle (DSTs 3401, 3422, 3428 (eastern English Channel), 3398 (Trevose), 5048 and 5072 (Channel “Block C”; South Devon offshore), and a further 3 crabs that recorded some of the egg-brooding period: DSTs 3388 (Trevose), 5058 and 5061 (South Devon). Metrics on the timing and duration of the brooding period are detailed in table AI.2.
Figure AI.7. Release (+), recapture (x) and brooding (*) locations and estimated geo-positions (based on tidal location method) of eight edible crabs, Cancer pagurus, tagged with electronic data storage tags. Eastern Channel: A03422 (355 d, red); A03428 (575 d, purple); A03401 (386 d, orange); Trevose: A03388 (141 d, light pink); A03398 (253 d, pink); South Devon: A05058 (269 d, navy); A05061 (221 d, blue); South Devon offshore: A05048 (384 d, green); A05072 (323 d, turquoise).
With the exception of the Trevose releases, all of the egg-brooding crabs in the current study migrated west following release and prior to brooding (Fig.AI.7). Unlike the other releases in this study (see section 4.1; Appendix III), Trevose crabs did not appear to demonstrate any directed migration, and mark-recapture data support the idea that there was limited dispersion of these crabs away from the point of release. Although the suggested brooding location for DST 3398 implies that the crab has moved east towards the coast, it is likely that the imprecision associated with tidal location method in this area (see section 3.2.3 above) provides some degree of distortion to the actual situation, and with little evidence of significant levels of activity by this particular crab, it is suggested that the true brooding locations of both of the Trevose brooding crabs was close to the position of release.
The results clearly demonstrate that Channel crabs are not restricted to a single, clearly defined brooding area, but that brooding occurred at various locations (and depths) throughout the Channel (Figure AI.7, Table AI.2). DST 3428, which was the only individual to have recorded data throughout two brooding seasons, settled down to brood in two locations in the two years recorded, the two locations separated by two degrees of longitude. As all of the brooding crabs were also recaptured to the west of their brooding locations (some significantly so, e.g. 3422), our results suggest that edible crab do not use the same brooding areas in successive years.
SID 5 (Rev. 07/10) Page 42 of 130
3428
With regards individual behaviour, some variability in depth between brooding females suggests that the crabs are not always completely immobile during brooding. It was also noted that during the second season recorded by DST 3428 (Figure AI.8), the “brooding” period terminates considerably earlier than in the previous year (Table AI.2), with the pot recapture of this individual on 24/03/10, suggesting that the crab had become active and started to feed. As the individual crabs were not returned, it is impossible to determine whether or not the crab was actually carrying eggs.
Figure AI.8. Average daily depth (black line) and temperature (red line) recorded by DST 3428, released in the eastern English Channel on 27/08/08 and recaptured 575 days later having migrated 196km
A1.4. Discussion
AI.4.1 Aquarium experiments
The tagging of crustaceans using DSTs presents considerable problems, not only in so far as the tag will usually be shed during ecdysis (moulting). The tag needs to be attached in such a way that the natural movements and behaviour of the animal remain unhindered, and that the tag is firmly attached, so as to withstand agonistic behaviour and burrowing in rocky crevices and substrata. Consequently, the aquarium conditions were designed to simulate, as far as was practically possible, field conditions allowing realistic amounts of interaction between individual crabs and to challenge the strength of the bond between carapace and tag by allowing abrasion due to burrowing in hard sediments and crawling in confined rocky areas.
The crabs tagged during the current series of experiments exhibited behaviours that directly counteracted successful adhesion of tags using adhesives. Specifically, crabs returned to the tanks post-tagging had a tendency to flip over onto their backs, and then sought refuge in crevices. When other crabs were present, the animals tended to slide laterally over one another, such that the tag was liable to become dislodged. The other main problem associated with tag attachment using epoxy resin was the relatively long curing time of the glue, probably accentuated in this case due to the relatively low operating temperature. Even under ideal conditions, the epoxy resin (specifically designed for use underwater) had a curing time of 2 hours.
Initial isolation of individuals post-tagging was designed to minimise tag-loss, however it was noted that where a large quantity of glue had been applied, slippage of the tag could occur within this period. However, in order to minimise stress to the animals, time out of water was limited to a maximum of 1 hour. It is perhaps surprising, therefore, that not more tags were lost during this early phase.
For those tags lost after the initial acclimation period, it was noted these were all sourced from the first eighteen crabs, all of which had been treated with acetone prior to the attachment of the tag. Furthermore, the glue was often tacky and slightly friable. It is suggested that the resin may not have been adequately mixed for these initial attachments, as it seems unlikely that infraction by seawater would occur at such an early stage. However these initial results do suggest that epoxy resin may not be best suited to in situ releases of DST-tagged crabs from commercial vessels.
By contrast, tagging using superglue provided a rapid, strong bond between the tag and carapace. However superglue is not resistant to seawater, and tags attached using superglue alone tended to detach within a month of initial attachment. Our third trial therefore used a combination of both resin and superglue. Although this technique was unsuccessful for the “cradle” tags, which had a smaller basal surface area, this approach was most successful with the larger lozenge tags, where a clear separation between the 2 glues could be achieved, and sufficient surface area was available to spread out on the rugose surface of the carapace. We suggest that this technique is the most promising for use with edible crabs, the superglue providing a rapid strong bond around which the epoxy resin can then harden and cure without risk of being dislodged. Although only 6 crabs were tagged in this way, all 6 tags were still firmly attached after 3 months, and it was necessary to lever the tags off the carapace in each case, using a screwdriver.
SID 5 (Rev. 07/10) Page 43 of 130
A03428
0
10
20
30
40
50
60
08/0
8/20
08
08/0
9/20
08
08/1
0/20
08
08/1
1/20
08
08/1
2/20
08
08/0
1/20
09
08/0
2/20
09
08/0
3/20
09
08/0
4/20
09
08/0
5/20
09
08/0
6/20
09
08/0
7/20
09
08/0
8/20
09
08/0
9/20
09
08/1
0/20
09
08/1
1/20
09
08/1
2/20
09
08/0
1/20
10
08/0
2/20
10
08/0
3/20
10
08/0
4/20
10
Dept
h (m
)
5
7
9
11
13
15
17
19
Tem
pera
ture
(deg
. C)
Previous studies requiring the application of tags to various types to marine animals have used glue, as the ease of application and low trauma properties can be beneficial (Trendall, 1989). Lemarie et al. used both epoxy resin and a cyanoacrylate to stick Floy and Hallprint tags to freshwater mussels (2000). Heald used a cyanoacrylate to stick various tags including metal, titanium and plastic tags to saucer scallops (1978). Unknown adhesives were used to adhere tags to abalone that gave the benefit of low predator attraction (McShane, 1989) and lobsters (Anon. In Permanent Labels, 1997).
For the limited number of published field experiments where spider crabs (González Gurriarán & Friere, 1994; Friere et al., 1999) and lobsters (Smith et al., 1998) have been tagged with telemetry tags and DSTs, all have used fast-setting epoxy resins. Although a vast amount of information is widely available regarding adhesives and their properties, most relate to specific or industrial applications rather than application to living chitin. Many commonly used adhesives are obviously unsuitable as their performance in the marine environment or lack of compatibility with living organisms precludes their consideration. However, as well as epoxy resins, toughened acrylic, cyanoacrylate, polyurethane and silicone-based ahesives may also have potential, however the scale of research required is outside the scope of the current study.
For future study, it is worth noting that dentistry, medical and offshore industries all provide examples of situations requiring adhesives sometimes for applications similar to the experimental protocol described above.
AI.4.2. Field experiment
The return rates in the current field release of edible crabs tagged with DSTs greatly surpassed expectations, with an overall return rate of 34% compared with 15% in the double T-bar tagging experiment. At 17%, DSTs were returned from the Trevose grounds, compared with return rates of 38% and 40% in other areas, reflecting the levels of fishing effort experienced in the different areas (see section 5.1; Appendix VI). The interrupted pattern of tag recapture, whereby approximately 50% of recaptures were made within the first 40 days following release, declining gradually up until approximately 120 days (depending on the time of release, but basically up until early to mid-November), with a hiatus in recaptures until the following summer, was very similar to that observed in other edible crab tagging programmes (see section 4;, Bennett & Brown, 1982; Ungfors et al., 2007), and was related to the reproductive activity of the crabs (see below).
The only release of male crabs was made just over a year ago in March 2010 in the eastern Channel. Although six of sixteen releases have already been recaptured, the longest data record was just eighty days. It is still conceivable that more male recaptures may be returned, however the relative paucity of data from this particular release means that the results focus predominantly on female behaviour.
Remarkably, only one crab was returned which was missing its DST. The tags proved robust, and recorded 4519 of 5540 days at liberty (82%). Data download was not possible from two tags (DSTs 5077 and 5098, both from South Devon offshore) and sensor failure prior to recapture occurred for a further 6 tags. This did negatively impact on our findings on the annual cycle of behaviour for 5 of these individuals, all located in the western Channel. DST 3401, one of the longest data records, and one of the furthest migrating individuals, was also one of the only individuals to move into water deeper than 100m. Our tags were programmed for use in up to 100m depth, and in this case the sensor functioned down until 113m, and recorded “113” until the crab came back up above this level. However, a second excursion below 113m resulted in a terminal failure of the tag; however a full annual cycle had already been captured. Several of our tagged crabs had been briefly recaptured and re-released, and it has been noted that it is widely believed amongst the crab fishers that the actual position can accurately be determined from the tag, which is not in fact the case.
There was relatively little within-release variability in temperature and depth experienced by the DST-tagged crabs, although most variability was associated with the longer records and hence migrating individuals. The data clearly demonstrate that migrating crabs do not follow defined depth bands. We did find some evidence that crabs released in the western Channel (South Devon inshore, South Devon offshore) may have sought out warmer water a time corresponding to the pre-brooding migration. While temperatures recorded by eastern Channel releases from late summer through autumn showed a declining temperatures from release (August) onwards, the temperatures recorded by the western releases continued to rise from August through to October before a decline was observed.
Track reconstruction using the two geolocation techniques employed suggested that movement of crabs between release and recapture positions showed relatively little deviation from a directed straight line movement. However neither technique (Tidal Location Method and Hidden Markov Model) performed as well as was initially expected. Although the relatively low-level movement recorded by crabs appears to provide a strong record of tidal conditions, this is, in fact deceptive. Even low-level activity by the crabs was adequate to distort the tidal information required by both track-reconstruction techniques (tidal range and time of high water). This is further
SID 5 (Rev. 07/10) Page 44 of 130
complicated by the relatively complex tidal situation in the English Channel, where two tidal “solutions” (i.e. the same time of high water and tidal range) can often occur within a single degree of latitude.
Consequently, the accuracy of TLM is greatly reduced, and a spread of “solutions” were often observed parallel to or around the area where the mark-recapture data suggested might be the more probable location of the crab. In addition, the relatively small scale of movement, compared, for example, with the fish species which have very successfully been studied using TLM (e.g. Metcalfe et al., 2008; Hunter et al., 2009, Righton et al., 2010), the very significant inter-individual differences in scales of migration, and the very gradual rates of change of depth in many parts of the Channel, made it very difficult to adequately discriminate between “correct” and “false” solutions. While HMM should have allowed for some correction by combining tidal location with bathymetry, we were unable to adequately constrain the HMM model to the low-levels of movement exhibited by most of our migrating crabs. Since the model was developed originally to aid in the reconstruction of migrating demersal fish (principally cod, Petersen et al., 2008), it seems likely that some of the model code will require re-modelling. We anticipate that this can be achieved, and will greatly enhance the “most probable tracks” generated within this particular project.
The real breakthrough in the current study has been with regards the timing, conditions and duration of egg-brooding. Previous observations on the brooding behaviour of crabs have been largely restricted to indirect observations on the occurrence and distribution of egg-bearing crabs, either in landings, scuba surveys or aquarium experiments (Appendix II, this study, Howard, 1982, Latrouite & Phillipe, 1993), and the histological examination of sexual maturity (Lawler et al., unpublished data). The onset of egg-brooding, or at least the time at which the crabs appeared to cease most activity, appeared to correspond well with previous observations by Latrouite & Phillipe (1993) which suggested that brooding commenced from around mid-November through to early January. The earliest onset of “brooding” observed in the current study was from late October in the South Devon offshore crabs. Indeed, there was some indication that the more westerly crabs started to brood slightly earlier than those in the eastern Channel, which tended not to commence brooding until mid- to late November onwards.
The average duration of brooding was 177±24 days, although by far the shortest brooding duration observed was 126 days during the second brooding season (recorded by DST 3428). Howard (1982) observed that the majority of ovigerous females sampled contained empty guts, and that the guts were “plugged”, and the hepatopancreas condition “poor” in 9/10 crabs. It is generally accepted that the general absence of berried female crab during winter is due to non-feeding during the egg-brooding period. Our results demonstrate that the brooding females became largely inactive for the duration of egg brooding, becoming active again at the end of the brooding period, when they started to forage for food, the majority having been recaptured in baited pots.
Interestingly, DST 3428, which is our only individual that recorded data over two full brooding seasons, migrated west prior to both her first and subsequent brooding period. The two brooding locations recorded were separated by two degrees of longitude. This observation, coupled with the fact that all of our other Channel brooding females were recaptured west of their brooding locations, provides some evidence that females do not show fidelity to the same brooding locations in successive years. However it is also notable that this female did not moult between broods, although it is thought that sperm retention by the females can facilitate multiple spawning from one mating event. Unfortunately, as the crab carcass is not generally recovered in these experiments, there is no way of knowing whether or not this female was indeed carrying eggs during her second season. Although the timing of the onset of “brooding” was similar in the first and second years, there was some evidence that the crab was slightly more active during the second brooding season, which lasted just 126 days, as opposed to 188 days during the first year.
Estimated brooding locations, based on TLM, suggested that brooding is not restricted to one single, clearly geographically defined brooding ground, but may occur at various locations throughout the Channel (although these are probably defined by substrate characteristics, Howard 1982). The average depth at which egg-brooding occurred was at 57±23m, but ranged from 19m in the shallower eastern Channel grounds, to 84m in the deeper South Devon grounds. Temperature at the onset of brooding was 13±1.5ºC, and 11±2ºC when the females became active again. The lowest temperatures at the onset of brooding were recorded on the Trevose grounds (10.7ºC), and the highest in the deepest (South Devon offshore) grounds (15ºC). By contrast, brooding appeared to stop earlier, and at lower temperatures in the western Channel (although due to tag failure, we have no data for South Devon). Again, the exception was DST 3428, which was the earliest to cease brooding (24/03/10), at the lowest temperature (6.7ºC), when she was recaptured just off Portland Bill.
Both the current study and previous mark-recapture experiments carried out in the 1970s (Bennett & Brown, 1983; Cuillandre et al., 1984; Latrouite & LeFoll, 1989) have demonstrated long distance, predominantly westward movements by crabs in the English Channel, particularly mature females. None of our DST tagged crabs exhibited west to east migration. It is noted however, that with the possible exception of the Trevose releases, all of our DST-tagged crabs that recorded the brooding period were migratory individuals. Work carried out under M1103 has allowed us to detail how these migrations proceeded in terms of crab behaviour, and the environmental conditions experienced during migration. Although further work will be required in order to refine the level of accuracy
SID 5 (Rev. 07/10) Page 45 of 130
associated with the geolocated crab-tracks produced as a result of this study, at the scale of fishery management, our study provides valuable new information on the rates and scale of movements.
The net westward movement of mature females has previously been interpreted as migration to spawning grounds located in the western Channel, in order to allow the hatching of planktonic larvae in the prevailing tidal currents that will ultimately facilitate the return of settling larvae to the areas of maternal origin. Recent surveys of larval distribution, and the interpretation of larval distribution using hydrodynamic modelling (Eaton et al., in prep.) have suggested that larval transport rates observed might not be adequate for larvae to be returned to the original maternal location. Furthermore, genetic studies (Defra MF0230, Paul Shaw, Royal Holloway, University of London, unpublished data) have provided some evidence of significant genetic variation in edible crabs at relatively local scales. Rationalisation of these otherwise apparently conflicting strings of evidence have proven difficult to formulate into a cohesive picture of stock identity. However the more widespread area over which brooding takes place suggested by the current study may go at least some way to help explaining these discrepancies.
Ultimately, the new results presented under M1103 provide significant new opportunities to re-examine and re-formulate and existing stock-dynamic models for application in the assessment and management of the stock. We have demonstrated that the large-scale application of archival electronic data storage tags (DSTs) to edible crabs has proved to be a highly successful and effective means of gathering information for direct application in the management of UK crab fisheries. The data gathered are now available to be mined for additional information on aspects of behaviour, including patterns of activity related to diurnal, tidal and reproductive cycles, and fine-scale habitat occupancy, which was outwith the scope of this project. It is anticipated therefore, that similar insights can also be gained from other heavily exploited shellfish fisheries.
Appendix II. Aquarium experiments carried out to support the double T-bar tagging programme
AII.1 Introduction
Quantitative analyses of tag return rates provide valuable insights in to exploitation and mortality rates, but are often weakened because data on tag loss are unavailable. Double T-bar tags have been successfully applied to crabs in other countries, but data relating to retention rates are scarce. Small scale aquarium experiments were therefore carried out to investigate tag retention rate, including through the moulting process. These experiments also provide some information on mortalities. However, high mortality rates most likely due to high temperatures in aquarium facilities tend to invalidate the mortality studies and made the interpretation of tag retention more difficult.
AII.2 Methods
A series of 6 small scale experiments were carried out using the aquarium facilities at Lowestoft to study the retention of the double T-bar tags used for the tagging programme. Crabs were kept individually in compartments during each trial which lasted between 100 and 200 days. At the end of each trial surviving crabs were moved to holding facilities where they were kept under observation in small groups. This permitted some further information to be gathered, while the subsequent experiment was underway.
In these experiments there are two response variables; survival (how long the crab survived in days) and retention (how long the tag was retained in days) and four possible states for a tagged crab (alive & tagged, dead & tagged, alive & not tagged, dead & not tagged), of which only the latter two are possible for the control animals. It is possible to look at the processes of survival and tag retention together, but as the primary aim of these experiments was to evaluate tag retention they were analysed separately. These data are statistically known as ‘right censored’ and recorded as two variables expressing the duration of the observation (survival or tag retention) and whether or not it was terminated by an event (crab death or tag loss).
Survival and tag retention data were analysed using methods from a field called survival analysis. All analysis was carried out in R 2.9.0 (R Development Core Team, 2009) using package “survival” (Therneau & Lumley, 2009). The commands used and theory behind them are summarised in Venables and Ripley (2002). The Kaplan-Meier estimator (Kaplan & Meier, 1958) was used to estimate the survivor function S(t), that is the probability of surviving (or retaining the tag) until at least time t. If there is no censoring then the estimator equals the proportion of crabs surviving, otherwise it accounts for the reduction in the number of crabs observed over time by multiplying together the probabilities of surviving at the time of each event. (R has the relevant equations built-in.) Differences in survivor curves between trials or between tagged and control crabs were tested using log-rank tests (Mantel, 1966). Although parametric models for survival data have fallen out of fashion (Venables & Ripley, 2002), a parametric model was fitted to the tag retention data to provide a rate of tag loss in the form of an exponentially declining model that could be directly applied within in the population dynamics modelling framework used to synthesise tagging results.
SID 5 (Rev. 07/10) Page 46 of 130
AII.3 Results
Tag retention
Tag retention was generally high (Table AII.1), with eight tags lost during the experimental periods and a further 5 lost during the subsequent periods in the holding tanks. The latter were included in the analysis to maximise the time period available.
Table AII.1. Number and percentage of tags lost during experimental and additional observation periodsExperiment
1 2 3 4 5 6 TotalTags lost 5 1 4 0 2 1 13
n tagged 19 16 17 13 13 17 95
% lost 26.3 6.3 23.5 0.0 15.4 5.8 13.7
Retention rates for the experimental periods only were not statistically different between experiments using the log-rank test (p=0.12) and this was also the case when using the additional holding tank data (p=0.13). However, individual curves (Figure AII.1) show that performance was poorest for experiment 1 and the individual comparisons showed experiment 1 was significantly different from the combined result (p=0.013), whilst other experiments were not (p=0.28, 0.46, 0.23, 0.67 and 0.51, respectively).
The retention curve for all experiments combined and including the time in the holding tanks (Figure AII.2) suggests that on average around 75% of tags will be retained for more than 200 days and this may be as high as 80% if experiment 1 is excluded.
A parametric curve for tag retention with exponential decline was fitted to estimate an instantaneous coefficient of daily rate of tag loss (Lambda=0.0010320) of around 0.031 per month. A more relaxed fit using a Weibull distribution had a scale parameter not dissimilar to 1 (0.78) and very similar log-likelihood suggesting the model was a reasonable approximation.
SID 5 (Rev. 07/10) Page 47 of 130
Figure AII.1. Tag retention curve for all individual experiments, and including subsequent periods in holding tanks. Dashed lines 95% C.I.
Figure AII.2. Combined tag retention curves for all experiments with parametric exponential curve superimposed (left) and experiments 2 to 6 (right), both including data from subsequent periods in holding tanks. Dashed lines 95% C.I.
Retention through the moult
Of the 7 crabs in experiment 1 that moulted, 3 were controls and 4 were tagged. All the tagged crabs retained the tags through the moult, although the hole in the carapace was enlarged and in one case the tag lost some 2 weeks
SID 5 (Rev. 07/10) Page 48 of 130
later. One crab moulted in each of experiments 4 and 6, with the tag lost during the moult in experiment 6. The crab in experiment 4 retained its tag through the moult and for 602 days in all, with the tag still in place when it died. Pooling these results 4 to 5 crabs of 6 in total retained tags through the moult, a tag retention rate of between 67%-83%.
Survival
Survival percentages at the end of each experiment (Table AII.2) were generally low and more so for tagged crabs. A log-rank test indicated that for all experiments combined survival for tagged crabs was significantly worse than for controls (p=0.0209). However, analysing experiments individually (survival curves illustrated in Fig AII.3), indicated that tagged crabs had a significantly worse survival in experiment 1 (p<0.01), but that the effect of tagging was not significant for experiments 2-6 (p=0.33, 0.91, 0.39, 0.24 and 0.98 respectively) and p=0.19 when results from these experiments were combined.
Table AII.2. % surviving in each aquarium experiment
% surviving at the end of each experimental periodExperiment
(total n) 1 2 3 4 5 6Control 100 (8) 77.8 (9) 50 (8) 50 (4) 100 (3) 33.3 (3)
Tagged 42.1 (19) 56.3 (16) 52.9 (17) 23.1 (13) 61.5 (13) 35.3 (17)
A log-rank test on all data for differences between experiments was marginally significant (p=0.0534), while for tagged crabs it was significant (p=0.02) and for controls it was not significant (p=.35), in part due to the low sample numbers for controls. For tagged crabs experiments 1 and 4 had lower than average survival.
Observations of spawning
During the aquarium experiments 24 crabs spawned, providing a limited dataset informing on the timing of this event under the ambient aquarium conditions. These showed highest frequencies spawning in December (10) followed by November (6), January (3), February (1) and March (1), with 3 for which the date of spawning was not recorded. These results are in good accordance with current knowledge regarding the timing of spawning.
SID 5 (Rev. 07/10) Page 49 of 130
Figure AII.3. Survival curves for the experimental periods in the six aquarium experiments. Thin, dotted lines are point-wise 95% confidence intervals. Crosses show censoring times without events. There is only one not at the end of the period, a crab observed for 115 not 129 days in experiment 2.
AII.4 Discussion
Aquarium trials were carried out with the primary aim of providing data that would enable estimation of the rate of double T-bar tag loss. The Lowestoft facilities are not well suited to long term experiments with crabs as water quality can be towards the edge of crabs’ tolerance. Also limitations on resources meant we were only able to run experiments of less than 20 crabs at a time, which allowing for some control animals meant the numbers of crabs tagged were quite small. Results were therefore pooled over experiments.
Statistical analyses suggest that experiment 1 exhibited both higher tag loss and mortality rates than other experiments. This experiment was timed to maximise the numbers of crabs likely to moult, and we believe the poorer performance may reflect our inexperience with the tagging technique at this time along with high water temperatures during this experiment and the occurrence of moulting in a number (7) of the crabs. Some other experiments (4 and 6) also took place over the summer months, when moulting is most likely, but few instances (1 in each) of moulting actually occurred in these trials. We therefore retain the results from experiment 1 in our analyses for tag retention, on the basis that they will provide an average result for moulting and non moulting periods. Although few instances of moulting occurred the rates of retention through the moult were very similar to the loss rates in general suggesting that the double T-bar tag is suitable for tagging crabs.
A number of factors were considered to have potentially affected survival rates, including the physical environment (salinity and temperature) and the expertise of the scientists carrying out the tagging. Edible crabs do not generally survive well for long periods of time in the Lowestoft facilities, which draws seawater from a buried sub-littoral intake near the laboratory. Salinities are typically in the high 20s ‰ and ambient environmental conditions can lead to temperatures that are extreme compared to those experienced by crabs on the seabed offshore. Experiment 1 suffered from high temperatures and was also the first time Cefas staff had carried out this type of tagging on crabs. We believe that both these factors may have contributed to the higher mortality rates in this experiment. Temperatures in experiment 1 were greater than 17oC from mid July though to mid September peaking at 18oC. French vivier vessels are reported to turn off their water pumps when returning to port and temperatures exceed 16oC (pers. comm. M. Laurans) so the temperatures experienced in the Lowestoft facilities are certainly towards
SID 5 (Rev. 07/10) Page 50 of 130
the upper limit of tolerance for edible crabs. Experiments 4 and 6 also experienced higher mortality rates, but unlike in experiment 1 for both tagged crabs and control crabs. These experiments also both took place over the summer and it is very likely that temperature was a major influence in the mortalities. The very high mortality rates observed in experiment 4 during August 2009 occurred during a period when the recorded daily temperatures were all above 17oC and frequently above 18oC and mortalities in experiment 6 occurred mainly during August and September after a prolonged period when temperatures were above 17oC during most of July and August.
Appendix III. Double T-bar tagging programme and results relating to movements
AIII.1 Introduction
It is 37 years since Bennett & Brown (1983) released ~15k tagged crabs in the English Channel as part of a 10 year long study to determine growth, mortality and movements of crabs. Their work was carried out at a time when the traditional inshore fisheries were expanding onto offshore grounds and one of the main objectives was to determine the relationship between these offshore and inshore components of the population. The remit of this study includes determining if the work done by Bennett & Brown (1983) is still relevant in the light of environmental changes and suspected continual increases in exploitation that have occurred since as well as more generally improving knowledge regarding crab stocks and their biology, in particular through the application of DSTs to crabs.
Conceptually the methodology relating to conventional tags has changed little, but improved technologies has led to more precise release and recapture positions being available for some of the crabs and much faster tagging procedures. The latter has enabled a similar number of marked crabs to be released over a much shorter time period (3 years and not 7 years) and with much less time being spent at sea by scientists. The potential for improved positional information has enabled analysis at a higher spatial resolution, however any mark and recapture study which is dependent on the industry for reporting recapture information is vulnerable to misreporting and non-reporting of recaptures.
This section presents results from the double T-bar tagging programme and analyses carried out on them that related to movements of crabs. The data and analyses are voluminous, so to aid clarity the methods and main results and discussion sections are ordered under the following sub-section headings:
Release sites and recapturesMagnitude and directions of movementsDescriptions of recoveries by tag group, standardised scatter and population movement vector plots and seasonalityThe effect of size, gender, shell condition, limb loss and release batch on crab movements
AIII.2 Methods
AIII.2.1 Double T-bar tagging methodology
Crabs were tagged using Super Heavy Duty monofilament Double T anchor tags (SHD FD-94 anchor tags) manufactured by Floy Tag and Manufacturing inc. in Seattle, USA. The tag had two 10mm long T-bars that were 10mm apart. The external monofilament of the tag was 55mm long surrounded by coloured polyolefin tubing which contained the numbering and legend, protected by a shrink lock covering. Each tag was serially inscribed with the tag batch and series numbers and requests for recovery information as follows:E04 0001 WWW.CEFAS.CO.UK PLEASE RECORDTAG NO, DATE, LAT & LONG, WIDTH, SEXThe last 4 digits of the tag number being sequentially incremented.
Avery Dennison Mark II SHD tagging guns, with a 2.3mm external diameter needle were used to insert the tags. The guns were modified to reduce possible damage to the crabs’ internal organs when tagged. The long point of the needle was ground off and a guard attached, limiting the maximum penetration of the needle to around 1cm. The needle was inserted into the branchial chamber through a hole made in the epimeral line at the back of the carapace of the crab and when triggered, the head of the tag was driven inside the carapace and anchored by the terminal T-bar. The second T-bar remains outside the carapace preventing the trailing tag section from being drawn inside the crab.
Tagging and release information for each crab was recorded using either a Thales Mobile Mapper, in which case the time, date and GPS position were automatically logged or a hand-held GPS was used. Tag number, sex and carapace width (CW) of the crab, together with an assessment of the shell condition (soft, new, hard or old) and any other observations, were also recorded before returning the crab to the sea. The tagging needle and bradawl used to pierce the carapace were disinfected between each tag insertion using 100% ethanol. This methodology
SID 5 (Rev. 07/10) Page 51 of 130
allowed approx. 700 crabs to be tagged daily compared with 200-300 using the Petersen disc suture tagging technique. Crabs were tagged through summer and autumn in major fisheries through the Channel and for the first time, on the Trevose grounds off the north Cornwall coast.
A small proportion (~4%) of crabs were double-tagged, with a double T-bar tag inserted in the epimeral line on both sides of the crab. Return rates of tags from these crabs provided an indication of the rate of tag loss in the tagged population.
The success or otherwise of any tagging exercise depends greatly on the rate and accuracy of tag reporting. An extensive campaign to advertise the tagging programme was carried out in all areas where tagging took place and where catches might be landed, both in England and Wales and abroad. Posters explaining the objectives of the exercise and how to report found tags (including Dutch and French versions) were displayed at landing points and in merchant and retail outlets. A variety of tag reporting methods advised, including by telephone, post or on-line through a link to the Cefas website, where regularly updated information on the project was available. Articles were published annually in the trade newspaper, “Fishing News”, along with results for annual lotteries (to encourage tag reporting, with a £500 prize) based on returned tag numbers. Articles were also carried in the local press and interviews conducted on local radio. A similar approach was used for the DST programme, except that a single £1,000 prize was drawn at the project’s end.
All crabs were obtained and tagged during commercial fishing operations at sea. Crabs which had lost more than 2 periopods from any side, or more than 3 in total, or which had suffered physical trauma during the catching process were not tagged. No size limits were imposed on crabs used for tagging.
AIII.2.2 Release sites and recaptures An overview of the releases and recaptures by release group is provided, followed by results summarising magnitude and direction by release area. In both cases sexes are treated separately because of the marked behavioural differences that occur due to the reproductive cycle.
Bennett & Brown (1983) worked with Decca coordinates or descriptive positions using line of sight from various landmarks or static features and they considered 10nm (18km) as the limit of spatial resolution that could be trusted. This is to assume that crabs moving less than 18km were equally likely not to have moved at all and that the observed movement was an artefact of reporting positional information with poor precision. In this report some tables use the same distance groupings as Bennett & Brown (1983) to permit direct comparison and this is specified in the title. However, most of the positional information from this study uses highly accurate positional information available from differential GPS systems. With a few exceptions due to mechanical failure, the release data were automatically and precisely recorded. Although there are few fishermen who do not employ accurate GPS positioning nowadays, the precision of return data is more variable, dependent on the circumstances of the recapture and the avidity of the fishermen concerned. Some positional data were missing due to human error or mechanical failures and in a few other cases scientists approximated a position where they felt confident to do so. Therefore, whenever possible data are presented and analysed with full precision.
AIII.2.3 Magnitude and directions of movements
Movement parameters generally calculated follow the methods used by Bennett & Brown (1983) and detailed by Jones (1976). However, mean individual speed is also presented in tables AIII.7 & AIII.8. Mean individual speed is the average of individual speeds and reflects the rate at which crabs are moving (i.e. the activity of individuals). Parameters following Jones (1976) include:
V: the mean velocity, which is the overall displacement of the population (centre), divided by the total time at liberty of all the individual fish in that population. Thus where fish move in opposite directions, these movements will tend to cancel one another out when considering the population as a whole. V is calculated trigonometrically by taking the square root of the squared sums of X and Y coordinates of each individual movement and dividing by the total time at liberty of all the individual fish in that population.
where r is the distance (displacement) moved by an individual, θ is the angle representing the direction of movement and t is the time at liberty for an individual.
a2: the mean square dispersion coefficient (Skellam 1951), which describes the extent to which individual fish move independently.
SID 5 (Rev. 07/10) Page 52 of 130
where V is the mean population velocity and n the number of individuals observed.
Typically, a high velocity and low dispersion characterise strongly directional movement with a low rate of dispersion about the group mean displacement.
Graphical presentations of a GIS summary plot and mean individual speeds by compass bearing are presented to highlight directionality of movements.
AIII.2.4 Descriptions of recoveries by tag group, standardised scatter and population movement vector plots and seasonalityAlthough the summary statistics can be informative in highlighting systematic population movements, the variable nature of these data means they can also be difficult to interpret and at times potentially misleading. Vectors of population displacement along with individual displacements, standardised for local variations in release position were therefore calculated and presented graphically by release group and recapture month. These charts can become cluttered when multiple months are presented together but are very voluminous when plotted separately by recapture month, so only a selection are presented herein, with the remainder retained in a separate annex (ANNEX I). However these form the basis of a textual description of the returns from each release group (see Descriptions of returns from each tag group).
Graphical presentations of mean individual speed and displacement through time identify potential seasonalities in the recovery data.
AIII.2.5 The effect of size, gender, shell condition, limb loss and release batch on crab movements
During tagging operations characteristics which could potentially influence crab mobility were recorded for each released crab. These were the size (carapace width), sex of the crab, the shell condition and the occurrence of limb loss. Shell condition varies depending on the time elapsed since moulting, which also dictates crab growth and the regeneration of lost limbs. Female crabs especially, often show prolonged periods of inactivity over the winter period, associated with the breeding cycle. To minimise any effect these might have on an analysis of the crabs’ movements, only recaptures within 60 days of release were analysed.
A general linear model (GLM) was used to test the influence of different factors on the mean speed and distance moved for each recaptured crab. Dependent variables were log transformed as the raw data were not normally distributed. Model selection was by sequential removal of insignificant factors based on the F test from the partial sums of squares. Pairwise Bonferroni comparisons were used to provide more information on differences in mean speed and distance moved for each release batch and characteristic, if these were shown to be influential. Release batch was included as a factor in the main model and the analysis was also repeated separately for each release batch. Analyses were carried out using the GLM procedure in SAS version 9.1.
The size range of released crabs was 125-236mm, but gear and fisher selection provided a distribution with relatively few animals below 140mm or above 200mm. Size distributions by sex for each batch of released crabs were broadly similar between release locations, but the sex ratios of marked animals varied widely between sites (ANNEX 2 - LDs). The proportion tagged by sex varied according to their availability in the catch, which in turn was determined by season, location and market forces. Analyses were carried out separately by sex, because of marked differences in behaviour.
Shell condition used a subjective assignment of individual crabs to one of four descriptions of shell condition at the time of tagging: soft, new, hard or old. Only 8 recaptured individuals had been described as “soft” at release and for this analysis these crabs were combined with the group of “new shelled” crabs. Only 73 crabs were described as “old shelled”, but this was deemed adequate for analysis.
Crabs damaged in the catching process were not tagged, but those with up to 3 missing walking legs, or one missing claw, were. Despite careful handling a small number of crabs autotomised (shed) walking legs during tagging; these individuals were still released. The absence of walking legs was recorded and crabs were assigned to one of two levels of limb loss (one or no legs missing, two or more legs missing). The absence of a claw was also recorded (no claw loss, one claw missing). Two aspects of mobility were considered, the mean speed of the crabs and the total distance moved (within the 60 day limit). The latter in particular, is strongly influenced by the level of fishing effort in and around the release location. Mean speed was computed for each recaptured crab as the straight line distance between the release and recapture positions divided by the time at liberty in weeks.
SID 5 (Rev. 07/10) Page 53 of 130
AIII.3 Results and discussion
AIII.3.1 Release sites and recaptures
Fifteen thousand four hundred and thirty eight crabs were measured, sexed and marked using uniquely numbered double T-bar tags and released at eleven sites in twelve main batches over a three year period. Up to the end of 2010 a total of 2475 had been recaptured and reported (not all with complete recapture details). Of these 159 were re-released, either by Cefas staff on tagging trips or by fishermen, and 33 of these were recaptured a second time. Three crabs were released a third time and one of these was recaptured.
Recapture rate was dependent on release site, but overall was 14% for females and 16% for males (crabs without recapture details excluded), similar to Bennett & Brown’s (1983) results where recapture rates varied from 1% to 43% and were 17% overall. Recapture rates by sex and by primary release site (re-releases included with original release site) ranged between just 6% and 7% for females at Hurd Deep (Mid-Channel) and South Devon Offshore (Channel Block 2) respectively, and 32% for males released on the Bullock Bank (Table AIII.1). In general release sites to the east of the major fisheries, for example, those at the eastern end of the English Channel, provided higher recapture rates, whilst those on the boundary of the English fisheries, specifically the offshore south Devon and mid-Channel sites yielded lower recapture rates. Although these offshore sites do not represent the boundary of the international fishery, recaptures and subsequent return details were dependent on cooperation from French fishers and despite publicising the work abroad in French, Irish and Belgian ports, fisheries institutes and fishermens’ organisations, inconvenience, mistrust and language difficulties were always likely to engender a relatively low return rate, especially from the French industry.
Table AIII.1. Numbers of releases and recaptures of crabs by release site Female tagged Male tagged
LocationDate
Released Returned % ReleasedReturne
d %
Bullock Bank 8-10 Oct 2007 828 171 21
245 78 32
Shingle Bank 9-11 Oct 2007 503 143 28
56 11 20
Sovereign Shoal 11 Oct 2007 574 56 10
230 44 19
Portland Bill 14-15 Nov 2007 1343 203 15
380 64 17
West of Lands End 4-5 Jun 2008 1292 217 17
86 8 9
Sth Devon inshore 10-11 Jun 2008 1494 225 15
100 13 13
Trevose grounds 14-15 Oct 2008 1680 169 10
115 8 7
Lands End - Longships 5-6 Nov 2008 1470 302 21
29 4 14
Lizard 20-21 May 2009 813 97 12
460 78 17
Sth Devon offshore 24 Jun 2009 316 21 7 1 0 0
Sth Devon inshore 23-25 Jun 2009 1472 211 14
5 1 20
Mid-Channel 10-16 Aug 2009 1376 89 6 570 38 7
Total 13161 1904 14
2277 353 16
The highest proportion of returned crabs was taken shortly after release and by the fishers used during the tagging programme as they continued to fish the same area. Between 87 and 100% of returned crabs were taken within the 1st year after release, up to 8% were taken in the 2nd year (Bullock Bank females) and only up to 2% were taken in
SID 5 (Rev. 07/10) Page 54 of 130
the 3rd year (Shingle Bank females) (Table AIII.2). Overall the number of crabs returned within the first month (including those re-released by scientists during the tagging field trips) was 1314 or 54% of the total recaptures.
Table AIII.2. Percentage of recaptures in 1st, 2nd, 3rd and later years from release by release site and total numbers
Years at libertyRelease location females
Total
males
Total1st
year2nd year
3rd year
>3 year
s1st
year2nd year
Bullock Bank 90 8 1 1 95 5
Sth Devon Offshore 95 5 0 0
Mid-Channel 93 7 0 0 97 3
Lands End - Longships 100 0 0 0 100 0
Lizard 99 1 0 0 100 0
Portland Bill 98 2 0 0 98 2
Sth Devon Inshore 95 5 0 0 100 0
Shingle Bank 96 2 2 0 100 0
Sovereign Shoal 87 7 4 2 95 5
Trevose grounds 100 0 0 0 100 0
West of Lands End 98 2 0 0 100 0
Total 1826 57 7 2 1892 344 8 352
AIII.3.2 Magnitude and directions of movements
Generally and in keeping with Bennett & Brown’s (1983) results, proportionally more females (13%) moved over 18km from the original release sites than males (5%). Nineteen female crabs travelled in excess of 181km, but no males travelled this distance (Tables AIII.3 - AIII.6).
Most recaptures occurred within a short time after release and in relatively close proximity to the release site. For example, of the crabs released at the Lands End – Longships release site 98% of the recaptured females and all of the males were taken within 18km (Tables AIII.5 & AIII.6). In contrast, over 25% and 18% of female crabs travelled between 18 and 36km and between 36 and 90km respectively from the Bullock Bank release site. Only 19 (5%) of the 354 male crabs that were recaptured had moved in excess of 18km, although low release numbers and modest return rates for males mean that results for males are less certain.
Table AIII.3. Numbers of female crabs moving certain distance groupings by release site
SID 5 (Rev. 07/10) Page 55 of 130
Release location Distance moved
Total<18 18-35 36-90 91-181 >181
Bullock Bank 93 43 30 2 3 171
Sth Devon Offshore 18 2 4 0 0 24
Mid-Channel 28 13 25 16 5 87
Lands End - Longships 295 7 0 0 0 302
Lizard 94 1 1 1 0 97
Portland Bill 162 1 23 13 0 199
Sth Devon Inshore 488 12 14 1 0 515
Shingle Bank 128 6 0 1 6 141
Sovereign Shoal 47 0 2 2 5 56
Trevose grounds 169 9 0 0 0 178
West of Lands End 192 5 0 0 0 197
Total 1714 99 99 36 19 1967
Table AIII.4. Numbers of male crabs moving certain distances groupings by release siteRelease location Distance moved
Total<18 18-35 36-90 91-181
Bullock Bank 71 3 4 0 78
Mid-Channel 35 2 0 1 38
Lands End - Longships 4 0 0 0 4
Lizard 78 0 0 0 78
Portland Bill 60 0 4 0 64
Sth Devon Inshore 18 0 1 0 19
Shingle Bank 11 0 0 0 11
Sovereign Shoal 42 1 1 0 44
Trevose grounds 8 2 0 0 10
West of Lands End 8 0 0 0 8
Total 335 8 10 1 354
SID 5 (Rev. 07/10) Page 56 of 130
Table AIII.5. Percentage of female crabs moving certain distance groupings by release siteRelease location Distance moved
<18 18-35 36-90 91-181 >181
Bullock Bank 54 25 18 1 2
Sth Devon Offshore 75 8 17 0 0
Mid-Channel 32 15 29 18 6
Lands End - Longships 98 2 0 0 0
Lizard 97 1 1 1 0
Portland Bill 81 1 12 7 0
Sth Devon Inshore 95 2 3 0 0
Shingle Bank 91 4 0 1 4
Sovereign Shoal 84 0 4 4 9
Trevose grounds 95 5 0 0 0
West of Lands End 97 3 0 0 0
Total 87 5 5 2 1
Table AIII.6. Percentage of male crabs moving certain distance groupings by release siteRelease location Distance moved
<18 18-35 36-90 91-181
Bullock Bank 91 4 5 0
Mid-Channel 92 5 0 3
Lands End - Longships 100 0 0 0
Lizard 100 0 0 0
Portland Bill 94 0 6 0
Sth Devon Inshore 95 0 5 0
Shingle Bank 100 0 0 0
Sovereign Shoal 95 2 2 0
Trevose grounds 80 20 0 0
West of Lands End 100 0 0 0
Total 95 2 2 1
Directions of movements varied depending on the release site but were predominantly in a west or southwest direction when released in the Channel, but were more variable and possibly more southerly for the release sites at the extreme west of the Channel and off the North Cornish coast (Figures AIII.1 & AIII.2). Although these directions are in general opposed to prevailing residual current flows it is important also to bear in mind the locations of the major crab fisheries, which for UK fisheries are similar to the release positions (black dots) in figure AIII.1., whilst significant French fisheries occur in mid Channel to the south of Cornwall, offshore of the North Cornwall coast and to the west of Brittany. Tags were returned by French fishermen from all of these locations, but return rates from the French fleet were generally low, possibly reflecting greater difficulty or reluctance to report tags.
Recovery of tagged animals is dependent on there being fishing activity where the animals have relocated. Crab fisheries are concentrated on grounds where densities of crabs are sufficient to
57
provide commercially viable catch rates, although some tagged crabs could be (and were) taken as by-catch by other trap fisheries, e.g. lobster fisheries, or by other gear types including fixed nets or beam trawls. Furthermore, the probability of recapture is likely proportional to the catchability of the gear and the amount of fishing effort in the vicinity. These points need to be borne in mind (and frequently re-iterated) when considering the results from this study.
Figure AIII.1. Summary of double T-bar and DST tagged crab movements
Movement parameters, summarised by release location and period (Tables AIII.7 and AIII.8), indicate that with just two exceptions the mean bearing for each release batch was to the south, south west or west. When summarised in this way, even the crabs released at the far west of the Channel and off North Cornwall generally appeared to move in a southerly or westerly direction, which may be contrary to the impression gained from figure AIII.1. However, the average distance moved in these cases can be very small (Figure AIII.2). Mean individual speed by release location and period ranged between 0.8 to 23.6 km/week for the females and 0.2 to 2.5 km/week for the males. However, two high values for females should be treated with extreme caution as they are very different from other results and both also stem from re-releases. Excepting re-releases, the maximum mean individual speed for females was 6.7km/week. These results are not directly comparable with Bennett & Brown (1983) who presented (population) velocity rather than mean individual speed, where the mean velocity is the average velocity shown by the population as a whole and is not the average of the individual speeds.
58
Figure AIII.2. Average distance moved by double T-bar tagged crab by sex and release group (averaged release position)
Table AIII.7. Movement parameters for female crabs (* indicates re-releases)
Releaselocation
Releaseperiod
Numbermoved
Distance(km)
Mean individualdistance
(km)
Mean individual
speed(km/week)
Meanbearing
Compasssector
Bullock Bank OCT07 168 4168 25 4.2 271 W
Bullock Bank* NOV07 1 1 1 0.8 180 S
Shingle Bank OCT07 141 2999 21 3.5 263 W
Sovereign Shoals OCT07 54 1754 32 2.6 258 W
Sovereign Shoals* NOV07 1 1 1 0.2 79 E
Sovereign Shoals* DEC07 1 316 316 3.6 260 W
Portland NOV07 197 3476 18 2.8 250 W
Portland* JUN08 1 26 26 20.1 247 SW
Portland* SEP08 1 0 0 0.1 240 SW
Sth Devon Inshore JUN08 245 1753 7 2.6 290 W
Sth Devon Inshore JUN09 269 2208 8 2.0 300 NW
Sth Devon Inshore* AUG09 1 7 7 23.6 103 E
Sth Devon Offshore JUN09 24 282 12 2.3 306 NW
Mid-Channel AUG09 87 5165 59 6.7 260 W
Lizard MAY09 92 418 5 1.6 190 S
Lizard* JUN09 4 5 1 0.8 343 N
59
Releaselocation
Releaseperiod
Numbermoved
Distance(km)
Mean individualdistance
(km)
Mean individual
speed(km/week)
Meanbearing
Compasssector
West of Lands End JUN08 197 648 3 0.8 213 SW
Lands End-Longships
NOV08 302 1653 5 2.5 206 SW
Trevose Grounds OCT08 178 869 5 1.5 218 SW
Table AIII.8. Movement parameters for male crabs
Releaselocation
Releaseperiod
Numbermoved
Distance(km)
Mean individual distance
(km)
Mean individual
speed(km/week)
Meanbearing
Compasssector
Bullock Bank OCT07 78 395 5 1.7 267 W
Lands End - Longships
NOV08 4 10 3 0.8 285 W
Lizard MAY09 76 130 2 1.5 215 SW
Lizard JUN09 2 2 1 1.0 336 NW
Mid-Channel AUG09 38 281 7 1.0 244 SW
Portland NOV07 64 385 6 1.3 245 SW
Shingle Bank OCT07 11 61 6 2.4 279 W
Sth Devon Inshore JUN08 18 118 7 2.5 276 W
Sth Devon Inshore JUN09 1 2 2 0.2 333 NW
Sovereign Shoals OCT07 44 268 6 1.8 68 E
Trevose Grounds OCT08 10 82 8 0.9 212 SW
West of Lands End JUN08 8 10 1 0.2 42 NE
For direct comparison with Bennett & Brown (1983) population movement statistics, including mean velocity and dispersion factors were computed as per Jones (1976) and using only movements in excess of 18km (Table AIII.9a). For completeness, statistics are also presented for all data (Table AIII.9b). Mean velocities ranged from 0.07 km/week for a site at the western end of the Channel to 0.72km/week for the Hurd Deep (Mid-Channel) release site. Dispersion measures the extent to which animals move independently of each other. Typically, a high value of mean velocity and a low dispersion coefficient is indicative of directional movement. Conversely, a low value for mean velocity and a high dispersion coefficient would suggest random movements with little directionality. In general, the dispersion parameter is lower for the females than the corresponding one for males (except mid-Channel), although any relationship between sex and mean velocity is less clear.
Scatter plots of standardised crab movements (next section) show that there is a very wide variation in individual distance moved, particularly for females, and this is one reason why these population movement statistics do not emphasise the systematic movements as much as might be expected. Thus female movements are primarily towards the west but vary from a few km to hundreds of km, so although the population movement is directional, the dispersion can be very high. Males tend to exhibit less directional movement, but because they do not move as far the population dispersion may not necessarily be as marked. This is particularly the case when the short range movements are included in the analysis (Table AIII.9b).
Table AIII.9a. Movement statistics for crabs as per Jones (1976). For comparison with Bennett & Brown (1983) crabs that moved <=18km have been excluded
60
Female Male
Releaselocation
Numbermoved
Meanvelocity
(km/week)Dispersion(kmsq/2)
Numbermoved
Meanvelocity
(km/week)Dispersion(kmsq/2)
Bullock Bank 77 0.37 33.08 7 0.35 88.62
Lands End 7 0.07 21.69
Lizard 3 0.24 50.91
Mid-Channel 59 0.72 57.50 3 0.31 32.27
Portland 37 0.33 12.50 4 0.22 38.84
Shingle Bank 13 0.37 66.08
Sth Devon Inshore 2008
12 0.19 13.38 1 0.61 0.00
Sth Devon Inshore 2009
15 0.22 24.64
Sth Devon Offshore 3 0.45 16.79
Sovereign Shoals 9 0.31 32.13 2 0.02 56.72
West of Lands End 5 0.08 6.11
Table AIII.9b. Movement statistics for crabs as per Jones (1976), including short range movements (<=18km)
Female Male
Releaselocation
Numbermoved
Meanvelocity
(km/week)Dispersion(kmsq/2)
Numbermoved
Meanvelocity
(km/week)Dispersion(kmsq/2)
Bullock Bank 171 0.27 24.75 78 0.08 9.04
Sth Devon Offshore 21 0.07 5.25
Mid-Channel 87 0.59 45.78 38 0.03 4.10
Lands End - Longships
303 0.03 2.69 4 0.01 0.28
Lizard 98 0.06 2.35 78 0.00 0.57
Portland Bill 199 0.24 4.29 64 0.05 3.22
Sth Devon inshore 453 0.06 3.15 19 0.15 3.24
Shingle Bank 141 0.21 11.67 11 0.23 3.57
Sovereign Shoal 55 0.23 9.49 44 0.03 3.57
Trevose grounds 169 0.02 1.15 8 0.05 0.56
West of Lands End 197 0.01 0.55 8 0.01 0.09
The variance of female crab movements by compass bearing from the Portland area (Figure AIII.3) was heteroscedastic (i.e. it changed with the magnitude of compass bearing, generally increasing up to around 320o (NW) then declining again. There was also a small group of high points around 20-60o (NE). The majority of crab movements and those with the highest speeds fell between around 240o (SW) and 325o (NW), indicating that most movements took place between the south-west to north-west directions, but also that these movements included both rapid and slower movements, whereas the other compass sectors had primarily slower movements. A similar perspective for all valid tag recoveries is provided by a box and whisker plot (Figure AIII.4) which shows that the highest average
61
of individual speeds occurred in a westerly direction, followed by northwest and southwest. Speeds in the directions of south, northeast and southeast were lower and also had lower variance.
Figure AIII.3. Mean speed by bearing of female crabs released off Portland Bill
Figure AIII.4. Distribution of female crab speed by compass bearing
AIII.3.3 Descriptions of returns from each tag group based on standardised scatter and population movement vector plots
This section describes the returns on the basis of standardised spatial scatter and population movement vector plots for each tag group, by recapture months and sex. The positions of recaptures were plotted in successive months after release, relative to a common release locus and the mean movement vector for all recaptures in that month calculated, indicating the mean direction and magnitude of movement. Combining data for all recapture months can make the plots cluttered, whilst presenting each month separately generates a large volume of output. The latter are available as a separate annex (ANNEX I), with a few selected examples of the former presented here.
62
Eastern Channel background
Tagging took place at three locations; Bullock Bank in mid-Channel, Hastings Shingle Bank approx. 35km to the west and the Sovereign Shoals area approx. 15km further west. Returns were heavily influenced by a single vivier vessel which, as well as being the major catcher in this area, was also used as the tagging vessel for both T-bar and DST releases. This vessel generally fishes on Hastings Shingle Bank and the Sovereign Shoals between April and December, setting gear on the Bullock Bank area furthest offshore in mid-Channel during the peak autumn fishery (October – December). In most years the vessel’s effort on crabs is greatly reduced in the first quarter of the year.
Bullock Bank October 2007 (Eastern English Channel)
FemaleAlthough initially the majority of returns were from the grounds on which the crabs had been released, by the end of October there had already been significant movements (30-50km) to the west and northwest. One month later, recaptures on the tagging grounds were exceeded by those on fishing grounds to the west of the release area. Recaptures were clustered in two groups showing the concentration of potting effort on the Hastings Shingle Bank and Sovereign Shoals areas. There were few recaptures in December, divided equally between the release area and the fishing grounds to the west and then no further recaptures on the Bullock Bank until 2 were taken in June 2009 and a single crab reported in August 2009, caught by a French vessel fishing approximately 10km to the southeast of the Bullock Bank. Prior to this time the monthly movement vectors had all been orientated approximately west-northwest and were <50km. Since August 2009 there have been 5 returns, all were >160km (furthest was 355km) and approximately southwest orientated. MaleRecaptures were generally within the release area. However, some longer movements were observed. Two crabs had travelled 50km south of west by the end of October 2007 and a third had gone the same distance to the northwest, all being recaptured in the Sovereign Shoals area. In November one male was recaptured on Shingle Bank and a second south of the Sovereign Shoals. There were 9 recaptures in December, all in the tagging area and then no further recaptures until autumn 2008, when single males were recaptured on Shingle Bank in October and November with just 2 males caught in the release area. There have been no further recaptures.
Shingle Bank October 2007 (Eastern English Channel)
FemaleBy the end of October, as well as recaptures in the release area there had been a noticeable dispersal to the west, with many recaptures on the Sovereign Shoals. Six crabs however had travelled 30 – 35km to the southeast, towards Bullock Bank. A similar pattern of westwards dispersal was still apparent by the end of November and there were no more recaptures until April 2008 when two crabs were taken on the Sovereign Shoals. A few recaptures in the release area were reported from July to August and then none until a single and the last female recapture on the Shingle Bank in June 2009. Long distance movements were first noted in September 2008 when 2 females were recaptured in the western Channel, over 300km distant. A further 4 long-distance recoveries were reported in 2009 and another in July 2010. All vectors were southwest into the western Channel.MaleOnly 56 males were tagged in this area and of those, 11 were recaptured, all within the tagging area and within two months of release.
Sovereign Shoal October 2007 (Eastern English Channel)
FemaleFive hundred and seventy four females were released in this area but only 10% were recaptured, a return rate less than half that seen in the two tagging areas to the east. The scattergrams show that in the month after release all recoveries are within, or to the west of, the release area with the exception of a single crab 45km to the east. This crab was caught soon after tagging and was re-released by the skipper, but there is some doubt about the accuracy of the second release position. By the end of November there is a greater scatter of recoveries, approximately 10km either side of the release locus and thereafter only 4 recaptures in the tagging area between January and December 2008. Given the
63
location of Sovereign Shoals, to the west of the other two release areas in the eastern Channel and the generally westwards movement observed in those areas, it is likely that a large part of the tagged population left the area in that direction accounting for the low return rate. There is little potting effort for crabs to the west along the west Sussex coastline for approx 100km. There were 6 long-distance movements (100 – 300km) reported between December 2008 and December 2010, all from fisheries in the western Channel. These long-distance movements were also shown by recoveries of DST’s released in the Sovereign Shoals area in August 2009. Of 11 DST recaptures, 4 were >50km west of the release position, 2 were recaptured off the Dorset coast and one in mid-Channel south of Start Point. MaleThere is no obvious discernable pattern to the distribution of male recaptures in the two months after release, with an apparently random scatter of recoveries in the tagging area. There was some doubt regarding the accuracy of the returned information for the male that apparently moved 45km to the southeast in the month of release. An indication of a north-eastwards movement in December 2007 is more likely to reflect the distribution of fishing effort and the small number (6) of returns than genuine directional movement.
Western English Channel - Devon and Dorset background
The coastal waters of south Devon and Dorset, extending offshore towards the mid-Channel potting grounds, are some of the most intensively fished potting grounds around the English coast. As such there is very good coverage in all directions around the release areas and the probability of recapture is high. Tagging in the early summer was carried out to investigate possible post-spawning movements of crabs and the claim of many local fishermen in the western Channel that crabs move eastwards at this time of year. Tagging was also carried out through the summer and autumn in this very important crab fishery and an additional study on the crab parasite Hematodinium was carried out in this area.
The fisheries in these areas cover both inshore and offshore areas and are essentially year-round, but with some reduction of effort over the winter months in the early part of the year, especially in the inshore fishery. The definition of inshore is imprecise, but is the area where the large majority of potting vessels <10m overall length operate and potentially broadly equivalent to the 6nm limit of IFCA jurisdiction. Seasonality is dictated by the practicality of operating small vessels in poor weather as well as by the abundance and catchability of crabs. Larger vessels operating offshore include the largest vivier vessels in the English fleet. These are less subject to weather effects, but may also be nomadic in their operation, fishing distant grounds according to the availability of the crabs. The peak of activity and landings occur from late summer to late autumn.
Tagging in the Devon and Dorset area took place at Portland, on the south Devon inshore grounds (twice), on the south Devon offshore grounds (small number of releases) and in mid-Channel.
Portland November 2007 (Western English Channel – Devon & Dorset inshore)
FemaleAlthough the majority of recaptures in the month of release were taken in the tagging area, there was an obvious bias towards recapture distributions between west and northwest of the release position. However, this may be a reflection of the distribution of fishing effort. By the end of the following month there were far fewer recaptures, but the majority appeared to follow the same pattern, with a few returns having travelled short distances to the northeast. A single female was recaptured on the west side of Lyme Bay, nearly 80km to the southwest. Through the winter months recaptures and movements were limited, but after June 2008 multiple recoveries were made on the south Devon potting grounds (Figure AIII.5).
64
Figure AIII.5. Movements of individual female crabs released off Portland by capture period and corresponding mean vector
MaleRecaptures were mainly within 5km of the release site with only 3 distant recoveries; two in the following spring/early summer, approx. 75km distant on the south Devon grounds, whilst the third was recaptured in March 2009 on inshore grounds 50km to the northeast.
South Devon inshore June 2008 & June 2009 (Western English Channel – Devon & Dorset inshore)
FemaleFollowing release in June 2008 the crabs had dispersed in all directions by the month’s end, some recaptures between 15 and 20km distant. Other than crabs recaptured in the release area, the dominant movement was to the northwest. This is interpreted as a movement inshore. The same pattern was observed throughout August, but then broke down as recaptures became fewer and distant recoveries were reported, up to 85km away near the Lizard peninsula. These had mostly been taken in areas to the west. A similar pattern of recoveries was observed after the 2009 releases MaleLow numbers were tagged compared with the females, but this was a reflection of their availability at the time. Of 20 recaptures from the 2008 releases, approx half were taken in the release area and with the exception of one male, which had travelled 42km to the WSW, the rest were taken within 15km of their release and had travelled generally inshore in a NW direction. There was only one recapture from the 2009 male releases which were even fewer than in 2008.
South Devon offshore June 2009 (Western English Channel – Devon & Dorset offshore)The primary purpose of this exercise was to release female crabs fitted with DST’s on offshore potting grounds approx 25km south of Start Point. Therefore only limited numbers of crabs tagged with double T-bars were released; 316 females and only 1 male.FemaleRecapture rates were low (7%) and during the first 2 months after release the majority were in the release area except one crab caught 42km to the WNW and another about 14km in a SSE direction from the release site. In September (2009), one female was recaptured 50km WSW and another had travelled 20km NNE. Two further recaptures the following year in October were both close to the release site (<6km).The single male tagged has not been reported as recaptured.
65
Mid-Channel (Hurd Deep) August 2009 (Western English Channel – Devon & Dorset offshore)
This extensive release area was located approx 100-150km east of the Devon offshore grounds and equidistant between Portland Bill and Cap de la Hague (see Figure. AIII.1). Nearly 2,000 crabs were tagged and released over a nine day period and half of these had haemolymph samples removed as part of a study of the incidence of hematodinium in edible crabs. The return rates were amongst the lowest observed during the study but the similar incidence of hematodinium in tagged and untagged crabs indicated that the low return rate was not connected with the extraction of haemolymph.FemaleRecaptures within the first four months showed that some crabs stayed within the vicinity of the release site (within 5km), but others moved about 100km away to the WSW or WNW (Figure AIII.6). Some of these larger movements took place within two months of release, demonstrating fast dispersion. In December (2009) one animal was caught 17km to the east of the release site. A few crabs were caught at the release location as late as January 2010 (<6months) but generally those few animals recaptured in 2010 were taken further away and up to 210km to the west, southwest or northwest. Although recapture rates were poor, crabs from this release generally seemed to disperse quickly.MaleThere did not appear to be any pattern to the male dispersal, recaptures coming from all points and generally within 20km of the release position. Two longer and quite rapid movements were seen however, with one animal recaptured 25km northeast of its release position and the other 90km to the southwest little over three months later.
Figure AIII.6. Movements of individual female crabs released on the Hurd Deep by capture period and corresponding mean vector
Western English Channel/Celtic Sea – Cornwall background
Releases around the Cornish coast took place to the west of the Lizard peninsula, between Land’s End and the Scilly Isles and on the Trevose Grounds off the North Cornwall coast. The proximity of the land will limit the potential movement in a shoreward direction for all crabs, but those released close to the shore will be more restricted. Releases at the Western end of the English Channel were examples of this and were also likely to be limited by the lack of fisheries surrounding the release sites, especially if any prevailing migrations were generally to the west.
Between Rame Head in the east and Lands End in the west the crab fishery is essentially inshore in nature, fished by <10m boats, subject to the vagaries of the weather as described earlier, with a hiatus in activity over the winter months which also coincides with the period of lowest crab
66
catchability. Larger vessels, such as that used for tagging off the Lizard, are mostly based at Newlyn and tend to work further off and west, especially the difficult waters between Lands End and the Isles of Scilly. Although a reduction in effort occurs over the winter period, the seasonality of the fishery is not as marked as inshore as the larger vessels are more able to fish in bad conditions and the crabs remain relatively catchable, having retreated to deeper offshore waters as the sea temperature dropped.
The Trevose Grounds are offshore potting grounds that have been fished since their discovery and prosecution by Breton fishermen over a century ago. The pattern of the fishery is dictated by the weather with less effort during the winter months and diversion to other targets (mainly whelks) when catch rates of crabs are low in the first half of the year. Autumn/early winter is the peak season. Inshore crab fisheries are widespread along the coast of north Cornwall, but effort is low compared with Channel coasts and the fishery is prosecuted by small cove boats, often single-handed.
Lizard May 2009 (Western English Channel – West Cornwall inshore)
This early season release was targetted to see if any easterly movements could be detected as had been suggested by some members of the industry.FemaleWithin the first four months from release (up to August 2009) recaptures were restricted within a 6km radius around the release site. In September 2009 one crab was recaptured around 14km to the NW in Mounts Bay and in October 2009 another was recaptured 160km to the south off the Brittany coast. In May 2010 a crab was taken 37km to the west, off Lands End. MaleAll male recaptures were within the release area (<7km) and showed no obvious directional movement.
West of Lands End June 2008 (Western English Channel – West Cornwall inshore)
Tagging took place in the Seven Stones Reef area, equidistant between Lands End and the Isles of Scilly, with smaller numbers released in the Wolf Rock area to the southeast of the Seven Stones. Although this has been nominally called inshore, this area of the coast is exposed to heavy seas and strong tides and the crab fishery is prosecuted mainly by large offshore vessels, working from Newlyn. FemaleRecaptures within the first three months of release (to August 2008) were typically dispersed within an area of about 5km from release site, although a few crabs had moved up to about 22km away. Of these larger movements there was no obvious prevailing direction (one was to the North, another to the East and four more to the WSW or SW). Over subsequent months recaptures were predominantly close to the original release site, indeed the last recapture in July 2009 was of a crab that was only 1.5km to the north of the release site.MaleFew males were tagged in this area and return rate was relatively low (9%). All returns were in the year of release and all were within an approximately 5km radius of the site, with no obvious direction of movement. Lands End – Longships November 2008 (Western English Channel – West Cornwall inshore)
The inshore designation is nominal (see text for previous release).FemaleThere was widespread dispersal throughout the release area in the month of tagging with 3 animals travelling over 20km to the northwest and one the same distance to the southwest. There were more recoveries in December from the southwestern quarter than any other, but all within 10km of their release position. There were no further recaptures until May 2009 when 4 were recaptured short distances east of their release position and 3 a similar distance to the west. There were more recoveries in June but all within the release area and the final recapture reported was in August 2009, 14km to the northwest.MaleVery few male crabs were tagged in this area and the 4 recaptures (14%) were all close to the release area.
67
Trevose grounds October 2008 (Celtic Sea – North Cornwall offshore)
FemaleRecaptures occurred over an eleven month period. In the first few months after release, recaptures occurred typically in a 5km radius of the release site, but with a few individuals moving further and up to 12km. The biggest movements were in the WSW direction and the majority of the movements had a southerly element to their direction. These may reflect the distribution of fishing gear on the grounds. Several returns were received in December from French vessels fishing between 20 and 25km to the southwest, although these were not included in this analysis due to lack of precise capture date. Recaptures during the summer of 2009 had not dispersed far (within 12km) and the southerly element to the direction observed in the previous season was obvious only in July. The last recapture in September 2009 was of a crab that had moved less than 1.5km.MaleThere were only eight recaptures from this release batch of 115 and no movements in excess of 5km during the first three months after release. One male crab caught 6 months after release (April 2009) had moved 15km to the WSW.
Mean individual speed and mean individual displacement for females by recapture period in the Sovereign Shoals area (Figure AIII.7) demonstrate that the first winter period was a period of low activity/movement reflecting the seasonality of the reproductive cycle. During this period female crabs would be expected to be ovigerous and movement would be very limited. Ovigerous female crabs do not actively forage and will therefore not be taken by baited trap, so return rates are also lower at this time. The points to the right of this graphic (later in time) incorporate displacements and speeds before and as such the picture becomes less clear as it incorporates successive stationary and mobile periods.
Figure AIII.7. Mean individual speed and mean individual displacement of female crabs released on Sovereign Shoal by month of recapture
Individual speeds by time at liberty and month of recapture for female crabs released off Portland (Figure AIII.8) show a similar picture. Individuals with higher average speeds in the first months after release were captured before the onset of the first spawning period, while those captures later have already had some periods of immobility, so average speed is reduced. This was not the case for the Sovereign Shoals example, possibly because the longer duration points in this plot had moved considerably further to the west than earlier recaptures, so maintaining high displacement and speeds despite previous periods of inactivity.
68
Figure AIII.8. Individual speed against days at liberty for female crabs released off Portland Bill in November 2007Box and whisker plots summarising distributions of individual crab speeds by sex and for each recapture period for all release sites combined (Figures AIII.9 & AIII.10) show that during the winter spawning periods both the return rate and mean speed for females were reduced, but this is not the case for males. They also show that the highest mean speeds for females occurred for recaptures taken in the autumn fishery, both in terms of the distributions means and upper margins, but again this pattern is not shown by males. Fahy et al. (2004) noted similar rates of movement for both sexes in south east Ireland, relatively high in spring, lowest in June then increasing to a maximum in September/October.
Figure AIII.9. Distribution of female crab mean individual speed by recapture period October 2007 to April 2009 (note this time series was truncated, later periods show a similar trend, but with decreasing numbers of returns)
69
Figure AIII.10. Distribution of male crab mean individual speed by recapture period October 2007 to April 2009
AIII.3.4 The effect of size, gender, shell condition, limb loss and release batch on crab movements
The mean distance moved by, and mean speed of, females was considerably greater than males (Figure AIII.11)
Figure AIII.11. Mean speed and mean distance of crabs recaptured within 60 days with 95% confidence limits and crab numbers by sex
Mean speed - femaleThe first model included all factors as explanatory variables (Table AIII.10), but sequential removal of the least significant factor as demonstrated by the F statistic based on the partial sum of squares showed that shell condition had no effect on mean crab speed and this factor was excluded from the analysis first. Subsequently the loss of a claw was also shown not to influence mean speed, leaving the size, leg loss and the release batch as significant factors (Table AIII.11). These three factors described 12.7% of the variance. The mean speed of female crabs released mid-Channel (Hurd) was significantly higher than those from the other release sites, and crabs released west of Land’s End were the slowest (Table AIII.12 & Figure AIII.12). Crabs with more than one leg missing were slower
70
than those missing only one or no walking legs (Table AIII.13). A larger size also appeared to offer a speed advantage (Figure. AIII.13).
Table AIII.10. Number of levels and values for each characteristic
Factor Levels Values
Shell condition
3 Hard, New, Old
Size Continuous 125 – 236mm Carapace width
Release batch
12 Bullock Bank, Hurd (Mid-Channel), Lands End – Longships, Lizard, Portland, Shingle Bank, South Devon inshore08, South Devon inshore09, South Devon offshore, Sovereign Shoal, Trevose grounds, West of Lands End
Missing claw
2 No, Yes
Missing legs
2 One or no legs missing, two or more legs missing
Table AIII.11. F statistics by factor for the initial model (mean speed as dependent variable - females) based on Partial sums of squares
Factor DF Type III SS F Value Pr > F
Shell condition 2 2.97 1.11 0.3282
Missing claw 1 1.88 1.41 0.2346
Release batch 11 175.29 11.98 <.0001
Missing legs 1 9.48 7.12 0.0077
Size 1 43.09 32.39 <.0001
Table AIII.12. Bonferroni grouping (same grouping denotes means are not significantly different), log transformed mean speed and the numbers of observations by release location
Bon Grouping Log mean speedkm/week N Release batch
A 1.4493 53 Mid-Channel
B 0.6661 111 Shingle Bank
B 0.5899 260 Lands End - Longships
B 0.5602 183 South Devon inshore08
B 0.5351 142 Bullock Bank
C B 0.4861 148 South Devon inshore09
C B 0.4705 147 Portland
C B 0.4242 111 Trevose grounds
C B 0.2509 39 Sovereign Shoal
C B 0.1928 17 South Devon offshore
C D -0.1563 75 Lizard
71
Bon Grouping Log mean speedkm/week N Release batch
D -0.5004 142 West of Lands End
Figure AIII.12. Mean speed by release batch for female crabs
Table AIII.13. Bonferroni grouping (same grouping denotes means are not significantly different), log transformed mean speed and the numbers of observations by missing leg grouping
Bon GroupingLog
mean speed
km/weekN Missing
legs
A 0.44 1365 One or no legs missing
B 0.05 63 Two or more legs missing
72
s ex=Femal e
111142
260
75
17
183
148
53
147
39
111142
Trevose grounds
West of Lands End
Lands End - Longships
Lizard
South Devon offshore
South Devon inshore08
South Devon inshore09
Hurd
Portland
Sovereign Shoal
Shingle Bank
Bullock Bank
Figure AIII.13. Relationship between female crab size and mean individual speed. Regression equation speed = -0.57971 + 0.005903*size
The strong influence of release batch suggested that repeating this methodology by each release batch would prove informative. Whereas the factors describing a missing claw and the shell condition were shown to be insignificant when the model included release batch as a factor, when the procedure was repeated for each release batch separately the absence of a claw was influential on mean speed for female crabs released in 2008 from South Devon inshore (crippled crabs moved more slowly). The factor describing severity of limb loss and crab size was also important for crabs from this release batch.
Similarly the shell condition was a significant factor at describing mean speed of female crabs released from South Devon offshore and Sovereign Shoals (however, which condition favoured higher mean speeds was different for the two sites).
Repeating the analysis by release batch showed that the two important effects in the main model (leg loss and crab size) were not significant for all release batches. Crab size was significant for the Hurd Deep (Mid-Channel), South Devon inshore (2008), Trevose and Portland releases, whereas leg loss was significant for South Devon inshore (2008) and Portland releases only.
The reason these significant effects were not apparent in the original model is likely because the variance explained by the release batch factor obscured the influence of these other factors. That is to say that the release batch factor may be acting as a proxy for a number of other factors including those describing a missing claw and severity of leg loss, but also some unknown factors not explicitly defined and available to our analysis.
Distance moved - female
The same methodology was applied using the log transformed distance moved as the dependent variable and with all potential characteristics put in the model initially, followed by sequential removal of the least significant factor. The factor describing a missing claw was removed first, followed by those for missing legs, shell condition and crab size (Table AIII.14). Release batch was the only significant factor that described distance moved in 60 days, accounting for 16.9% of the variance.
73
Female crabs released from mid-Channel (Hurd) moved significantly further than those from all other release sites (Table AIII.15 & Figure AIII.14).
Table AIII.14. F statistics by factor for the initial model (distance as dependent variable - female) based on Partial sums of squares
Factor DF Type III SS F Value Pr > F
Size 1 5.17 3.64 0.0567
Shell condition 2 6.91 2.43 0.0883
Release batch 11 383.66 24.54 <.0001
Missing claw 1 1.51 1.06 0.3023
Missing legs 1 2.55 1.80 0.1803
Table AIII.15. Bonferroni grouping (same grouping denotes means are not significantly different), log transformed mean distance and the numbers of observations by release batch
Bon Grouping Log mean distancekm N local
A 2.9379 53 Mid-Channel
B 1.5696 142 Bullock Bank
C B 1.4535 194 South Devon inshore09
C B 1.4014 111 Shingle Bank
C B 1.2327 199 South Devon inshore08
C B 1.2157 20 South Devon offshore
C B 1.2061 260 Lands End - Longships
C B D 1.0442 40 Sovereign Shoal
C E D 0.8574 120 Trevose grounds
E D 0.5215 142 West of Lands End
E D 0.3949 75 Lizard
E 0.3336 147 Portland
74
Figure AIII.14. Mean distance between release and recapture positions by release month (female)
Repeating the analysis by release batch showed that larger female crabs moved further than smaller crabs during the 60 days of liberty at the Lizard and Hurd Deep (Mid-Channel) sites.
Crippled female crabs (missing claw) moved significantly less distance than their dual clawed counterparts for the 2008 South Devon inshore release batch.
Mean speed - maleFactors describing missing claws, shell condition and missing legs were removed from the full model in that order, leaving only crab size and release batch as significant factors (Table AIII.16). These two factors described 13.1% of the variance. Male crabs released from West of Land’s End moved significantly more slowly than those from the Trevose, South Devon inshore (2008) and Sovereign Shoals releases (Table AIII.17 & Figure AIII.15). There is a positive relationship between the size of crab and distance moved (Figure AIII.16).
Table AIII.16. F statistics by factor for the initial model (mean speed as dependent variable - males) based on Partial sums of squares
Factor DF Type III SS F Value Pr > F
Size 1 12.51 8.03 <.0050
Release batch 9 44.26 3.16 <.0013
Shell condition 2 1.26 0.40 0.6681
Missing claw 1 0.02 0.01 0.9061
Missing legs 1 0.66 0.42 0.5155
Table AIII.17. Bonferroni grouping (same grouping denotes means are not significantly different), log transformed mean distance and the numbers of observations by release batch
75
s ex=Femal e
120
142
260
75
20
199 194
53
147
40
111
142
Trevose grounds
West of Lands End
Lands End - Longships
Lizard
South Devon offshore
South Devon inshore08
South Devon inshore09
Hurd
Portland
Sovereign Shoal
Shingle Bank
Bullock Bank
Bon Grouping Log mean speed km/week N Release batch
A 0.5918 3 Trevose grounds
A 0.3838 14 South Devon inshore08
A 0.2834 29 Sovereign Shoal
B A -0.0844 10 Shingle Bank
B A -0.0988 3 Lands End - Longships
B A -0.2624 47 Portland
B A -0.4318 67 Lizard
B A -0.4436 19 Mid-Channel
B A -0.7857 65 Bullock Bank
B -1.8840 3 West of Lands End
Figure AIII.15. Mean speed by release batch for male crabs
76
sex=Mal e
3
3
367
1419
47 29 10
65
Trevose grounds
West of Lands End
Lands End - Longships
Lizard
South Devon offshore
South Devon inshore08
South Devon inshore09
Hurd
Portland
Sovereign Shoal
Shingle Bank
Bullock Bank
Figure AIII.16. Relationship between male crab size and speed. Regression equation speed = -0.104807 + 0.002135*size
Repeating this methodology by release batch shows that shell condition influences mean speed for male crabs released in 2008 South Devon (new shelled crabs moving faster than hard shelled crabs). The influence of crab size as shown in the main model is only significant for the Portland release batch. For all other release batches none of the characteristics appear to influence the speed of male crabs within 60 days of liberty. Low numbers of observations is likely to have reduced the power of this analysis.
Distance moved - male
The factor describing leg loss was removed first followed by those describing shell condition, crab size and missing claws. Release batch was the only significant factor describing the distance moved by male crabs in 60 days (Table AIII.18). This model described 18.4% of the variance in the log transformed distance moved. Crabs released on the Trevose ground moved the furthest whilst those released from the West of Land’s End, Lizard and Portland sites moved the shortest distances before recapture (Table AIII.19 & Figure AIII.17).
Table AIII.18. F statistics by factor for the initial model (distance as dependent variable - male) based on Partial sums of squares
Factor DF Type III SS F Value Pr > F
Size 1 0.18 0.15 0.6948
Shell condition 2 0.75 0.31 0.7306
Release batch 9 63.96 5.97 <.0001
Missing claw 1 0.24 0.20 0.6555
Missing legs 1 0.11 0.10 0.7568
Table AIII.19. Bonferroni grouping (same grouping denotes means are not significantly different), log transformed mean distance and the numbers of observations by release month
77
sex=Mal e
Bon Grouping Log meandistance km N Release batch
A 2.0151 5 Trevose grounds
B A 1.3263 14 South Devon inshore08
B A 1.1544 29 Sovereign Shoal
B A 0.8428 19 Mid-Channel
B A 0.7920 3 Lands End - Longships
B A 0.7743 10 Shingle Bank
B A 0.4348 65 Bullock Bank
B 0.0342 47 Portland
B -0.0222 67 Lizard
B -0.3856 3 West of Lands End
Figure AIII.17. Mean distance between release and recapture positions by release group (male)
Repeating this analysis by each release batch suggested that none of the factors was important for determining how far the male crabs moved in the 60 day period.
Conclusions
There were sometimes differences between the ranks of factors when log-, or un-, transformed, which indicate that the distributions of the data varied between factors.
Both the distance moved and speed of male and female crabs in the first 60 days of liberty was strongly influenced by the release batch.
78
sex=Mal e
5
3 3
67
14
19
47
29
10
65
Trevose grounds
West of Lands End
Lands End - Longships
Lizard
South Devon offshore
South Devon inshore08
South Devon inshore09
Hurd
Portland
Sovereign Shoal
Shingle Bank
Bullock Bank
Within the 60 day period larger crabs of both sexes moved faster and further than smaller crabs.
The severity of limb loss also influenced the mean speed of female crabs with those with one or no legs missing moving faster than those with more severe leg loss.
The mean speed of movement and the mean distance moved was higher for female crabs.
When release batch in included in the model this factor may obscure the potential affects of claw loss and shell condition (i.e. release batch may act as a proxy variable for other combined factors for example season and shell condition).
When this methodology is repeated separately for each release batch the factor describing shell condition was significant in describing mean speed of female crabs for the South Devon offshore and Sovereign Shoal releases and for the male South Devon Offshore releases. However, whereas new shelled male crabs and female crabs from the Sovereign Shoal releases moved faster than the hard shelled crabs, it was new shelled female crabs from the South Devon offshore area which moved slowest.
The absence of a claw reduced the mean speed and magnitude of the movements of female crabs released from the South Devon inshore site in 2008.
The severity of limb loss was a significant factor for describing speed of movements for female crabs released off Portland and South Devon inshore in 2008 only.
Larger female crabs released from Portland, South Devon inshore 2008 and the Trevose ground moved faster than smaller crabs, and both faster and further for the Hurd Deep (Mid-Channel) releases. In addition crab size influenced the distance moved within the 60 day limit for female crabs released off the Lizard. The speed of male crabs was faster for larger crabs released off Portland.
The sex of the crab and the release batch strongly influenced the movements of those animals recaptured within 60 days of release. The influence of the four characteristics deemed likely to influence mobility is less clear. They all appear to affect some aspects of crab movement in some of the release batches but not all. Where it is easy to understand the potential positive affect of increased crab size and negative influence of limb loss (claw or walking leg), we are not able to explain the contradictory result for shell condition.
AIII.4. Summary of main findings
The overall recapture rate for the double T-bar tagging programme was around 15%, very similar to that achieved using suture tags by Bennett & Brown (1983).
Typically >90% of recaptures were in the first year of release and most of these in the first few months, with very low numbers of recaptures occurring in subsequent years.
Recaptured female crabs s had moved predominantly in a NW, W or SW direction. Movements of recaptured male crabs included directional components in a NW, W and SW
direction, but proportionally more local and undirected movements. Females generally moved faster and further than their male counterparts. Highest rates of movement occurred in October and November. Crabs released early on in the study at the eastern end of the Channel moved the furthest,
whilst those released at the western end were predominantly recaptured close to their release site. However, this may reflect an absence of major UK fisheries surrounding these release areas.
There was no evidence to support the hypothesis by fishers in the South Devon and Cornwall area that crabs move in from the west in an easterly migration at the onset of each fishing season.
There was some suggestion of crabs moving around coastal bays, which could infer that an inshore migration is combined with the observed westerly movements, but this could also reflect the local distribution of fishing gear.
79
Seasonal rates of movement support the biological knowledge relating to a sedentary period from December to May, during which time female crabs spawn and incubate eggs.
Rates and magnitude of movement were strongly influenced by the sex of the crabs and the release location.
Physical characteristics, such as shell condition and missing legs or claws, influence the rate, and sometimes, the magnitude of movements in some cases.
Appendix IV. Modelling the movements and exploitation of edible crabs in the English Channel, Western Approaches and Celtic Sea
AIV.1 Introduction
Results from the double T-bar tagging programme have been used to describe general patterns of movement in section 4.3 and Appendix III, while results from the DST programme are presented in section 3 and Appendix I. In this Appendix we describe a population dynamics modelling framework used to synthesise data from both double T-bar and DST tagging programmes, together with results from the aquarium experiments and commercially reported effort data in an attempt to quantify rates of movement by edible crabs in the study area. The framework is based on the model of Hilborn (1990) and includes modifications suggested by Aires-da-Silva et al. (2005). The use of this type of population dynamics model also permits estimation of exploitation rates which are presented and discussed in the sections on exploitation (5.1 & Appendix V).
AIV.2 Methods and data
Initially twenty spatial units were defined based around the distributions of tagged crab releases and recaptures and using some knowledge of the main fishing areas as the basis for the modelling framework. A population dynamics model developed during the EU POORFISH project (Smith, 2008, unpublished) was considered as the potential basis for modelling the system dynamics, but it soon became apparent that with 3 seasons, 2 sexes and a matrix of 20 spatial units, the number of parameters to estimate was unmanageable. This model was therefore simplified to a single sex model with 2 seasons and 12 spatial units and applied separately for males and females.
Populations of tagged and released edible crabs were projected forward using monthly time steps. This provided 39 time steps for the first ‘tag group’, with subsequent tag groups introduced into the model at the appropriate time and location. Tag groups included both double T-bar tags, data storage tags (DSTs) and small numbers of re-releases made by some fishermen (Table AIV.4). The latter were considered as normal recaptures and their release as a new introduction into the population. Releases of crabs tagged with data storage tags (DSTs) had much higher return rates than double T-bar tags, so a parameter for differential return rate by tag type was included in the model.
Fishing effort for each spatial unit and time step were retrieved from reported commercial data aggregated to ICES rectangles and allocated to each spatial unit (Figure AV.1). Spatial units were nominal, of different sizes and did not correspond exactly with ICES rectangles, so in some cases effort for a single rectangle was apportioned into different spatial units, whilst in other cases a spatial unit consisted of effort from several rectangles. It was not possible to obtain fishing effort data for foreign vessels and effort allocated to the ‘catch all’ spatial unit for neighbouring areas is irrelevant because these crabs are ‘lost’ from the analysis and not included in the fitting objective function. The mobility of crabs is limited and we therefore follow the rationale, used by Hilborn (1990), of limiting movement to contiguous spatial units in any one time step. This also has the advantage of substantially reducing the complexity and the number of potential permutations for movement between areas.
Edible crab biology displays substantial seasonality as well as differences between sexes, which complicates the modelling process. One notable feature is that ovigerous females have very limited mobility and therefore low catchability whilst in this condition, due to the large size of the egg mass. The population model assumes 2 seasons, one corresponding to the main ovigerous season for females (December to May) and the other corresponding to the remainder of the year (June to November), during which females crabs are mobile and which also includes the main period of fishing activity. Although male crabs do not show the same seasonality of catchability as females, the same 2
80
season model was run for males. Other biological features such as the moulting and mating cycle may impact on mortality, movement and catchability of crabs, but the biology of these aspects is insufficiently quantified to meaningfully parameterise a model and therefore in order to enhance parsimony they were not modelled at this stage.
In order to retain the correct ratios between numbers of tags released and recaptured, all recaptures were used, although some did not have full temporal or positional data relating to recapture. These were apportioned among the possible spatio-temporal strata by utilising all available information about them and/or according to the proportions of valid returns for these strata.
The basic population equation of Hilborn (1990) was used along with modifications suggested by Aires-da-Silva et al. (2005) for including terms for tag loss and natural mortality
where is the predicted number of tagged fish if tag group i present in area a at time (month) t,qj,s is catchability for area j and season s, Ej,t is fishing effort in area j at time t,Pj,a is the probability of movement from area j to area a,Ti,a,t is the number of tagged crabs and released from group i, area a and time t.
The rate of tag loss for double T-bar tags was estimated (monthly λ=0.031) from 6 aquarium experiments (see section 4.2 and Appendix II) and the rate of loss of data storage tags was assumed zero within the timeframe of the field tagging experiments. A double T-bar tag loss rate estimated simplistically from field double tagging experiments was very similar, but slightly lower in magnitude (0.019). However, this methodology did not take account of the potential loss of both tags, so will be an under-estimate of the overall double T-bar tag loss rate.
Monthly harvest rate in each area (hj,t) is expressed as the product of seasonal catchability (qj,s) and monthly effort (Ej,t)
Return rates between the 2 types of tag used in our study differed widely and in common with Aires-da-Silva et al. (2005) a parameter for tag reporting rate ( double T-bar tag return rate relative to data storage tag return rate) was included and estimated as a parameter in the model fit. The observation model therefore becomes
where is the predicted number of tags recovered from tag group i in area a at time t.We use the Hilborn (1990) Poisson likelihood function which gives a total likelihood of
but for minimisation we use the simplified negative log-likelihood function suggested by Aires-da-Silva et al. (2005), in which the factorial R term has been dropped because it is constant.
Using the logarithmic transformation also reduces problems with overflow that were encountered with some of the terms in the full likelihood function. The model was implemented in Excel workbooks supported by Visual Basic for Applications (VBA). Minimisation was carried out by iteratively using the Solver add-in under VBA control.
AIV.3 Results
The use of Excel and VBA for modelling was very useful in visualising the dynamics of the model in this development phase, but much less effective in terms of minimisation, which was very time consuming and it was difficult to be certain that a global minimum was reached. However,
81
convergence of the objective functions for males and females suggested that relatively stable solutions had been reached and comparisons of predicted recoveries against observed recoveries also suggested that the results were broadly plausible.
Summaries of rates of movement between areas (Tables AIV.2 & AIV.3, Figures AIV.1 to AIV.4) show that in most areas most crabs of both sexes remained within their ‘current’ area in any one month. This was the case for females in 10 of 12 areas in summer & autumn and 7 of 12 areas in winter & spring. Those areas where it occurred in both seasons were: the Trevose Grounds, mid Channel, Portland, Wight & Sussex, Shingle Bank & Sovereign Shoals and Bullock Bank. It was also the case for female crabs during winter & spring in Lands End & Scilly, south Cornwall offshore and during summer & autumn in Lizard & west Cornwall, south Cornwall inshore, south Devon offshore.
Substantial amounts of inshore - offshore transfer of female crabs occurred in the more spatially complex areas of the model around south Devon and south Cornwall. In summer & autumn inshore-offshore transfers were 21% and 19% off south Cornwall and 18% and 47% between south Cornwall offshore and south Devon inshore, but negligible between south Devon offshore, Portland and mid Channel and other areas. In winter & spring female crabs from south Devon inshore moved offshore (69%) and to and from south Cornwall offshore (31% and 18%), while transfers between inshore and offshore Cornwall were 50% and 66% and between inshore Cornwall and offshore Devon 13% and 53%.
The westerly movements that are very striking in the raw tagging data were not very apparent in the model outputs for female crabs, with a few exceptions. In summer & autumn significant monthly rates of movement in a westerly or south-westerly direction occurred from Bullock Bank, Wight & Sussex, mid Channel and South Devon inshore, while in winter & spring they occurred from Wight & Sussex, Portland, South Devon inshore and offshore and Lizard & west Cornwall. The number of areas indicating movements and the proportion of the female population moving was slightly higher during winter & spring.
Significant losses to the ‘outside’ catchall (which can also account for additional mortality as well as movement) occurred during both seasons in the Trevose Grounds, Lizard & west Cornwall, south Devon inshore and to a lesser extent Portland and during winter & spring in Lizard & west Cornwall, Mid Channel and to a lesser extent Lands End & Scilly, Portland and Bullock Bank. These losses from the model system appear to be related to low rates of tag return, hence their higher prevalence during winter & spring, and they may also indicate areas where the model is not fitting well.
Far fewer male crabs were tagged and recovered than females, so the results for males need to be treated with more caution and far more were ‘lost from the system’ in the modelling outputs. The majority of male crabs stayed in their ‘current’ area in 6 of 12 areas in summer & autumn 7 of 12 areas in winter & spring. Those areas where it occurred in both seasons were: Lands End & Scilly, south Devon inshore, Portland, Shingle Bank & Sovereign Shoals and Bullock Bank, whilst this was the case in the winter only for the Trevose Grounds, Lizard & west Cornwall (49% in summer & autumn).
Inshore offshore transfer of male crabs was not apparent to any extent during summer & autumn, but losses from the system in most of the western areas were high at this time reflecting low levels of tag returns. In contrast, during winter & spring inshore offshore transfers were widespread in the western Channel; 79% from mid Channel to Portland, 37% and 26% between inshore and offshore south Devon, 69% and 10% between inshore and offshore south Cornwall and 39% and 67% between Lizard & west Cornwall and south Cornwall offshore. Losses from the system were lower during the winter & spring season for males.
For the Wight & Sussex and area all male crabs were predicted to move into the Portland area during both seasons, but quite a high reverse movement rate was also estimated for summer & autumn. The former area is the only route (in the model) for crabs to take between the eastern and western Channel therefore although returns for this area were low the population is forced to move through it in order to fulfil the expectations for returns (from eastern Channel releases)made further in the western Channel. The majority of males in south Cornwall offshore were predicted to move west into Lizard & west Cornwall in both seasons. Losses from the system were high during summer & autumn with the south Cornwall inshore (100%), Trevose Grounds, Lizard & west Cornwall, south Cornwall
82
offshore, mid Channel, Lands End & Scilly South Devon inshore and Shingle Bank & Sovereign Shoals all high or significant, whilst in winter & spring they were generally very low with the exception of Portland, Bullock Bank and Lands End & Scilly.
A few results were counter intuitive to the general impression provided by the data, including in particular the high rates of movement of female crabs from the Lands End & Scilly fishery area into the Lizard area during the summer & autumn and of male crabs from the south Devon offshore area into the mid Channel area in both seasons, which were both against the general westward movement seen in the tag recoveries and not supported by any physical evidence (see discussion). Other such counter intuitive movements for females included 18% from South Cornwall offshore to South Devon inshore during both seasons, although this was exceeded by movements in the opposite direction of 47% during summer & autumn and 31% in winter & spring.
Catchability parameters estimated by spatial area and season (Table AIV.1) in separate sex models provide scalars for the effort data including taking account of the different spatial extent of the areas concerned. These were multiplied by the monthly effort data to provide estimates of exploitation rate over the study period (see section 5.1 and Appendix V). Zero values for catchability were returned for a number of areas for male crabs, reflecting zero returns for males in these areas.
Table AIV.1. Estimated catchability parameters by area, season and sexCatchability
Fishery area Female MaleSummer Autumn
Winter Spring
Summer Autumn
Winter Spring
Trevose Grounds 2.24E-06 3.20E-07 2.56E-06 9.88E-06Lands End & Scilly 1.07E-05 1.01E-06 9.73E-07 9.88E-07Lizard & west Cornwall 4.81E-07 1.78E-05 3.96E-06 1.54E-05South Cornwall inshore 3.72E-07 9.75E-08 1.31E-05 0South Cornwall offshore 8.38E-07 2.32E-07 0 0South Devon inshore 4.68E-06 3.19E-07 1.21E-06 3.6E-06South Devon offshore 5.02E-07 9.81E-08 5.92E-07 0Mid Channel (E7) 4.58E-07 2.30E-07 1.6E-06 1.15E-05Portland 4.99E-06 1.25E-06 3.85E-06 1.46E-06Wight & west Sussex 3.70E-07 9.06E-08 5.92E-07 6.49E-06Shingle Sovereign 1.53E-05 2.94E-06 1.54E-05 1.77E-05Bullock Bank 3.50E-05 5.86E-06 4.29E-05 2.21E-05
The only other parameters estimated were reporting rates for double T-bar tags relative to DSTs, the latter of which were assumed to be 100% reported. These parameters were estimated to be 0.629 for females and 0.815 for males. It should be borne in mid that very few DSTs were applied to male crabs and these were the last release made in the eastern Channel only and within the project’s duration this only left scope for in-year returns (Table AIV.4). Also the numbers of double T-bar tags applied to male crabs was substantially lower than for females. The value for females is therefore likely to be better estimated on both counts.
83
Table AIV.2. Estimated movement parameters for female crabs (red cells not included in the objective function)Rate of movement into this cell from host cell (column 1)
Fishery area No. 1 2 3 4 5 6 7 8 9 10 11 13 14Summer & AutumnOutside catchall 1 1 0 0 0 0 0 0 0 0 0 0 0 0Trevose grounds 2 0.27 0.7323 0.0 0 0 0 0 0 0 0 0 0 0Lands End & Scilly 3 0 0.0 0.1646 0.8354 0 0 0 0 0 0 0 0 0Lizard & west Cornwall 4 0.35 0 0.0505 0.6000 0.0 0.0 0 0 0 0 0 0 0South Cornwall inshore 5 0 0 0 0.0064 0.7611 0.1908 0.0290 0.0127 0 0 0 0 0South Cornwall offshore 6 0 0 0 0.0 0.2089 0.5597 0.1793 0.0522 0 0 0 0 0South Devon inshore 7 0.25 0 0 0 0.0 0.4688 0.2790 1E-05 0.0 0.0 0 0 0South Devon offshore 8 0 0 0 0 0.0106 0.0357 0.0 0.9537 0.0 0 0 0 0Mid Channel (E7) 9 0.04 0 0 0 0 0 0.004 0.1274 0.8279 0.0 0 0 0Portland 10 0.14 0 0 0 0 0 0.0 0 2E-14 0.8559 0.0024 0 0Wight & west Sussex 11 0 0 0 0 0 0 0 0 0 0.3114 0.6886 0.0 0Shingle Sovereign 13 0 0 0 0 0 0 0 0 0 0 0.0202 0.9496 0.0302Bullock Bank 14 0 0 0 0 0 0 0 0 0 0 0 0.2109 0.7891Winter & SpringOutside catchall 1 1 0 0 0 0 0 0 0 0 0 0 0 0Trevose grounds 2 0.03 0.9735 0.0 0 0 0 0 0 0 0 0 0 0Lands End & Scilly 3 0.12 0.0 0.8763 0.0 0 0 0 0 0 0 0 0 0Lizard & west Cornwall 4 0.34 0 0.5966 0.0667 0.0 0.0 0 0 0 0 0 0 0South Cornwall inshore 5 0 0 0 5E-14 0.2696 0.5000 0.0955 0.135 0 0 0 0 0South Cornwall offshore 6 0 0 0 2E-13 0.6628 0.0997 0.1813 0.0562 0 0 0 0 0South Devon inshore 7 0 0 0 0 2E-05 0.3147 1E-05 0.6853 2E-07 7E-14 0 0 0South Devon offshore 8 0 0 0 0 0.5346 0.0319 0.0 0.3147 0.1187 0 0 0 0Mid Channel (E7) 9 0.45 0 0 0 0 0 1E-13 1E-13 0.5541 0.0 0 0 0Portland 10 0.11 0 0 0 0 0 0.2544 0 0.0 0.6337 0.003 0 0Wight & west Sussex 11 0 0 0 0 0 0 0 0 0 0.4454 0.5546 2E-07 0Shingle Sovereign 13 0.2 0 0 0 0 0 0 0 0 0 0.0417 0.7027 0.0536Bullock Bank 14 0.11 0 0 0 0 0 0 0 0 0 0 0.0 0.8864
84
Table AIV.3. Estimated movement parameters for male crabs (red cells not included in the objective function)Rate of movement into this cell from host cell (column 1)
Fishery area No. 1 2 3 4 5 6 7 8 9 10 11 13 14Summer & AutumnOutside catchall 1 1 0 0 0 0 0 0 0 0 0 0 0 0Trevose grounds 2 0.683 0.3168 0.0 0 0 0 0 0 0 0 0 0 0Lands End & Scilly 3 0.303 0.0 0.6970 0.0 0 0 0 0 0 0 0 0 0Lizard & west Cornwall 4 0.514 0 0.0 0.4851 0.0 0.0004 0 0 0 0 0 0 0South Cornwall inshore 5 1 0 0 0.0 0.0 0.0 0.0 0.0 0 0 0 0 0South Cornwall offshore 6 0.442 0 0 0.558 0.0 0.0 0.0 0.0 0 0 0 0 0South Devon inshore 7 0.3 0 0 0 0.0073 0.0 0.6930 0.0 0.0 0.0 0 0 0South Devon offshore 8 2E-07 0 0 0 0.0 0.0 0.0 0.1768 0.8232 0 0 0 0Mid Channel (E7) 9 0.405 0 0 0 0 0 0.0 0.0917 0.5036 0.0 0 0 0Portland 10 5E-09 0 0 0 0 0 0.0 0 0.0276 0.7063 0.2661 0 0Wight & west Sussex 11 7E-08 0 0 0 0 0 0 0 0 1.0000 6E-07 0.0 0Shingle Sovereign 13 0.242 0 0 0 0 0 0 0 0 0 0.0 0.7491 0.0086Bullock Bank 14 5E-08 0 0 0 0 0 0 0 0 0 0 0.0695 0.9305Winter & SpringOutside catchall 1 1 0 0 0 0 0 0 0 0 0 0 0 0Trevose grounds 2 6E-07 1.0000 0.0 0 0 0 0 0 0 0 0 0 0Lands End & Scilly 3 0.146 0.0 0.8541 0.0 0 0 0 0 0 0 0 0 0Lizard & west Cornwall 4 1E-07 0 0.0 0.5003 0.1080 0.3917 0 0 0 0 0 0 0South Cornwall inshore 5 5E-08 0 0 3E-07 0.3636 0.6364 0.0 0.0 0 0 0 0 0South Cornwall offshore 6 4E-08 0 0 0.6741 0.0997 0.2262 0.0 0.0 0 0 0 0 0South Devon inshore 7 5E-08 0 0 0 0.0 0.0 0.5353 0.3739 0.0 0.0908 0 0 0South Devon offshore 8 2E-08 0 0 0 0.0 0.0 0.2619 1E-07 0.7381 0 0 0 0Mid Channel (E7) 9 9E-08 0 0 0 0 0 0.0 0.2065 1E-07 0.7935 0 0 0Portland 10 0.315 0 0 0 0 0 0.0091 0 0.0 0.6762 0.0 0 0Wight & west Sussex 11 1E-08 0 0 0 0 0 0 0 0 1.0000 0.0 0.0 0Shingle Sovereign 13 3E-09 0 0 0 0 0 0 0 0 0 0.0328 0.6071 0.3600Bullock Bank 14 0.203 0 0 0 0 0 0 0 0 0 0 0.0347 0.7622
85
Figure AIV.1. Expected monthly rates of movement predicted by the dynamics model for female crabs during summer and autumn
86
Figure AIV.2. Expected monthly rates of movement predicted by the dynamics model for female crabs during winter and spring
87
Figure AIV.3. Expected monthly rates of movement predicted by the dynamics model for male crabs during summer and autumn
88
Figure AIV.4. Expected monthly rates of movement predicted by the dynamics model for male crabs during winter and spring
89
90
AIV.4 Discussion Fishing effort data (pot hauls per month) were extracted from a database of officially reported commercial data. There are some concerns regarding some aspects of the quality of the reported commercial data, particularly regarding effort data. Effort data were selected only where a catch of crabs was taken, but in reality this is likely to include a large amount of lobster targetted effort as well as that targeted at edible crabs, since a significant by-catch of crab is likely in the lobster fisheries. The effort data should therefore be regarded as a broad indication rather than comprehensive, precise and accurate. In some areas, particularly the more offshore ones (e.g. South Cornwall offshore E5), there may be significant amounts of effort from French fishermen which is not accounted for in this study. However, with one or two exceptions seasonal and spatial trends in the effort data (Figure AV.1) were generally in accordance with expectations over the study area. The nominal fishery areas used for this modelling were not all the same size and do result in some areas having higher effort than might be expected, for example the Trevose Grounds and Wight & Sussex areas seem high relative to some other areas, but this can be explained both by the fact that they comprise several rectangles and also that both areas contain important lobster fisheries that may account for large amounts of summer-time effort. However because catchability coefficients are modelled regionally these parameters should provide an appropriate scaling that accounts for the difference in spatial extent and relative level of crab targetting assuming that these effects are consistent within the seasons used, which is the case for the spatial area, although possibly slightly less so for targeted lobster fishing. The population dynamics model did not explicitly model the moult process and used a fixed estimate of tag loss rate that did not take any account of seasonality and the process of moulting when tag loss is much more likely. Aquarium trials, although limited, did not suggest particularly higher tag loss rates during moulting and many crabs in the adult population will not moult every year so the estimate of tag loss provided by the aquarium experiments may be considered provide an average over moulting and non moulting individuals. However, the constant rate of loss model did not appear particularly appropriate and the aquarium experiments suggested a higher initial loss rate for up to 200 days followed by a period when retention remained relatively constant with around 75% of tags will be retained during the medium to long term. Analysis of returns from the double tagged crabs under field conditions will only account for loss through ‘wear and tear’ rather than explicitly associated with moulting, but analysis of these data (Figure AIV.5) suggested a similar pattern of high initial tag loss tag loss, stabilising at around 75% retention. A constant instantaneous rate of loss estimated from these data (0.019 per month) was lower (as expected because it does not take account of occurrences where both tags were lost), but broadly similar to that for our aquarium experiments (0.031), although this model does not seem particularly appropriate given the pattern of tag loss through time. Other authors (e.g. Stevenson, 1954 in Schweigert & Schwarz, 1993) have noted that tag loss and tag mortality (for finfish) is often high for an initial period and then lower over the medium term and this assumption is made for the Brownie model estimations of mortality (see section 5.1 and Appendix V). Future work using the population dynamics approach discussed here might benefit from using a different model for tag loss which more closely captures this pattern. A two-line approach might be a strong candidate. However although both our experiments on tag loss showed similar early initial loss rates stabilising at around 75% retention in the medium to long term the duration of the initial higher loss rate period was quite different, up to 200 days in the aquarium experiments and up to 100 days in the field double tagging experiments.
Outputs from the population dynamics model were quite difficult to interpret and not always in concordance with the general impressions given by the tagging data summary. They did support some westerly movements, but not as strongly as is suggested by the tagging summary plot (FigureA.III.1). Model outputs suggested higher proportions of the female population moving and in a more systematic westerly direction during winter and spring, the time when as individuals most females would be ovigerous, relatively immobile and therefore not likely to exhibit these systematic movements. This may indicate that the seasons within the population dynamics model were not correctly aligned with the timing of the crab life cycle or possibly that individually the duration of incubating eggs does not take the full season and movement occurs before and/or after the period the female is actually ovigerous.
91
Figure AIV.5. Tag retention curves for the field double tagging experiment
The model highlighted the potential for substantial inshore offshore movements of crabs of both sexes where sufficient strata were available for this to occur. Inshore offshore interactions were most apparent during winter and spring, which is a time when inshore activity for both crabs and lobsters is reduced, but larger offshore vessels are still active and this may influence the distribution of tag returns at this time of the year. Also, substantial losses from the model system were noted at certain times and particularly for males and in some cases these suggest that the dynamics of the model are insufficient to explain the tag returns adequately.
Although the tagging summary plot can be criticised because it tends to focus attention on the longer movements, there is other evidence that strongly supports systematic westerly movements at the population level. The input data for the population dynamics model included matrices of tag returns for each tag release group by time and space. Some of these proved extremely informative, in particular those for releases made early on during the project and in the more easterly regions of the Channel. Comparing the pattern of observed data for the first two release batches with model outputs helps to understand the model dynamics and clarifies whether or not the population dynamics are explaining the observations plausibly. The first two panels on the top row of figure AIV.6, show the recovery matrices (converted to percentage of numbers released) for the Shingle Sovereign and Bullock Bank tag groups for females released in October 2007. In the first 2-3 months all recaptures are taken relatively locally, although there is evidence for stronger movement from Bullock Bank to Shingle Sovereign than vice versa (i.e. in a westerly direction). The population dynamics model captures these effects well, with 21% movement from Bullock Bank to Shingle Sovereign and a reverse movement of 3% during the summer & autumn period as well as a 2% movement from Shingle Sovereign to Wight and west Sussex in this period. There follows a period of reduced recaptures over the winter & spring, which is reflected in lower catchabilities estimated for this season in the model as well as in the lower fishing effort data for this season (Figure AV.1) and significant levels of losses from the system from both Shingle Sovereign (20%) and Bullock Bank (11%). In the second year of liberty most recaptures for Shingle Sovereign are still in the release locality with a few in the nearest neighbour to the west and further west. Again, this is well captured by the model in that the westerly movement rates from Shingle Sovereign are relatively low in both seasons (2% summers & autumn, 4% winter and spring) so most recaptures should still be local. Observations of recaptures from Bullock Bank releases are all made in Shingle Sovereign, also well captured by the model with the 21% monthly export from Bullock Bank to Shingle Sovereign in summer & autumn and no movement during winter & spring allowing complete export during 2 summer & autumn months in 2007 and 3-6 in 2008 (this population will also be subject to fishing and natural mortality), whilst the relatively low movement rates from Shingle Sovereign mean few of these crabs would be expected to be caught further west. There is another reduction in recaptures over the second winter, again well reflected by the low catchabilities for the winter & spring season in addition to the low effort input data as well as the losses from the system in this season. In the third year of liberty few observations occur in the
92
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
0 100 200 300 400 500 600 700 800
Prop
ortio
n of
tag
s re
tain
ed
Days at liberty
of total double tag returnsof retuns to capture dateConstant instantaneous loss
Shingle Sovereign area, more recaptures are made in areas to the west of the release area than in the release location. The model outputs of high summer & autumn (31%) and winter & spring (45%) rates of westerly movement out of Wight and west Sussex permit movement into the Portland area which accounts for a few recaptures from both the Shingle Sovereign and Bullock Bank releases. There is also a high rate of movement from Portland to S. Devon inshore during winter & spring which permits the model to match observations of recaptures from this area. However, more observations of recaptures from Portland might be expected and in order to accommodate the lack of these, losses from the system in Portland are significant in both seasons (14% and 11%). After another winter with no recaptures all the recaptures in the 4th season at liberty are in areas well to the west of the release area. This is after a period of 33 -39 months at liberty, so even at the relatively low 2-4% westward rate movement from Shingle Sovereign this would permit sufficient transfer for all the population to have been removed from this area, especially taking account of other sources of loss (fishing and natural mortality and losses from the system). Thus, although initially movement is limited in both distance and the proportion of the population moving, after a number of years all of the recaptures are made from areas further west in the Channel. This is very apparent from the observations and although less striking in the model outputs, the dynamics of the model are able to accommodate this outcome as described in the discussion above.
A simplistic consideration of the input observations implies that all of the surviving population has moved away from the release area after a period of 3 to 4 years. This pattern was also apparent in the Portland release and in the mid Channel release even though this took place much later in the project, so less time at liberty was available. Other areas show more intermediate results with some, but not complete dispersal to the west. Releases in areas 8 and 9 (S. Devon offshore and mid Channel, respectively) showed quite high levels of returns in the inshore areas of S. Devon and S. Cornwall (areas 7 and 5 respectively) in subsequent years. However, in some other areas, for example, the Trevose Grounds (area 2, Figure AIV.6, lower row, second panel), although the seasonality of recaptures is marked there is no evidence of movement elsewhere. This may be indicative of no surrounding fisheries in which recaptures can be made or that the crabs are not moving substantially. Interestingly, in this example the rate of recaptures in the second season at large is quite high, possibly suggesting that the crabs have a tendency to remain in this area. This is reflected in the dynamics model output which indicates high monthly rates of crab retention in both seasons, with some losses from the system, but no transfer to other areas. Recapture rates for females were only above 10% during the first month for the Shingle Sovereign release and for the mid Channel DST release (Tag group 42). Recapture rates for double T- bar tags were rarely above 1% after the first season at liberty, with the exception of some recaptures from south Devon inshore that were released off Portland, and some recaptures on the Trevose Grounds released in the same area. Recapture rates for DSTs were always above 1%.
Far fewer males were released and no recaptures were reported beyond the third season at liberty, with these occurring for releases made in the Shingle Sovereign area and Portland. Movement to other areas was less marked for males and was restricted to the neighbouring westerly area (i.e. Bullock Bank to Shingle Sovereign, Shingle Sovereign to Wight & west Sussex, Portland to S. Devon inshore, S. Devon inshore to S. Cornwall inshore and mid Channel to S. Devon offshore) (Figure AIV.7). Recapture rates for males were above 10% for the first 2 months for the Bullock bank release only and were only above 1% during the second season for a Lands End release. All recapture rates were above 1% for DSTs released late on in the project that were only at liberty for 1 season.
93
Figure AIV.6. Selection of observed tag recovery matrices expressed as % of released numbers for female edible crabs showing recoveries by recapture area (columns) and time (rows). Red area title: area of release. Blue text: tag group. Red number: number released. Cell shading red: 10%+, orange: 1-9.9%, yellow: <1%
94
Figure AIV.7. Selection of observed tag recovery matrices expressed as % of released numbers for male edible crabs showing recoveries by recapture area (columns) and time (rows). Red area title: area of release. Blue text: tag group. Red number: number released. Cell shading red: 10%+, orange: 1-9.9%, yellow: <1%
95
It is also useful to consider and try to explain some of the less plausible results from the model in order to gain better understanding of the dynamics of the system. For example, there were two instances of model outputs suggesting substantial movement from west to east that are not directly supported by any physical data. These were high rates of movement of female crabs from the Lands End & Scilly fishery area into the Lizard area during the summer and autumn and of male crabs from the south Devon offshore area into the mid Channel area in both seasons. Figure AIV.8 shows the expected against observed recaptures (for positive recaptures only) from the main tag groups including 2 releases at Lands End & Scilly and 1 at Lizard & west Cornwall. These appear to fit poorly compared with the other tag groups. There were some re-releases in these areas but no recaptures from any of these. For tag group 4 in summer (2008) observations of recoveries are lower than expectations so crabs needed to be moved out from Lands End & Scilly (although there are no observations of recaptures here or in the other neighbour, Trevose Grounds). Also for tag group 4 subsequent winter and summer (2009) observations of recoveries in Lands End & Scilly are higher than the predictions so crab would need to be moved back into Lands End & Scilly at this time. These trends are consistent with the model output. For tag group 7 the late summer (2008) and first winter month (based on the last of the summer movement) observations are much higher than the expectation, which should reduce the level of movement out of Lands End & Scilly at this time, but does not appear to do so. Subsequent observations over the winter period (and first summer month) are also higher than expectation in Lands End & Scilly which would tend to move crabs back into this area at this time, again a pressure consistent with the model output. For tag group 8 released in Lizard & west Cornwall observations over the winter period are much higher than expectations which would tend to require that crabs were retained in this area, inconsistent with the model output. Further there is around a 35% monthly loss to the system from the Lizard and west Cornwall area in both seasons so this area is the main source of loss to the model system for females. Overall it appears that the influence of low initial summer recapture rates for tag group 4, followed by higher than expected winter recapture rates for both tag group 4 and 7 in Lands End & Scilly outweigh the high observed late summer recaptures for tag group 7 in Lands End & Scilly and the higher than expected winter recapture rates for tag group 8 in Lizard & west Cornwall. This results in the latter tag group fitting particularly poorly and that an influx of crabs is required (during summer) to offset the ongoing losses to the system that occur during both systems.
The other strong predicted easterly movement was of male crabs from the South Devon offshore area into the mid Channel area in both seasons. Figure AIV.9 shows that for tag group 33 released in mid Channel, observations in this area were lower than expectations, a pressure for lower residence in this area or reduced catchabilities. Only 1 male crab was released in the south Devon offshore area and this was not reported as being recaptured. There is no physical evidence for movement from the south Devon offshore area eastwards to the mid Channel area, but the model predicts substantial movement (74-82%) in this direction in both seasons. In the mid Channel area in summer & autumn 50% of crabs were retained, while 9% move west and 41% were lost from the system, whilst in winter & spring 21% moved west to south Devon offshore and 79% moved into the Portland area (where there was subsequently a 32% loss from the system. It therefore appears that a large proportion of crabs were lost from the model system in the model either in the Mid Channel area or subsequently from the Portland area. Crabs lost from the model system cannot re-enter so it appears some replacement is required to support the model dynamics and accommodate this sink.
Both the areas where movements were predicted that do not concord with physical observations also act as sinks in the model system, losing a large proportion of crabs each month. Bearing this in mind these unsupported results should be treated with extreme caution. It is again worth reiterating that far fewer male crabs were tagged and recovered than females and the results for males therefore need to be treated with more caution.
In general, and more particularly for females, the expectations produced by the model are informative and may help to quantify the rates of transfer of crabs between some areas, but they still need to be treated with caution. They confirm that the majority of female crabs remained in their current area in any one month in almost all areas and although they do not always highlight strongly the westward patterns of movements suggested by many of the tag recaptures, close consideration of the outputs shows that the model dynamics can accommodate these westerly migrations which occur over a protracted time scale. Other, model outputs suggest some easterly movements that are not supported by any physical evidence from either the double T-bar or DST tagging programmes and are almost
96
certain to be erroneous. In its current form it appears that the model may at times force population movements to accommodate changes in recapture rates and distribution that may be due to changes in catchability related to the reproductive cycle or result from anomalies in the data such as different reporting rates between areas and through time. Hilborn (1990) notes that although there is considerable scope for parameter confounding, in his case study there was remarkably little, because the conditions of mark and release in each area and a time series of effort data for each area were largely met. However, in contrast Schwarz & Arnason (1990, in Hilborn, 1990) found enormous confounding of parameters in a case where the release and recapture areas were not the same. In the case of our study, we have effort data for all areas, with some caveats regarding its quality and target species and releases did occur in most areas. However, we had low return rates for some areas and areas without releases and lower project visibility (Wight & west Sussex, S. Cornwall inshore and S. Cornwall offshore) did prove to be somewhat problematic. The limited potential for movement of crabs and highly spatially focussed nature of the fisheries may also pose some problems relating to dynamic pool assumptions for population dynamics models such as this, which could be problematic particularly immediately following releases of tagged crabs and given potentially different reporting behaviours by fishermen. Other areas of uncertainty relate to poor reporting of tags that was apparent on some occasions, but is also unknown in the main as well as accurate estimation and modelling of tag loss, that was investigated and modelled, but still has high uncertainty and potential for better modelling, and tag induced mortality that was investigated but not modelled.We considered further simplifying the model with regards to spatial and temporal disaggregation.
We chose a spatial scale broadly equivalent to the scale of our tag release groups, with some intermediate areas and simplifying the spatial scale may be a useful approach, but would still utilise the data at the level of the tag group. We used a monthly time step with two seasons to capture the biology. Changing to a broader scale for the time step reduces the number of data points available, whilst not changing the number of parameters to be fitted, so does not offer immediate advantages. Also, changing to a longer time step would make it possible for crabs to move further than to the contiguous area which would greatly increase the complexity of the movement matrix and increase the number of parameters to be estimated. Any broadening of temporal scale would therefore require a broadening of spatial scale if movement is to be restricted to contiguous areas.
Future investigations could focus initially on developing a similar model but in a more powerful minimisation framework (possibly AD model builder) before increasing model complexity. This would permit more efficient minimisation and provide fit diagnostics. This would permit subsequent investigations into model performance under different assumptions regarding the population dynamics to be carried out efficiently.
97
Figure AIV.8. Expected recoveries against observations for the main female tag groups 1 to 10 and 12 (for positive observations only). Rows 2 and 3, column 1 are releases in the Lands End & Scilly area and Row 3, column 2 shows the Lizard & west Cornwall release
98
0
20
40
60
80
100
120
140
160
180
200
0 20 40 60 80 100 120 140 160 180 200
Expe
cted
Observed
0
20
40
60
80
100
120
140
160
180
200
0 20 40 60 80 100 120 140 160 180 200
Expe
cted
Observed
0
20
40
60
80
100
120
140
0 20 40 60 80 100 120 140
Expe
cted
Observed
0
20
40
60
80
100
120
140
0 20 40 60 80 100 120 140
Expe
cted
Observed
0
10
20
30
40
50
60
70
80
0 20 40 60 80
Expe
cted
Observed
0
20
40
60
80
100
120
140
0 20 40 60 80 100 120 140
Expe
cted
Observed
0
20
40
60
80
100
120
140
160
180
0 20 40 60 80 100 120 140 160 180
Expe
cted
Observed
0
20
40
60
80
100
120
140
0 20 40 60 80 100 120 140
Expe
cted
Observed
0
20
40
60
80
100
120
140
0 20 40 60 80 100 120 140
Expe
cted
Observed
0
10
20
0 10 20
Expe
cted
Observed
0
20
40
0 20 40
Expe
cted
Observed
Figure AIV.9. Expected verses observed recoveries for tag group 33, males released in mid Channel
Table AIV.4. Summary of tag groups used in the population dynamics modelTag
groupSex Tag
typeLocation Month
& yearReleases Recapturenumbers numbers %
1
F
2T
Shingle Bank/Sovereign Shoals 10/07 1095 240 21.92 Bullock Bank 10/07 832 197 23.73 Portland 11/07 1344 201 15.04 Land’s End/Scilly 6/08 1293 217 16.85 South Devon inshore 6/08 1495 259 17.36 Trevose Grounds 10/08 1680 178 10.67 Land’s End/Scilly 11/08 1470 302 20.58 Lizard/West Cornwall 5/09 841 93 11.19 South Devon inshore 6/09 1472 279 19.0
10 South Devon offshore E6 6/09 316 25 7.911 South Devon inshore 8/09 3 1 33.312 Mid Channel E7 8/09 1376 90 6.513 Bullock Bank 11/07 2 2 100.014 Shingle Bank/Sovereign Shoals 11/07 2 2 100.015 Shingle Bank/Sovereign Shoals 12/07 1 1 100.016 South Devon inshore 1/08 1 0.017 South Devon inshore 7/08 1 0.018 South Devon inshore 8/08 1 0.019 South Devon inshore 9/08 1 1 100.020 Lizard/West Cornwall 6/09 31 4 12.921 Lizard/West Cornwall 7/09 2 2 100.022 Land’s End/Scilly 12/09 1 1 100.039
DST
Shingle Bank/Sovereign Shoals 8/08 32 12 37.540 Trevose Grounds 10/08 29 5 17.241 South Devon offshore E6 6/09 37 14 37.842 South Devon inshore 6/09 30 11 36.723 M 2T Shingle Bank/Sovereign Shoals 10/07 294 63 21.424 Bullock Bank 10/07 247 92 37.225 Portland 11/07 382 64 16.826 Land’s End/Scilly 6/08 86 8 9.327 South Devon inshore 6/08 100 19 19.0
99
0
10
20
30
40
0 10 20 30 40
Expe
cted
Observed
28 Trevose Grounds 10/08 115 10 8.729 Land’s End/Scilly 11/08 29 4 13.830 Lizard/West Cornwall 5/09 491 76 15.531 South Devon inshore 6/09 5 1 20.032 South Devon offshore E6 6/09 1 0.033 Mid Channel E7 8/09 570 42 7.434 Portland 12/07 4 0.035 Portland 2/08 1 0.036 Lizard/West Cornwall 6/09 11 2 18.237 Lizard/West Cornwall 7/09 1 0.038 Lizard/West Cornwall 5/10 1 0.044 DST Shingle Bank/Sovereign Shoals 3/10 16 7 43.8
Appendix V. Using data from the tagging programmes to provide estimates of exploitation
AV.1 Introduction
In this section we replicate Bennett’s 1979 analysis applying a slicing methodology to length frequency distributions taken during the current tagging programme and using catch curves to derive total mortality. We also take outputs from the dynamic population model (Hilborn, 1990; Aires-da-Silva et al., 2005) we fitted to data from both double T-bar and data storage tagging programmes to provide estimates of exploitation rate for the spatially defined fishing units we used in this model. We also considered a number of other (modern) methods for estimating mortality rates from tagging data that have been described in the literature, but many of these were age structured (e.g. McAllister et al., 2004; Michielsens et al., 2006; Polacheck, 2006) or based on intensive short term mark recapture experiments (Bell et al., 2003) and thus not particularly suited to our study. However, Brownie models (Brownie et al., 1978; 1985) provide a means of estimating mortality rates from tagging data and we utilise the age-independent instantaneous rates model (Hoenig et al., 1998), an updated Brownie model, applied to our tagging data using monthly time steps as a third approach for providing tag based estimates of mortality rates.
AV.2 Catch curve analysis
Methods
Bennett (1979) used estimates of moult increment and moult frequency to calculate an average growth curve and subsequently used this to split length frequency distributions into arbitrary sequential ages and applied catch curve methodology to estimate total mortality rate (Z). We replicated Bennett’s analysis for calculating average growth curves and moult slicing and apply his method (and parameters, see below) to length data obtained during our tagging programme. During our analysis of growth we noted that Bennett’s data were more comprehensive, especially so for males and for moult increment and we found no statistical difference between the parameters for fits to our moult increment data and Bennett’s fits to his data. We did note some possibility for differences in moult frequency, although not conclusive and we subsequently used the generalised anniversary method which gave slightly higher moult frequency rates again. We therefore use Bennett’s (1974; 1979) moult increment and frequency parameters using weights initially, but subsequently for females only (because insufficient growth data were available for males in our study) we apply moult frequency parameters obtained, from our moult frequency analysis and the generalised anniversary approach (McGarvey et al., 2002) using regressions on width along with moult increment parameters based on width and based on Bennett’s (1974) data. We apply these parameters to length frequency data for the fishery units used in the population dynamics model we used to analyse the tagging data and ‘slice’ these to produce age distributions to which we apply catch curve analysis.Catch curves provide a simplistic approach to estimating total mortality rate under steady state conditions by fitting a regression though the descending limb of log catch at age frequencies against age (or time). In Bennett’s (1979) approach the absolute age is undetermined, but the regression over relative ages still holds as a steady state estimation of Z. In theory the descending limb should
100
approximate a straight line, but this is often not the case so interpretation of catch curves becomes rather subjective. We use VBA code to estimate catch curves which calculates Z estimates for all permutations of points on the descending limb of the catch curve and we present the mode, maximum, minimum and a selected approximation of the Z resulting from the first approximately linear decline observed after the maximum, which we label 1st. This 1st estimate is influenced both by the growth parameters and the decline in frequencies, while later points (often the minimum) may be influenced by the growth parameters alone since multiple age slices may result from a single frequency at length class.
Results
Estimates of Z based on Bennett’s (1979) growth parameters and length distributions from this tagging study suggest relatively low total mortalities for female crabs, ranging from around 0.15 to 0.5 and relatively high total mortalities for male crabs with a range around 0.5 to 0.85 (Table AV.1). For both sexes total mortalities were lower in the eastern Channel, intermediate of the Devon coast and higher in west Cornwall.
Table AV.1. Estimates of total mortality, derived from age distributions obtained by slicing length distributions from this tagging programme using Bennett’s moult increment and frequency parameters
Z estimatesFemale Male
Area Mode Max Min 1st Mode Max Min 1st
Bullock Bank 0.2 0.22 0.08 0.15 0.35 0.65 0.1 0.5Shingle Sovereign
0.2 0.35 0.1 0.25 0.45 1.1 0.1 0.7
Portland 0.15 0.45 0.1 0.38 0.25 0.95 0.15 0.7S. Devon inshore 0.2 0.25 0.1 0.2 0.15 0.97 0.1 0.75S. Devon offshore 0.15 0.4 0.05 0.2Mid Channel 0.2 0.3 0.1 0.15 0.2 0.85 0.1 0.7Lizard 0.3 0.56 0.1 0.5 0.3 1 0.1 0.85Lands End 0.25 0.45 0.1 0.3 0.5 1.1 0.3 0.8Trevose 0.2 0.33 0.1 0.2 0.4 1.1 0.1 0.8
Applying the moult frequency parameters estimated for females using the generalised anniversary method with a growth season from May to July (moulting occurred at larger sizes in this scenario) results in considerably higher estimates for Z, with the modes approximately doubled and the 1st estimate in the range of 0.3 to 0.75 (Table AV.2). These estimates are based on the same length distributions used above so it is unsurprising that the pattern of lower mortalities in the eastern Channels and higher mortalities in west Cornwall is repeated.
Table AV.2. Estimates of total mortality for female edible crabs only, derived from age distributions obtained by slicing length distributions from this tagging programme using parameters for moult increment (on width) derived from Bennett’s (1974) data and moult frequency parameters (based on width) derived from this programmes data using the generalised anniversary method (McGarvey et al., 2002)
Z estimatesArea Mode Max Min 1st
Bullock Bank 0.4 0.6 0.1 0.34Shingle Sovereign 0.4 0.65 0.1 0.32Portland 0.4 0.7 0.05 0.45S. Devon inshore 0.35 0.65 0.1 0.37S. Devon offshore 0.35 0.8 0.1 0.32Mid Channel 0.35 0.6 0.1 0.34Lizard 0.55 0.9 0 0.75Lands End 0.45 0.8 0.1 0.6Trevose 0.4 0.6 0.1 0.43
101
Discussion
Estimates of Z derived using length distributions from this study and Bennett’s (1974; 1979) growth parameters (Table AV.I) were lower than those obtained by Bennett (1979) for females during spring (0.54 and 0.42 for inshore and offshore, respectively), but comparable with his figures for autumn (0.25 and 0.19 for inshore and offshore, respectively). Bennett’s lower estimate of total mortality during autumn for females is also somewhat counter intuitive, given that the peak of fishing activity and catches of females occur at this time, but he notes that this was due to changes in the distribution of larger females. Estimates of Z for males derived in the same manner (Table AV.I) had a lower mode than those estimated by Bennett (1979) (0.44-0.63), but a higher ‘1st’ estimate. It is unfortunate that the catch curve methodology permits an element of subjectivity and it is therefore difficult to make a direct comparison with this historical work. Using growth parameters estimated during this project which allowed for growth to continue at larger sizes for females resulted in estimates of Z that were generally comparable or higher than Bennett’s (1979) estimates.
The relatively low mortality estimates for females obtained from the initial catch curve analysis were somewhat counter intuitive, given the high proportion of females in the catch, particularly in the western Channel and Celtic Sea. This primarily reflects the fact that the moult frequency parameters for females suggest relatively little growth at moderate to large sizes which are common in the length frequency distribution. Thus overall the frequencies in the length distribution have not declined much by the time the crabs reach a relatively old age. As mentioned in the section on growth, most of the moult frequency estimates suggest that moulting ceases for females at around 180mm, but crabs of 203mm and greater occur with some regularity in the catch suggesting that growth continues above this size and that growth rates for larger crabs are under estimated. By contrast Bennett’s (1974; 1979) parameters indicate that growth of male crabs continues to greater sizes and male crabs were also sparsely sampled in this study with the result that decline in the frequency at age is sharp and estimates of Z are high. Fishing mortality estimates obtained from length based VPA (LVPA) carried out as part of routine stock assessments and using von Bertalanffy growth parameters where L∞ was fixed a priori at 240mm for both sexes (Addison & Bennett, 1992) are also high for edible crab stocks in these areas, typically in the range of 0.6-1.0 for females and 0.4 to 1.0 for males (Cefas, 2010).
Changing the moult frequency parameters used in the analysis to a set estimated by the generalised anniversary method which permitted moulting to occur at larger sizes highlights the sensitivity of the method to the growth parameters as total mortality estimates for females were approximately doubled. This is because under these growth conditions more large crabs can be produced so mortality is estimated to be higher in order to achieve the same decay pattern in the pseudo-age distribution. Relatively frequent observations of large crabs in the catch suggest that growth does continue at these large sizes and mortality estimates based on these growth parameters may therefore be more realistic, however it is important also to take them with some caution and not to ‘cherry pick’ results on the basis of prior perceptions. These results for total mortality are still low relative to those obtained using LVPA and the Addison & Bennett (1992) von Bertalanffy growth curve in the Cefas annual stock assessments for crabs (Cefas, 2010).
It is also important to note that in many cases these estimates of Z might be expected to include elements of emigration and immigration and the balance of these over time could also lead to biases if there is a difference in rates and sizes of animals migrating. Further the length distributions obtained during the tagging programme may not be representative of the population because crabs were fed to the tagging operatives by members of the crew and although they were frequently reminded to pass on crabs ‘as they came in the catch’ there will have been some temptation to land the more valuable larger crabs and pass on the less valuable smaller ones. Nonetheless the size range of tagged crabs includes significant numbers of large crabs.
AV.3 Exploitation rates derived from fitting a population dynamics model to the tagging data
Appendix V.2 considered estimates of total mortality derived from growth models and length data, whilst this section derives exploitation rates from outputs of a dynamic model fitting movement and catchability parameters to spatio-temporal tag return and commercial fishing effort data. These estimates of mortality rates are thus derived from completely different information sources.
102
Methods
The population dynamics model used (Hilborn, 1990; Aires-da-Silva et al., 2005) and its implementation has been described in Appendix IV.
Results
Model outputs relating to movement and catchability are provided in the section on the dynamic model. Here we provide further results specifically relating to exploitation, derived as the product of catchability and fishing effort. These estimates should be treated with some caution bearing mind that this model is still in relatively early phases of development and that while it seems to fit well in many instances, some of the movement results commented on in section AIV.4 (i.e. easterly movements) appear to be erroneous.
Time series of exploitation rate estimates (Figure AV.2) are similar to fishing effort in overall trend (Figure AV.1), showing marked seasonal variations with 3 clear peaks during the study period. However, there are also clear differences in exploitation rates between the areas modelled. High exploitation rates for females are apparent in the Lands End & Scilly and south Devon inshore areas with relatively low or moderate exploitation on males in these areas. By contrast exploitation rate is estimated to be higher for males in the Trevose Grounds, south Cornwall, mid Channel and Wight and west Sussex areas, while females and males have similar levels of exploitation in the Lizard & west Cornwall, Portland, Shingle Sovereign and Bullock Bank areas. It is noticeable that in some areas (Trevose Grounds, Lizard & west Cornwall and south Devon inshore) male exploitation rate remains at moderate levels between peaks or throughout the year. In the Trevose area the peaks for males occur at different times of the year, but this does not seems to be the case in any other areas. Exploitation rates were estimated to be negligible in the south Cornwall offshore and south Devon offshore areas, reflecting low effort recorded from the former and relatively low tag return rates in these areas.
Annual exploitation rates (averaged over months) varied widely between areas and sexes (Table AV.3). Exploitation rates for females were very low or negligible during winter & spring in all areas except the Lizard & west Cornwall which was previously noted as fitting very poorly and seemingly providing erroneous results. They were higher during the summer & autumn season except for the south Cornwall offshore area where exploitation for females was estimated as zero in both seasons and the Lizard & west Cornwall where it was very low (negligible) in summer & autumn. Exploitation rates for females were also negligible in both seasons for the South Devon offshore, mid Channel and Wight & west Sussex areas. Non-negligible summer & autumn female exploitation rates occurred in Bullock Bank (0.15), Shingle & Sovereign (0.34), Portland (0.25), south Devon inshore (0.67), Lands End & Scilly (0.94) and Trevose Grounds (0.22).
Estimated exploitation rates for males were higher during winter & spring than during summer & autumn, although this was not the case for south Cornwall inshore, Portland, Shingle Sovereign and Bullock Bank where summer & autumn exploitation rates ranged between 0.19 and 0.6. Winter & spring exploitation rates in the 0.4-0.5 range were apparent for the Trevose Grounds, Lizard & west Cornwall, mid Channel and Wight & west Sussex. Summer & autumn exploitation rates were also moderate (0.2-0.25) for the Trevose Grounds, Lizard & west Cornwall and for were similar for males in both seasons in south Devon inshore (0.17 and 0.23).
Table AV.3. Average annual exploitation rates during the study period by sex and fishery areaAverage exploitation rate
Fishery area Female MaleSummer Autumn
Winter Spring
Summer Autumn
Winter Spring
Trevose Grounds 0.22 0.02 0.25 0.53Lands End & Scilly 0.94 0.06 0.09 0.06Lizard & west Cornwall 0.02 0.49 0.20 0.43
103
South Cornwall inshore 0.02 0.00 0.60 0.00South Cornwall offshore 0.00 0.00 0.00 0.00South Devon inshore 0.67 0.02 0.17 0.23South Devon offshore 0.02 0.00 0.03 0.00Mid Channel (E7) 0.02 0.01 0.07 0.48Portland 0.25 0.05 0.20 0.05Wight & west Sussex 0.03 0.01 0.05 0.50Shingle Sovereign 0.34 0.02 0.34 0.14Bullock Bank 0.15 0.01 0.19 0.04 Discussion
Fishing effort data (pot hauls per month) extracted from a database of officially reported commercial data (Figure AV.1) show the highly seasonal nature of the pot fisheries in the area which target edible crabs primarily during the late summer and into autumn, with females making up the majority of the catch. In most of the fishery areas 3 clear peaks can be seen following the last three study years (2008, 2009, 2010), with the start of the study field programme (October 2007) corresponding with the downward trend of effort as the main season for that year drew towards its close. Historically there have been difficulties in obtaining reliable data on fishing effort in terms of pot hauls. The targeting of different species (e.g. lobsters, edible crabs, spider crabs) in different seasons and areas, the use of different pot designs (e.g. inkwell pots, parlour pots, soft eyed creels) and variations in soak time all add further complexities to consideration of fishing effort, so these data need to be treated with some caution. Nonetheless, they present plausible overall trends.
Figure AV.1. Spatial and temporal distribution of fishing effort for the study area and duration
The majority of female edible crabs spawn in late autumn and remain ovigerous over the winter and spring during which time they are relatively inactive and catchability in pots is low. This feature is captured by the tag returns and population dynamics model, with winter and spring exploitation rates for females very low with the exception of the Lizard & west Cornwall area, where we believe there are data and model fitting problems. However, female crabs are still captured over the winter, although vessels active at this time of the year may focus more towards capturing males, which may
104
involve utilising slightly different fishing grounds in some cases. The population dynamics model suggested high winter and spring exploitation rates for males in five areas and although summer and autumn rates were also higher (than winter and spring) in five other areas, the absolute level of these was generally lower.
Spatial consideration of exploitation rates suggests that the south Devon inshore and Lands End & Scilly fisheries are exerting high levels of exploitation on females, which ties in with these two areas supporting substantial fisheries on pre-spawning females and high crab larvae densities being found in these areas indicating them as proximal to major spawning locations. However, plankton surveys also indicate high levels of early stage crab larvae around the Lizard and Trevose headlands. The Lizard & west Cornwall area does indicate high exploitation on both females and males, but the Trevose Grounds area exhibits much higher exploitation for males than females and also indicates a disparity between seasons, with the highest exploitation rates for males occurring during winter and spring. The North Cornish coast is another area where lobster fisheries are important and these may impact on the effort figures in this area.
The high exploitation rates for males in some areas of the western Channel and Celtic Sea (Trevose Grounds, Lizard & west Cornwall, south Cornwall inshore) were somewhat surprising given the highly skewed sex ratio that occurs in these fisheries with the annual landings consisting of around 80% females. However, we have already commented on potential problems with data and model fitting in the Lizard area and return rates from the Trevose Gounds were relatively low, whilst the south Cornwall inshore area was not an area where the project was active, and had specific tag releases. In the eastern Channel, the Wight & west Sussex area was considered poorly sampled as the project had no presence in this area, there was no release of tagged crabs and the area is generally more important for lobster fisheries than for crabs. The population dynamics models produced generally unconvincing outputs for the offshore areas, which suffered from low return rates, in some cases poor effort data and were considered to fit relatively poorly in the movements section, where we noted a population sink in the mid-Channel area. Further east, the Sovereign & Shingle and Bullock Bank areas were considered generally well sampled as relatively few vessels fish the area and the main fisherman in the area supported the project and played an active part in the tagging programme. Here, there is less bias in sex ratio in the landings and exploitation rates were similar for both sexes and moderate in level, peaking at around 0.5.
105
Figure AV.2. Spatial and temporal distribution of estimated exploitation rate for the study area and duration
The results for females in LandsEnd & Scilly and Lizard & west Cornwall need to be treated with extreme caution as clear problems with the results for the model were noted in this area with respect to movements. Similarly results from the offshore areas may be poorly determined as in these areas return rates were generally low and the effort data may also not be comprehensive as it did not include French activity. The remaining results suggest that during summer and autumn moderate exploitation levels occur in the Shingle Sovereign area, lower on Bullock Bank and very low in the Wight and west Sussex area, high exploitation occurs in the south Devon fishery, and moderate exploitation on the Trevose Grounds and at Portland. Winter & spring exploitation rates were very low for females. These results are all plausible and not dissimilar in distribution to those of the catch curve analysis. The results for males need to be treated with more caution as far fewer were tagged, but the occurrence of high winter & spring mortality rates in many areas is in keeping with the behavioural aspects of edible crabs and the fisheries prosecuting them.
As a first attempt at fitting complex population dynamics models of this type, it is difficult to be conclusive about the results, which were plausible in some instances but clearly erroneous in others (see movement section). Overall, key features such as the seasonality of exploitation and very high exploitation rates in areas where pre-spawning females congregate are captured, but in other areas the results are less convincing. Losses to the system were high in some areas and the fact that there is a large boundary from which crabs can be lost from the system but not return, whilst in reality they may move in and out or through this area causes problems in modelling the dynamics. Further, we assumed a constant reporting rate for all areas which we believe is unlikely as return rates varied widely, however, introducing more complexity into the model is likely to lead to confounding, in particular between catchability, reporting rates and tag loss rates. Other complexities such as potential seasonal patterns in natural mortality were also not modelled and might also confound with the factors listed above. The seasonality of crab catchability is a key feature of the fisheries, but capturing precise information regarding the exact timing of the different behaviours, particularly for females remains problematic. We had initially considered a 3 season model to try and explicitly capture biological aspects associated with the moult cycle, but had to simplify the model for implementation and minimisation reasons. Potential future application of this modelling approach is considered further in the movements section.
AV.4 Brownie multi-year tagging model
Section AV.2 derived estimates of total mortality from growth models and length data, and Section AV.3 used outputs from a dynamic model fitting movement and catchability parameters to spatio-temporal tag return and fishing effort data to estimate exploitation rates. In this section we apply the age-independent rates model (Hoenig et al., 1998) to temporally structured tag release and recapture data only to derive a further set of mortality estimates.
Methods
Brownie multi-year tagging models (Brownie et al., 1978; 1985) generalise earlier work by Seber (1970) and Robson & Youngs (1971) and consider survival and tag recovery rates through time. The latter consists fishing mortality, tag reporting and tag shedding and mortality components, which can be separated if prior information is available for some of them, for example, information on retention and survival rate and reporting rate can permit estimation of exploitation rate (Pollock et al., 1991).
For this evaluation we use the implementation provided by the irm_h function, written by Gary A. Nelson (Massachusetts Division of Marine Fisheries) in the R package ‘fishmethods’. This function implements the age-independent instantaneous rates tag return model of Hoenig et al. (1998) following the methods of Burnham & Anderson (2002) to compare observed and expected recovery matrices, assuming recoveries follow a multinomial distribution and to calculate summary statistics. The program permits different configurations of model structure for the estimation of fishing and natural mortalities to try and accommodate biological realism.
106
The Hoenig et al. (1998) model assumes that all tagged fish are fully recruited to the fishery and that an initial tag loss rate and reporting rate are known. These assumptions are largely met in our case by using data from our tag retention experiments and outputs from the population dynamics model as estimates of the latter two parameters. We use the reporting rate estimated for females (from our dynamics model) since there is no reason to believe that reporting rate would be different between crab sexes and more data were available to fit the model for females. For all runs the initial tag loss rate was therefore set at 0.2 and the reporting rate set at 0.629.The influence of these parameters was not investigated at this stage, because they are both based on data, however it would be preferable to investigate a wider range of possible parameters in future work.
The irm_h function includes 3 sets of parameters to ‘seed’ and constrain the estimates of fishing and natural mortality and these can impact on the results. We ran the function for 102 permutations of individual and combined tag release batches, using 3 different sets of constraining parameters of which 88 converged successfully. A fourth set of more highly constraining parameters was also applied to a small subset of data. In all runs we used F=0.05 and M=0.01 as the initial parameters for the model and estimated a constant natural mortality rate over the entire period.
Diagnostics from the function include pooled and un-pooled parameters for over-dispersion (c-hat). Burnham & Anderson (2002) note that the over-dispersion factor (c-hat) should normally be relatively small, ranging from just over 1 to around 4 due to small violations of assumptions such as independence and parameter homogeneity over individuals. Larger values for over-dispersion (6-10) are usually caused by inadequate model structure; the fitted model does not explain an acceptable amount of variation. The model authors (Hoenig & Nelson) advise that the pooled c-hat value should be used (pers. comm. G. Nelson).
Results
There were sometimes problems with convergence of the model depending on the data set and starting parameters specified. Convergence and goodness of fit criteria for the model runs (Table AV.4) unsurprisingly suggest that Run 1, which is slightly less constrained, may have slightly better convergence properties. On the basis of pooled estimates for over-dispersion most of the model runs were acceptable, however, when using the pooled estimates the number of acceptable models was much reduced (Table AV.4).
Plots of 3 ‘reference’ data sets for which the model was run with alternative constraining parameters (Figure AV.3), show that in many cases the solution is not very sensitive to these unless the estimate of F actually lies outside the bounds in which case it is constrained. For the eastern Channel we used the Bullock Bank tag group as a reference as convergence was successful for all the constraint parameters used. In Run 1 when F was largely unconstrained F was very high in the second month (the first month was only partial) and for the final positive tag return. As the constraints on F were tightened these estimates were constrained within the bounds, but estimates of F derived from other returns (which were within bounds were relatively unaffected. Only in run 4 (the most constrained) was there a change in F amounting to more than 0.1 in these estimates. In most runs the estimated M, which is constant through time, tended to the lower bound constraint. The second ‘reference’ dataset utilised 2 tag groups released off south Devon on the inshore grounds. In this case for run 1 the estimates of F for the first recaptures appeared more consistent with the other data, whilst estimates for the final tag returns were again at the upper F bound. Changing the constraints (on F and M) did seem to make more difference to this data set with Run 2 resulting in lower F estimates towards for the later tag returns, whilst Run 3 was more similar to run 1. However, as with the Bullock Bank tag group the estimates of F for the early and intermediate seasons were generally consistent between runs. The third dataset was based on a tag group released around Lands End and again had similar features. All runs except Run 4 (most constrained) were consistent with the exception of the final tag returns which tended to the upper F boundary (Figure AV.3).
107
Figure AV.3. Fishing mortality estimates for female crabs estimated using the irm_h function with 3 datasets under 4 different sets of parameter constraints. All runs: initial F=0.05 & M=0.01, Run 1: lower limit F=0.0001 & M=0.0001, upper limit F=5.0 & M=0.5 , Run 2: lower limit F=0.0001 & M=0.01, upper limit F=1.5 & M=0.5, Run 3: lower limit F=0.0001 & M=0.01, upper limit F=1.0 & M0.2, Run 4: lower limit F=0.0001 & M=0.01, upper limit F=0.5 & M=0.1 Note of these runs, those for Bullock Bank had unacceptable over-dispersion values, whilst the others were within recommended c-hat limits
Plots of fishing mortality from all ‘successfully’ converged model runs where the pooled over-dispersion factor was <4 and using the constraint parameter set giving the best diagnostics (Figures AV.4-AV.5) are highly variable possibly, indicating that the data may not always be conclusive. The failure of some runs to converge is also likely to be an indication of this. In particular, the first month (or two) tend to give high estimates of F and the estimate of F based on the final tag recovery is often (but not always) at the upper limit of F. Estimates of natural mortality (M) were estimated as constant for all time periods and tended to be at the minimum constraint level set. However, the plots are also consistent in some features, clearly the highly seasonal nature of the crab fisheries and suggesting strongly that during the fishing season, fishing mortality on crabs may be very high. The pooled over-dispersion parameters for all runs on females in the eastern Channel were high, although the un-pooled values were within recommended limits and the different data sets gave highly consistent results (Figure AV.6). The plots for the Devon and Cornwall fisheries were very noisy, possibly
108
reflecting the greater complexity of spatial units than in the eastern Channel and shorter times at liberty. They tend to suggest higher fishing mortalities than in the eastern Channel although the latter should be treated with caution due to problems with over-dispersion. Not surprisingly, because releases and returns of males were far fewer, the plots for males tend to be very noisy, but may show less marked seasonality and also suggest high fishing mortalities.
Discussion
Hoenig et al. (1998) suggest that most tag loss and tag associated mortality occurs soon after release and that long term or chronic tag shedding is minimal and the ‘fishmethods’ irm_h function models the initial tag loss in this way. This may not necessarily be the case for animals such as crabs that grow by moult, but our aquarium experiments and double tagging data both suggest that initially tag loss is quite high, but that this stabilises with retention rates of around 75-80% subsequently persisting over a long period of time. Our aquarium experiments suggested retention rates through the moult were similar, although this was for a very limited dataset. However, the higher rate of initial tag loss occurred over a period of 180 days in the aquarium experiments and over around 100 days in the double tagging field experiments, which at our monthly time step is not truly initial. Nonetheless, modelling tag loss in this way in this way is no worse, and may be better, than the continuous rate of loss approximation we used in our population dynamics model approach.
As with the population dynamics model approach this Brownie model analysis required utilisation of all tag returns, so where data relating date of recapture were assigned using best possible assumptions or pro-rated according to existing returns from the same release. This leaves the potential for the introduction of errors, so as with many of the results these should be treated with a significant degree of caution. The noisy nature of the outputs also gives cause for concern in this regard, however the approach provides a further indication that fishing mortality on edible crabs is high during the main periods of exploitation.
Additional model runs to evaluate the sensitivity of the model outputs to the input parameter for initial tag loss/tagging mortality, derived from the aquarium and field double tagging experiments in the populations dynamics models or the reporting rate input parameter derived from the population dynamics models, were not carried out. These parameters may influence the level of the mortality estimates derived and should be further evaluated as part of future ongoing work to utilise the tagging data more thoroughly.
Although the Brownie models can be used to estimate fishing mortality and natural mortality separately, in these investigations, natural mortality always tended to the lower boundary limit. We used a constant natural mortality for all time steps in the model and it would be interesting in future work to see if by including other information such as fishing effort we could meaningfully estimate seasonal patterns in natural mortality. Such an investigation might be informative with regards to clarifying any relationship between natural mortality and the moult cycle.
109
Figure AV.4. Estimates of female monthly fishing mortality for individual, and combinations of, tag groups where irm_h model over-dispersion parameters were within acceptable limits
110
Figure AV.5. Estimates of male monthly fishing mortality for individual, and combinations of, tag groups where irm_h model over-dispersion parameters were within acceptable limits
Figure AV.6. Estimates of female monthly fishing mortality for individual, and combinations of, tag groups in the eastern Channel. Note. irm_h model pooled over-dispersion parameters were outside acceptable limits, whilst un-pooled parameters were within limits
111
Table AV.4. Diagnostic outputs for the instantaneous rates model runsRun Area Neg. Log-
LikelihoodK AIC AICc Eff. Sample
SizeUnpooled Chi-square
Unpooled df Unpooled c-hat
Pooled Chi-square
Pooled df Pooled c-hat
F R1 ShingleSov -938 40 1956 1959 1095 84 38 2.2 83.6 12.0 7.0F R2 ShingleSov -942 40 1964 1967 1095 94 38 2.5 93.5 11.0 8.5F R3 ShingleSov -942 40 1964 1967 1095 94 38 2.5 93.8 11.0 8.5F R1 Bullock -758 40 1596 1600 832 93 38 2.5 93.3 9.0 10.4F R2 Bullock -761 40 1602 1607 832 102 38 2.7 102.2 7.0 14.6F R3 Bullock -763 40 1605 1609 832 107 38 2.8 106.7 8.0 13.3F R1 EastChanDST Failed to convergeF R2 EastChanDST -54 30 168 2028 32 79 28 2.8 77.7 1.0 77.7F R3 EastChanDST -54 30 168 2028 32 82 28 2.9 78.7 1.0 78.7F R1 EastChan Failed to convergeF R2 EastChan -1736 40 3553 3554 1927 194 38 5.1 193.7 15.0 12.9F R3 EastChan Failed to convergeF R1 Portland -868 39 1814 1816 1344 7 37 0.2 6.5 11.0 0.6F R2 Portland -869 39 1816 1819 1344 9 37 0.2 9.1 11.0 0.8F R3 Portland -869 39 1816 1819 1344 9 37 0.3 9.4 11.0 0.9F R1 SDevonIn -1132 32 2327 2329 1495 32 30 1.1 31.5 11.0 2.9F R2 SDevonIn -1134 32 2332 2334 1495 38 30 1.3 37.8 11.0 3.4F R3 SDevonIn -1134 32 2333 2334 1495 38 30 1.3 38.5 11.0 3.5F R1 SDevonIn2 -1231 20 2502 2503 1472 53 18 3.0 53.3 10.0 5.3F R2 SDevonIn2 -1234 20 2508 2509 1472 61 18 3.4 61.1 10.0 6.1F R3 SDevonIn2 -1234 20 2509 2509 1472 61 18 3.4 61.3 10.0 6.1F R1 SDevonInComb -2367 32 4799 4800 2967 96 618 0.2 95.3 328.0 0.3F R2 SDevonInComb -2396 32 4857 4858 2967 193 618 0.3 191.4 328.0 0.6F R3 SDevonInComb -2373 32 4810 4811 2967 111 618 0.2 109.8 328.0 0.3F R1 SDevonOff -123 20 285 288 316 0 18 0.0 0.0 4.0 0.0F R2 SDevonOff -123 20 285 288 316 0 18 0.0 0.0 4.0 0.0F R3 SDevonOff -123 20 285 288 316 0 18 0.0 0.0 4.0 0.0F R1 MidChannel Failed to convergeF R2 MidChannel -504 18 1045 1045 1376 0 16 0.0 0.0 9.0 0.0F R3 MidChannel -504 18 1045 1045 1376 0 16 0.0 0.0 9.0 0.0F R1 SDevonInDST Failed to convergeF R2 SDevonInDST -49 20 138 231 30 70 18 3.9 69.0 1.0 69.0F R3 SDevonInDST -49 20 138 232 30 72 18 4.0 70.3 1.0 70.3F R1 SDevonOffDST -78 20 196 248 37 99 18 5.5 98.5 1.0 98.5F R2 SDevonOffDST -79 20 198 250 37 109 18 6.1 108.3 1.0 108.3F R3 SDevonOffDST -79 20 198 251 37 112 18 6.2 109.6 1.0 109.6F R1 DevonDorset Failed to convergeF R2 DevonDorset Failed to convergeF R3 DevonDorset -3931 39 7940 7940 6003 197 1171 0.2 190.1 634.0 0.3
112
F R1 LandsEnd -892 32 1849 1850 1293 21 30 0.7 20.8 7.0 3.0F R2 LandsEnd -893 32 1851 1853 1293 23 30 0.8 23.3 7.0 3.3F R3 LandsEnd -894 32 1851 1853 1293 23 30 0.8 23.5 7.0 3.4F R1 LandsEnd2 -1090 27 2235 2236 1470 85 25 3.4 84.9 3.0 28.3F R2 LandsEnd2 -1096 27 2247 2248 1470 101 25 4.0 100.5 4.0 25.1F R3 LandsEnd2 -1100 27 2255 2256 1470 112 25 4.5 111.9 4.0 28.0F R1 LandsEndComb -1995 32 4055 4055 2763 206 310 0.7 120.0 156.0 0.8F R2 LandsEndComb -2002 32 4068 4068 2763 208 310 0.7 140.0 157.0 0.9F R3 LandsEndComb -2006 32 4076 4077 2763 220 310 0.7 138.6 156.0 0.9F R1 Lizard Failed to convergeF R2 Lizard -419 21 879 881 841 0 19 0.0 0.0 7.0 0.0F R3 Lizard -419 21 879 881 841 0 19 0.0 0.0 7.0 0.0F R1 Trevose -833 28 1721 1722 1680 0 26 0.0 0.0 7.0 0.0F R2 Trevose -832 28 1721 1722 1680 0 26 0.0 0.0 6.0 0.0F R3 Trevose -833 28 1721 1722 1680 0 26 0.0 0.0 7.0 0.0F R1 TrevoseDST -20 28 95 Inf 29 9 26 0.4 9.2 1.0 9.2F R2 TrevoseDST -20 28 96 Inf 29 12 26 0.5 11.7 1.0 11.7F R3 TrevoseDST -21 28 97 Inf 29 14 26 0.5 13.0 1.0 13.0F R1 Cornwall -3292 32 6648 6648 5284 640 580 1.1 469.2 310.0 1.5F R2 Cornwall -3298 32 6660 6660 5284 673 580 1.2 504.0 310.0 1.6F R3 Cornwall -3298 32 6660 6660 5284 673 580 1.2 504.0 310.0 1.6
Run Area Neg. Log-Likelihood
K AIC AICc Eff. Sample Size
Unpooled Chi-square
Unpooled df Unpooled c-hat
Pooled Chi-square
Pooled df Pooled c-hat
M R1 ShingleSov Failed to convergeM R2 ShingleSov -256 40 592 605 294 22 38 0.6 21.6 6.0 3.6M R3 ShingleSov Failed to convergeM R1 Bullock -323 40 725 741 247 183 38 4.8 181.8 2.0 90.9M R2 Bullock -317 40 713 729 247 142 38 3.7 141.8 3.0 47.3M R3 Bullock -319 40 718 734 247 154 38 4.1 154.1 3.0 51.4M R1 EastChanDST -30 11 83 149 16 51 9 5.6 50.7 0.0 InfM R2 EastChanDST -31 11 83 149 16 54 9 6.0 53.9 0.0 InfM R3 EastChanDST -31 11 84 150 16 57 9 6.4 57.0 0.0 InfM R1 EastChan -581 40 1243 1249 541 124 38 3.3 124.2 6.0 20.7M R2 EastChan Failed to convergeM R3 EastChan -584 40 1249 1255 541 134 38 3.5 134.0 6.0 22.3M R1 Portland -278 39 635 644 382 6 37 0.2 6.1 7.0 0.9M R2 Portland -279 39 636 645 382 7 37 0.2 7.2 7.0 1.0M R3 Portland -279 39 636 645 382 7 37 0.2 7.4 7.0 1.1M R1 SDevonIn -76 32 216 248 100 5 30 0.2 5.0 2.0 2.5M R2 SDevonIn -76 32 217 248 100 6 30 0.2 5.6 2.0 2.8M R3 SDevonIn -77 32 217 249 100 6 30 0.2 6.2 2.0 3.1M R1 MidChannel -221 18 479 480 570 0 16 0.0 0.0 6.0 0.0
113
M R2 MidChannel -221 18 479 480 570 0 16 0.0 0.0 5.0 0.0M R3 MidChannel -221 18 479 480.16 570 0 16 0.0 0.0 6.0 0.0M R1 DevonDorset -581 39 1240 1243 1052 26 1171 0.0 15.1 604.0 0.0M R2 DevonDorset -582 39 1241 1244 1052 30 1171 0.0 16.6 604.0 0.0M R3 DevonDorset -582 39 1242 1245 1052 31.33 1171 0.0 16.7 604.0 0.0M R1 LandsEnd -39 32 141 181 86 0 30 0.0 0.0 3.0 0.0M R2 LandsEnd -39 32 141 181 86 0 30 0.0 0.0 3.0 0.0M R3 LandsEnd -39 32 141 181 86 0 30 0.0 0.0 3.0 0.0M R1 LandsEnd2 -16 27 86 1598 29 0 25 0.0 0.0 1.0 0.0M R2 LandsEnd2 -16 27 86 1598 29 0 25 0.0 0.2 1.0 0.2M R3 LandsEnd2 -16 27 86 1598 29 0 25 0.0 0.3 1.0 0.3M R1 LandsEndComb -55 32 174 200 115 1 310 0.0 0.7 150.0 0.0M R2 LandsEndComb -55 32 174 200 115 1 310 0.0 0.8 150.0 0.0M R3 LandsEndComb Failed to convergeM R1 Lizard Failed to convergeM R2 Lizard -308 21 658 660 491 4 19 0.2 4.4 4.0 1.1M R3 Lizard -308 21 658 660 491 5 19 0.2 4.7 4.0 1.2M R1 Trevose Failed to convergeM R2 Trevose -45 28 147 166 115 36 26 1.4 1.5 2.0 0.7M R3 Trevose -43 28 141 160 115 0 26 0.0 0.0 1.0 0.0M R1 Cornwall -411 32 886 889 721 18 580 0.0 8.1 298.0 0.0M R2 Cornwall -411 32 886 889 721 18 580 0.0 8.5 298.0 0.0M R3 Cornwall -411 32 887 890 721 17.4 580 0.0 9.5 299.0 0.0
114
Appendix VI. Using the double T-bar tagging data to estimate growth rates
AVI.1 Introduction
Estimation of growth rate for crustaceans is difficult because they grow by moulting, casting off their old exo-skeleton and absorbing water to stretch the exo-skeleton while it is soft. This means that growth is discontinuous and that there are no hard parts (such as scales or otoliths in fish) that can provide a record of age and growth. Tagging studies can be informative with regards growth and we therefore utilise the data collected in this programme together, and in comparison, with data collected from crabs in the English Channel during the 1970s (Bennett, 1974). Growth can be by explicitly modelling the moulting process, usually through moult frequency (i.e. the probability of an individual moulting annually or the proportion of the population moulting annually) and moult increment (the absolute or relative increase in size occurring during a moult) functions. Both of the latter are usually modelled as a function of pre-moult size.
Although individually crustaceans grow by moult and traditional continuous functions such as the von Bertalanffy (1938) model are therefore not strictly appropriate, when averaged across individuals continuous growth models are considered useful and appropriate for explaining growth at the population level. Further, continuous growth models are important in supporting stock assessment methods such as length converted catch curves (Sparre & Venema, 1998) and length cohort analysis (Jones, 1984) where they provide a means to transform length structured data into pseudo-age structured data.
AVI.2 Discontinuous growth
Methods
Moult increment
Mean growth increment (G) can be modelled as a linear function of size prior to moulting (l).
Variation in growth increment per moult was assumed to follow a beta distribution by Bergh & Johnston (1992), but has also be described using a gamma distribution, which is versatile and can approximate several functional forms (Sullivan et al., 1990; Zheng et al., 1995; 1996; 1998; 2002). where x is growth increment per moult and l and are parameters. Mean growth increment is given by l and is equal to Gl for a given length class l. Therefore
and growth can be represented by two parameters Gl and . The expected proportion of crabs moulting from length class l to length class l' is given by the integral of the gamma function over the receiving length class range (Zheng et al., 1995).
Other authors (Bennett, 1974, Latrouite & Morizur, 1988) have assumed linear models with normal errors, sometimes using proportional (relative to pre moult size) rather than absolute increments and Bennett (1970) used weight rather than carapace width for the majority of his growth modelling. In order to facilitate comparison we transform Bennett’s data and results in to crab width.
Moult frequency
In order to obtain annual moulting rate we initially apply the anniversary method (Hancock & Edwards, 1967), sampling those returns with a time at liberty approximating a year with a 12 week tolerance (281 - 449 days) as suggested by Bennett (1974), but subsequently we also apply the generalised observed moulting rate approach of McGarvey et al, (2002).
Zheng et al.( 1995; 1998) used a reverse logistic function to model moult frequency.
115
Gl=a+bl
g (x|αl , β )= xαl−1e
− xβ
βα lΓ (α l)
α l=Glβ
Pl ,l '
= ∫lmin' −lmid
lmax' −lmid
g (x|αl , β )dx
Pml=1
1+φ e−ϖι
where and are parameters and is the mean length of the length class. This is appropriate for binomial data, e.g. where a crab either moults or does not in any one year. However, it restricts moult frequency to between 0 and 1 and is not suited to modelling the occurrence of multiple moults in any one year. Our data set contained 4 reported growth increments that were considered double moults, however, for the purposes of fitting this model we assume these simply as a single moult occurrence.
Bennett (1974) also estimated moult frequencies from the tagging data and again he preferred to use weight rather than size and fitted a log linear relationship between moult frequency and weight using a regression on the proportions moulting weighted by the number of returns in each length class. where a and b are the slope and intercept of the weighted regression, Wt is weight at length calculated using a weight length relationship and a constant of 1 has been added to prevent problems taking logarithms of zero. Bennett (1974) expressed his moult frequencies as percentages.
Latrouite and Morizur (1988) similar fitted a linear model with normal errors to proportions moulting annually by length class as an estimate of moult frequency.
McGarvey et al. (2002) generalise the anniversary method by excluding data that fall within a known moulting season, then calculating the proportion moulting by the number of growth seasons the crabs are at large and fitting a regression line weighted by the proportion of observations in each growth opportunity category. This permits the inclusion of data for crabs that have been at liberty over a wider range of time periods and allows for the inclusion of double moulting animals. Due to the scarcity of our data relating to males we only apply this approach to the data for females. The method consists of excluding recaptures taken during the moulting period and then calculating the number of growth seasons experienced by each recapture during its time at liberty. The ‘observed moulting rate’ is calculated for each number of growth opportunities (0, 1, 2 or 3 for our data) and a range of size classes. For each size class the slope of a regression (fixed at the origin) of observed moulting rate against number of growth opportunities provides an estimate of annual moulting rate and the reciprocal of this is the intermoult period.
Models were fitted using the R statistical modelling environment (R 2.10.1, R Development Core Team, 2009).
Data selection and quality control
Information on sizes of recaptured crabs was obtained from the fishing industry, with a highly varying degree of completion and accuracy. For growth studies the full tagging dataset was screened and records were excluded for any of the following reasons: no release measurement, no recapture measurement, return details differed from release details in tag colour or crab sex or the crab appeared to have moved faster than was possible. This resulted in a dataset of just over 1700 records of which a substantial proportion were negative and potentially provide an indication of the level of error in recording and reporting crab size. A histogram of increment distribution (Figure AVI.1) indicated some positive bias, but also a large peak at zero. The large peak at zero consisted of many returns from fishermen that had helped in the tagging programme. They were highly motivated to contribute and had interacted with the scientists measuring crabs while tagging and releasing and therefore inclined to measure relatively accurately. The secondary peak around 4mm suggests a slight measurement bias in the distribution may represent slight over-estimation of carapace width. This could result from the curvature of a crab’s back; possibly resulting from the use of tape measures we had distributed to fishermen to help with measuring. The remaining negative errors are either very poor estimates of size or occasions where the tag numbers have been misreported or misrecorded.
116
Pml=exp (aWt+b )−1
Figure AVI.1. Histogram of data available with information on size of recapture
If the distribution consisted of measurement error alone then it would be expected to be symmetrical and the negative values would provide an indication as to where the positive values were measurement errors and where they were real data. Although, this bias appears inconsistent; in some parts of the data, but relative accuracy of other parts (peaks at 0 - 1mm) we carry out analyses on both the ‘raw’ data and data adjusted for this bias. Taking the measurements at face value the negative increments suggest that data from around 17mm or alternatively 20 - 25mm are likely to represent real growth increments. When adjusted for bias we suggest there is a natural break in the data at around 18mm and therefore we use data for values of 19mm or above as moult increments. Observing the positive ‘raw’ values for increments, there is a natural break in the data also at around 20-25mm and in particular at around 22mm. This equates to using the same set of data points for moult increment, but adjusting the increment down by 4mm when assuming a constant bias.
Figure AVI.2. Positive ‘raw’ data available with information on size of recapture showing potential beaks between numbers of moults. Left panel absolute increment, right panel relative incrementMcGarvey et al. (2002) suggest methodology to statistically separate the points which are borderline for 0 or 1 (and 1 or 2) moults. They suggest rigorously checking the data to ensure the time at liberty is consistent with allowing moulting and subsequently assume that borderline points have 3 possible states, 0 moults, 1 moult or outlier (e.g. recording error). From our screening we decided on borders of 21mm < one moult <58mm. Probability density functions were fitted to these to evaluate the likelihood of obtaining these 3 fits and others for
117
different permutations of borderline points. Although we did individually check the data for consistency with moult season as suggested by McGarvey et al. (2002), this is not conclusive as some edible crabs may moult outside the main moulting season. For short time at liberty moults we also checked that moulting was consistent with the shell condition recorded at release time (i.e. not ‘soft’ or ‘new’). These checks resulted in the exclusion of two points previously recorded as moults.
Results
Moult increment
On the basis of these screening plots and other data checks ‘raw’ increments of > 21mm were considered to be valid moult increments. For the upper margin, 5 increments between 60mm and 80mm occur at relatively small size and are considered either double moults (3 occurring during 2 to 3 years at large) or errors (1 occurring during 18 days at liberty) or unclear a male with a 75mm increment during 75 days at liberty). An increment of >100mm was considered an error as multiple moulting by such a large crab in a time period of 1.2 years would be extremely unlikely. These points were all excluded from this analysis of moult increment. This results in datasets consisting of 54 points for females and just 7 points for males.
A linear relationship of relative moult increment to premoult size for female crabs declines with increasing size, with most data falling in the range of 15-30% increase in size during moulting (Figure AVI.3, left panel). A linear relationship of absolute moult increment with premoult size for female crabs increases very slightly but is approximately level (Figure AVI.3, right panel). Most data for absolute moult increment fall between around 25mm and 47mm. Adjusting for bias shifts the relationship for absolute increments down by 4mm, whilst the proportional relationship is also shifted down, but changes slightly because the bias adjustment is applied to the increment but not the premoult size.
Estimated linear relationships for male moult increments (Figure AVI.4) should be treated with extreme caution as they are based on very sparse data, but they suggest both relative and absolute moult increment may increase with increasing size.
Figure AVI.3. Estimated linear relationships and 95% confidence intervals for moult increment with size for female edible crabs. Left panel: normal errors and relative moult increment, right panel: gamma errors and absolute moult increment
118
Figure AVI.4. Estimated linear relationships and 95% confidence intervals for moult increment with size for male edible crabs. Left panel: normal errors and relative moult increment, right panel: gamma errors and absolute moult increment
Table AVI.1. Parameters estimated for linear models of moult incrementData type Error
structureSex Intercept Probability
(intercept)Slope Pr.
(slope)n
Proportional Normal F 39.75 5.84e-07 -0.11348 0.0109 54Absolute Gamma F 32.71 0.00622 0.0096 0.8924 54Proportional Normal M 3.44 0.913 0.1106 0.571 7Absolute Gamma M -39.20 0.473 0.4558 0.208 7Proportional bias adjusted
Normal F 34.89 7.03e-06 -0.09233 0.0255 54
Absolute bias adjusted
Gamma F 28.68 0.0156 0.0098 0.8906 54
Figure AVI.5. Left panel: base line probability distributions for moult increment, right panel, moult increment distributions modified to include more data for one moult at the lower boundary (distributions for 1 and 2 moults scaled for the number of points fitted rather than the total)
A normal distribution fitted to the zero moult points and gamma distributions fitted to the one and two moult points for females on the basis of our initial screening suggests some overlap of the distributions at both 0-1 and 1-2 borders (Figure AVI.5, left panel). Carapace width was not taken into account because the slope of the fit to absolute increments with gamma errors was not significantly different from zero.
119
On the basis of these fits there is a less than 1‰ chance that 21mm is zero moults while there is more than 3% probability that 22mm is 1 moult. There is a 1‰ chance that 16mm is 1 moult, but a 1% probability that it is zero moults. An 18mm point has a 4‰ probability of being 1 moult and a 5‰ probability of having not moulted.
At the margin of one and two moults we refitted for two moulst to exclude the borderline point at 60mm. The probability that this (60mm) point falls into one moult is <2‰, while the probability that it falls into 2 moults is >1%. On the basis of this the 60mm point should be considered a double moult as was our original conclusion.
On this basis there would be scope for including more data at the lower boundary in the fitting of moult increment. Defining a new lower boundary of 19mm < one moult <58mm results makes slight differences to distributions (Figure AVI.5, right panel) and suggests that the mean absolute moult increment for female crabs is 31.6mm. However, it also broadens the overlap at both margins of the probability distribution, which is not considered advantageous.
Moult frequency
In order to model the annual moult frequency all the data where size information was available including negative values that were considered measurement error ( i.e. all data >-21mm) were initially selected, but subsequently restricted to recaptures that took place around the first anniversary of release with a tolerance of 12 weeks on either side. This resulted in data sets of 84 records for females and 7 for males. In the moult increment analysis we adjusted for a potential constant bias, but judged growth to have occurred in the same set of records, so for moult frequency analysis the data are unchanged under either scenario.
Estimated reverse logistic models for both females and males are presented (Figure AVI.6), but as with the moult increment results for males the relationship for moult frequency is based on too few data to be informative.
For comparative purposes, models similar to those used for edible crabs in the same region by other authors were also fitted (Figure AVI.7, Table AVI.2). These included a linear fit with normal errors (Latrouite & Morizur, 1988) and a log linear model with normal errors fitted against premoult weight (Bennett, 1974). Although it is possible to include double moults in this model, the double increments noted in the moult increment analysis did not fall within the anniversary period and therefore were not utilised. The log linear model was heavily influenced by the occurrence of a moult (proportion moulting = 0.5) for the largest size class which has high leverage. This return was individually checked in the database and appears to be a valid size measurement returned from a French fisherman during July after almost a year at liberty.
Figure AVI.6. Estimated reverse logistic models for annual moult frequency with 95% confidence intervals. Left panel females, right panel males
120
Figure AVI.7. Estimated linear models for female annual moult frequency with 95% confidence intervals. Left panel proportion moulting against width with Normal errors, right panel logarithmic transformation of proportion moulting against weight (back transformed to width) with Normal error structure
Table AVI.2. Parameters estimated for models of moult frequencyModel Error structure Sex Intercept Probability
(intercept)Slope Pr.
(slope)n
Reverse logistic
Binomial F 9.13116 0.01975 -0.06354 0.00815 84
Linear Normal F 1.638226 0.00701 -0.008521 0.01469 15Log linear on weight
Normal F 6.345077 0.000292 -0.005468 0.007506 15
Reverse logistic
Binomial M 3.13272 0.717 -0.02423 0.643 7
Generalised model for growth opportunities
The generalisation for observed moulting rate (McGarvey et al., 2002) requires that moulting season is known, so that recaptures during this time can be excluded and provide a clear categorisation of how many growth opportunities have been experienced. Female edible crabs are thought to moult (and mate) primarily during the summer, but there appears to be wide variation in this with historic datasets suggesting significant proportions of female edible crabs with ‘new shells’ as early as March in the western Channel and as late as December in the eastern Channel (Figure AVI.8). During the summer it is possible to find soft female crabs attended by a male in rock crevices at the lower extreme of the littoral zone (pers. comm. Smith). Whilst tagging during this study we recorded shell condition, for over 13000 female crabs in 5 different months. Proportion ‘new shell’ was highest in August (28%), followed by May (26%) and lowest in June (4%), with intermediate values in October (9%) and November (14%). The high proportion of recently moulted in August supports early summer moulting and the high value in May may reflect low catchability of the majority females which may still be ovigerous at this time. The low proportion of new shell in June suggests that either moulting may not have started or it may be taking place at this time, but the fully soft crab are not catchable. Whist acknowledging the variability in moult season we therefore assumed May to August as a baseline and evaluated sensitivity to a number of shorter permutations of these months as well as whether all or any of the moult season should count as a growth opportunity.
121
Figure AVI.8. Historic data on shell condition in the eastern (left panel) and western (right panel) Channel
Initially 5mm size classes were used, but this resulted in low frequencies in many size classes, so data were subsequently pooled to 20mm size classes, resulting in six classes with lower limits at 120mm to 220mm, the largest three of which had only two, two and one points, respectively (Figure AVI.9, left panel). Estimated intermoult frequency for female edible crabs was sensitive to the different scenarios regarding moult season and varied between 1.0-1.5 years for crabs in the 120-139mm size class, 1.7- 4.7 for the 140-160mm size class, 3.8-10.1 for the 160-180mm size class and 6.5-infinity for the 180-199mm size class (Table AVI.3). Results for the baseline scenario of moult season from May to August and the whole period required to constitute a growth opportunity suggest the female crab population is moulting annually between 120mm and 139mm, annual moult frequency decreases to around 0.6 per annum between 140mm and 159mm and between 160mm and 179mm crabs are moulting after around 3.8 years, annual moult frequency is around 0.26. The size class 180mm to 199mm are estimated not to moult with this scenario for moult season. However, restricting the growth season to May to July (Figure AVI.9, right panel) would result in growth continuing into this size range with an intermoult interval of 6.5 years, equivalent to an annual moult frequency of 0.15.
Table AVI.3. Estimates of intermoult period for different combinations of moult season and requirements to constitute a growth opportunityMoult season Requirement for growth Estimated intermoult period by size
class120-139mm
140-159mm
160-179mm
180-199mm
May to August Any part of growth season 1.0 3.3 8.3 InfinityMay to August All of growth season (base) 1.0 1.7 3.8 InfinityJune to August Any part of growth season 1.0 3.4 8.5 InfinityJune to August All of growth season 1.0 2.4 5.3 InfinityMay to July Any part of growth season 1.2 3.5 9.9 20.3May to July All of growth season 1.0 2.0 4.5 6.5June to July Any part of growth season 1.2 3.5 10.1 20.3June to July All of growth season 1.2 3.2 8.6 17.5May to June Any part of growth season 1.5 4.7 10.1 32.0May to June All of growth season 1.4 2.5 5.3 10.0
122
Figure AVI.9. Left panel: proportion moulting by number of moulting seasons and size class (baseline scenario), right panel: data and fitted regressions for size classes 120-139mm (blue), 140-159mm (red), 160-179mm (green) and 180-199mm (orange). Growth period scenario - May to July, whole growth period required
Discussion
Although a significant effort was made to publicise the need to collect size data through posters, press articles, the provision of tape measures and conversations with staff when in the field, many fishermen (around a third of all returns) did not report size data with their tag return data. Further, the quality of those size data obtained was highly variable, with some records very accurately replicating the release data, others showing a small positive bias as well as many appearing inaccurately measured both positively and negatively. This meant substantial screening of the data was required and reduced the amount of data available for analysis. For a similar overall number of tag returns in the 1970s, Bennett (1974) obtained substantially more information relating to growth. This may reflect a number of factors, including that staff were on the fishing grounds for much longer during Bennett’s programme, which also covered a smaller geographic range, and had a longer overall duration allowing more time to observe growth. There may also be differences in tag retention between the suture tagging method used by Bennett and our double T-bar tags, but our data did not indicate substantial tag loss associated with moulting. Both our tank experiments and the field double tagging programme suggested that tag loss is initially quite high but stabilises after up to 6 months at a level around 70%-80% retention, which then appears to be relatively stable in the medium term. Relatively few animals moulted in our aquarium experiment, but of those that did around 75% retained their double T-bar tags through the moult.
Our results provided a basis to estimate moult increment and moult frequency relationships for female crabs, but for males the data were insufficient to permit the estimation of meaningful relationships.
Moult increment
Results for moult increment from this study were similar to those of Bennett (1974), although this study sampled generally larger crabs, had a larger absolute size estimate for mean moult increment and a wider dispersion in moult increment (Figure AVI.10). Trends in moult increment from the current work were slightly lower than those estimated from Bennett’s (1974) data, a slight upward trend in absolute moult increment with increasing size and a downward trend in relative moult increment (Figure AVI.10). The largest increment in our data considered a single moult event was 52mm for a female crab of release size 165mm and that was at liberty for exactly a year October to October. Although this could represent a double moult, the duration and large size of the crab make this seem highly unlikely. It is not clear from Bennett’s data exactly what the criteria for determining that moulting had occurred or the boundary between single and double moulting. The lower margin of the distribution of points in Figure AVI.10 (right panel) is very similar to that chosen in our study as described previously. Another explanation for our slightly larger estimates of moult increment could be the small positive measurement bias that we noted in our data screening investigation. Adjusting our reported measurements down by 4mm to compensate for this bias reduces the differences between our data and fitted relationships and those of Bennett 1974. Latrouite & Morizur (1988) estimated parameters for a linear model of absolute size, fitted with normal errors, obtaining a relationship very similar to the fit (with gamma errors) to Bennett’s data.
123
Student’s t tests indicated statistically non-significant differences between both the slopes and intercepts for linear models of relative increments for data from this study compared with those of Bennett (1974). Similar tests for difference between the slopes and intercepts of relationships for the bias adjusted data and Bennett’s (1974) data were also non-significant.
Figure AVI.10. Comparison of data (unadjusted for bias) and estimated relationships for moult increment in this study with those estimated using Bennett’s (1974) data. Left panel relative moult increment, right panel absolute moult increment. Red: Bennett’s(1974) data, black: this study, blue: this study, fit to bias adjusted data, green: Latrouite & Morizur, 1988
Data from this study were very noisy and when following the rationale of McGarvey et al, (2002) there was overlap between the data that represent zero moults (i.e. measurement error and bias) and one moult. This approach suggested more data should be included at the lower margin of 1 moult and doing so reduced the mean moult increment (estimated with gamma errors and no relationship with size) from 34.26mm to 31.62mm again very similar to results obtained by Bennett (1974) and Latrouite & Morizur (1988). However it also increased overlap of the distribution with the neighbouring distributions on both upper and lower margins. Ultimately the data were very noisy and probably biased (in some cases). Although sophisticated statistical methods can be applied, the level of uncertainty in the raw data negates advantages in precision that these methods could bring.
Moult frequency
Moult frequency is rather more difficult to estimate than moult increment and a wider range of models have been used, i.e. non-linear as well as linear models are applied. Data may be very scarce particularly at larger sizes and where records (of moulting) are aggregated to form proportions. Data from this study suggested slightly higher moult frequency rates than Bennett’s (1974) data and this was reflected in the estimated log linear relationship on weight (Figure AVI.11, left panel). Student’s t tests indicated statistically non-significant differences between the slopes for loglinear models of moult frequency on premoult weight for data from this study compared with those of Bennett (1974), but the intercepts were statistically significant. There may therefore be some evidence for a difference in moult frequency between these data sets. A change in growth rate of crabs over time would be more likely to manifest itself through changes to moult frequency rather than moult increment, so it could be tempting to suggest that growth may have changed potentially due to changes in environment and/or high and increasing exploitation. However, the quality of the data need to be given due consideration and it is clear from the extensive pre-screening that was required for our data that there is considerable uncertainty, and potentially both bias and error in them. Further, a single observation in our dataset leads to a 50% moult frequency observation at 205mm premoult size and the analysis is sensitive to this. Removing this point impacts substantially on the curve and when compared against Bennett’s data and log linear relationship using the Student’s t test, neither the slope or intercept are statistically significantly different. Given this uncertainty in the size related data generally and in the moult frequency data at large sizes it was not possible to detect with certainty any change in growth rates between data collected in this study and those of Bennett (1974).
124
Figure AVI.11. Left panel: Comparison of loglinear relationships; black: this study, red: Bennett, 1974. Right panel: Comparison of data and models extended over a wide size range: black: this study, red: Bennett, 1974, blue: Latrouite & Morizur 1988, dashed: this study with moult frequency data point at 205mm excluded, green: McGarvey et al. (2002) generalised method results for baseline scenario
The generalisation of the anniversary method suggested by McGarvey et al. (2002) was applied under a variety of scenarios for the moult season and it was found to be a useful new method. However, the results were sensitive to selection of the moult season and the data excluded as a result of these assumptions. It was also sensitive with regards to whether all or just part of the moult season was required to constitute a growth opportunity. McGarvey et al. (2002) recognise that natural mortality in crabs is likely to be related to moulting through physiological stress and increased vulnerability to predation and infection by disease or parasites when soft. They acknowledge that this violates one of the assumptions made by Willoughby & Hurley (1987) when generalising the anniversary method and may bias the estimation of observed moult rate downwards and thus intermoult duration upwards. However, they point out that although this is true for the growth of an individual, it does not bias (or causes much less bias for) the estimate of average growth in biomass for a cohort to which both survival and growth contribute. Although this is true for mortality, it is not necessarily so for tag loss, so if tag loss is higher during moulting than at other times then this is also likely to lead to an underestimate of observed moult rate.
Experiments to evaluate tag loss demonstrated that tags were retained through the moult with similar rates to retention over a wider timescale, but for a very small sample of crabs. Nonetheless it is difficult to be sure that tag loss is not higher during moulting and in that case some under estimation of observed moult rate is likely. Further, the incidence of mortality and/or tag loss associated with moulting may lead to a non-linear relationship between moult rate and growth opportunities. For example, consider the following example for a size class with an intermoult period of 3 years: with no growth opportunities mortality (and or tag loss) will be the same for the whole population, so the sample will be representative, after 1 year a third of the population will have moulted and thus been subject to higher mortality (or tag loss), so the moulted animals will be under-represented and observed moult rate underestimated, after 2 years two thirds will have moulted and again been subjected to higher mortality (or tag loss), while after 3 moult seasons all the population should have moulted so the population will again be representative. This would suggest that points intermediate in the intermoult period may under estimate observed moult rate and there may be some evidence for this (Figure AVI.9, right panel). The numbers of records for each growth opportunity will decline with increasing numbers of growth opportunity and as the intercept is fixed at the origin and the last point likely to be very poorly sampled the weighting used in the regression will tend to weight the intermediate points more highly. Whilst the final point might have reduced bias, and increasing its weight might potentially seem desirable, increasing the weight on the most poorly sampled point cannot be recommended. Results from this approach were generally similar to those obtained by other means and authors suggesting slightly higher moulting rates up to 170mm (size class mid-point), but zero thereafter for the base case. The baseline scenario results for the size classes 120-139mm and 140-159mm and 160-179mm were slightly higher than the other estimates and broadly in-line with expectations, particularly given that, if anything, they were likely to be underestimates of moult rate. However, above 180mm and for many of the other scenarios estimated moult frequencies are lower than expected.
Female crabs of 210mm carapace width and above are not uncommon in the catch, so it would be expected that crabs do moult at sizes of 180mm and potentially larger within a time frame which with high exploitation can lead
125
to these numbers. The base line scenario of the generalised anniversary method results in a highly linear relationship between moult frequency and size (Figure AVI.12) (MF= -0.01507*PremltWdth+2.841163) with high r2 (0.991). Restricting the growth period to May to July (Figure AVI.9, right panel) results in slightly lower moult frequencies for the intermediate size classes but increases the moult frequency for crabs above 180mm and results in a more curvilinear relationship (Figure AVI.12). This also includes more observations to be retained resulting higher frequencies available for the regressions. A linear regression on these data fits less well (r2=0.827). A loglinear model similar to Bennett’s (1974), but using width as the predictor (Ln(%MF+1)=-0.029422*PremltWdth+8.25317423) is a better fit (r2=0.936). Such a model includes the potential for female edible crabs to continue moulting at low frequency at sizes well above 200mm. An extremely long term recovery from an Irish claw tagging programme (where the tag would be lost on moulting) in the Celtic Sea, indicated moulting had not occurred during the 4.8 years for which a female crab of 189mm carapace width was at liberty. This provides some direct evidence for extended intermoult period for large crabs and is not at odds with the rates suggested by the generalised method anniversary method with a May-July growth season.
Figure AVI.12. Moult frequency data derived using the McGarvey (2002) generalised method and linear relationships weighted by numbers of observations. Solid lines: linear models, dashed line: log linear model for Ln(% moult frequency +1) on width
The different models behave very differently outside the range of the data as is illustrated by Figure AVI.11, right panel. The logistic model is inappropriate for smaller sized crabs as it limits moult frequency to 1, but captures the features of the data well at larger sizes and has a theoretically suitable error structure in this range of size. Linear models do not appear capture features in the data over the observed range particularly effectively. The log linear models to weight do appear to capture features in the observed data, can accommodate more than one moult per year, but are somewhat less transparent that the more direct fits on carapace width. Although a model would not necessarily be expected to be appropriate over the entire length range due to processes such as maturation at which growth is likely to change in character, when the log linear models on weight are extrapolated to small sizes they provide broadly plausible results, with Bennett’s data potentially around the lower limits of growth and our data around a maximum. For example, a newly settled crab of around 6mm would be expected to moult several times during its first year, but by the time it reaches 100mm it is unlikely to moult more than twice and once recruited to the fishery moult frequency is likely to be 1 or less.
Estimation of moult frequency from tagging data remains problematic because of generally low levels of return for larger crabs and a number of possible sources that can bias estimation of the proportion moulting including:
1) Tag loss is most likely to occur during moulting so a recaptured animal that has moulted may have a high probability of having shed its tag and is therefore not recorded. On the other hand a recaptured crab that has not moulted has a high probability of having retained its tag and is recorded. This will bias downwards the proportion of animals recorded as having moulted.
2) Size dependent selection could also influence the estimation of proportions moulted (Hancock & Edwards, 1967) if small crabs are able to escape more easily and/or very large crabs have difficulty entering pots. In the first case small crabs which moult will be larger and therefore more catchable, while in the latter case large crabs which moult will be less catchable. Once again both these factors would tend to increase the negative slope of the relationship and decrease the size at which moult frequency is zero.
3) Higher natural mortality rate during moulting has also been cited as a reason for under-estimating moult frequency by tagging experiments (Hancock & Edwards, 1967). It seems likely that during or immediately after moulting crabs may be more vulnerable to death than when hard shelled. This would result in a
126
0
0.2
0.4
0.6
0.8
1
1.2
100 150 200 250 300
Annu
al m
oult
freq
uenc
y
Size (mm)
McGarveyMethodBase
McGarveyMethodMayJul
reduced recapture rate for those crabs that have moulted hence under-estimation of the proportion moulting.
4) Differential migration out of the area (to spawning grounds) by crabs that have moulted might occur for mature females after fertilisation while moulting (Hancock & Edwards, 1967). However, there is evidence that female crabs store sperm and therefore hard-shelled females that have not moulted would also migrate to spawning grounds.
The majority of these biases will lead to under-estimation of moulting rates for large crabs. The generalised anniversary method of McGarvey et al. (2002) does seem to offer some advantages in permitting more data to be used to determine moult frequency.
Bennett obtained better data during his programme during the 1970s, but this was carried out over a smaller spatial scale, for a longer duration, had scientists ‘on the ground’ for more of the time and greater access to the returned crabs. In this programme we took advantage of electronic communications technology to simplify the submission of tag recapture details permitting reporting remotely (by telephone hotline, email and internet website), introducing some quality checks, but not requiring the return of the captured crab. If growth data are to be a key feature of future programmes then we would recommend that efforts are made to submit the recaptured crabs or their carapace so ensure that measurements of carapace width are carried out consistently and accurately.
AVI.3 Continuous growth
Although discontinuous growth models capture the process of moulting by individual crabs more effectively than continuous growth models, the latter are considered appropriate at the population scale and are integral to a number of stock assessment methods for estimating mortality rates (e.g. length converted catch curves, length based virtual population analysis, length slicing and aged based assessments). It is therefore useful to use the data obtained to estimate continuous growth model parameters, in particular those of the von Bertalanffy (1938) model. However, taking the scarcity of data available for males in to account, the following analyses estimating continuous growth parameters were carried out only for female crabs.
Methods
The discontinuous growth data available from the tagging programme do not conform straightforwardly to the data requirements of the graphical and regression methods used to estimate von Bertalanffy growth curves. In this instance the data selected for the anniversary method approach to estimating moult frequency were used with average growth increments (including non-moulting crabs) for all the records in 5mm size classes to provide an average annual growth increment.
Combinations of average increment, premoult, intermoult and postmoult size were used to support graphical and regression based methods for estimating von Bertalanffy (1938) growth parameters. The von Bertalanffy (1938) growth model is expressed as
with parameters L∞, the asymptotic size of an infinitely old fish, K, a curvature parameter determining how quickly a fish approaches L∞ and t0, the theretical time (or age at which a fish has zero size, which is sometimes not estimated. Lt is the size at time t.
Average annual increment ( where t = 1 year) was plotted against mid-moult size ( ) to produce a Gulland and Holt (1959) plot where a linear regression (of the form y=a +bx) crosses the x axis at L∞ and the slope is minus K.
K =-b and L∞=-a/b
Strictly speaking this is only valid for small increments of ∆t and other methods have also been developed.
The Ford-Walford plot (Ford, 1933; Walford, 1946) has been widely used to estimate von Bertalanffy growth parameters and is often applied to constant ∆t intervals such as the 1 year anniversary in the data. For our data average annual post-moult size ( ) was plotted against premoult size (Lt) for 5mm size classes. is a constant of 1 year so the parameters are estimated from the slope ( ) and intercept ( )
K=-ln(b) and L∞=a/(1-b)
127
Chapman ‘s (1961) plot would consist of plotting the mean increment ( ) against the premoult size (Lt) for 5mm length classes and is a direct transformation of the Ford Walford method when based on the average annual data for 5mm length classes used in this study.
Results
Results from Gulland and Holt, Ford Walford plots applied to tagging data where time at liberty approximated to one year and change in size estimated as the product of average moult frequency and proportion moulting for 5 mm size classes were highly consistent (Table AVI.4, Figure AVI.13). Chapman’s plot using the same data is a transformation of the Ford Walford plot, therefore giving identical results.
Figure. AVI.13. Gulland and Holt (left panel) and Ford Walford plots (right panel) of average annual growth data for female crabs using 5mm size classes
Table AVI.4. Von Bertalanffy growth parameters estimates from averaged annual growth data Method K L∞
Gulland and Holt (1959) 0.2431777 211.6987Ford (1933), Walford (1946), Chapman (1961) 0.3251917 203.1144
Discussion
Estimating von Bertalanffy growth parameters obtained from these growth data are slightly lower than those suggested by Addison & Bennett (1992), which were set on the basis of fixing L∞ (at 240mm) and then estimating K based on a range of lines drawn by eye. Crabs of both sexes larger than 200mm carapace width are encountered quite regularly in sampling so it is likely that L∞ is significantly higher than this and likely negative bias in estimating moult frequency noted in the discussion for discontinuous growth, will also lead to under estimation of L∞. Negative auto correlation between parameters is common in the von Bertalanffy growth model hence underestimating L∞ may lead to an over estimation of K.
Comparing parameter estimates from this study with those estimated by a wider range of authors (Figure AVI.14) indicates that they conform very well with all the estimates that are based entirely on data, whilst the curve estimated by fixing one parameter on the basis of expertise (i.e. Addison & Bennett, 1992) has a higher L∞. Although conforming well with other estimates it is likely that these parameters under estimate growth, particularly at larger sizes, because of the biases associated with tagging data and growth by moult and given the frequency of observations of large crabs in the catch.
128
Figure AVI.14. Comparison of von Bertalanffy growth curves for female crabs estimated from data in this study (MF1103a &b) with other author’s results (Addison & Bennett, 1992; Latrouite & Morizur, 1988; Tallack, 2002; Sheehy & Prior, 2008).
Overall conclusions on growth
Our data on growth were very noisy and potentially slightly biased due to measurement error. We did not have sufficient size information on male crabs to meaningfully estimate growth parameters for
males. Estimates of moult increment from this study were very similar in magnitude to those estimated by other
authors for edible crabs in this region. Estimates of the variance in moult increment from our data are likely to be very heavily influenced by measurement error, and may not therefore provide a useful indication of variation in moult increment between individuals.
Estimates of moult frequency were also consistent with those of other authors for female edible crabs in this region. The generalisation of the anniversary method provided a useful alternative method of estimating moult frequency and utilising more of the available data. It was however sensitive to assumptions regarding the duration of the moult season.
Estimates of continuous growth rate were consistent with those estimated by other authors for female edible crabs in this region.
Estimation of growth rate for edible crabs remains problematic, particularly for the larger sizes, with many of the estimates produced appearing lower than suggested by a range of other anecdotal and circumstantial information. There are a number of sources of potential bias in growth data from tagging studies on crustaceans that are likely to cause such underestimation.
129
References to published material9. This section should be used to record links (hypertext links where possible) or references to other
published material generated by, or relating to this project.
Fishing News, Sep. 2007. Major tagging programme for English Channel crabs.
Fishing News, Apr. 2008. Crab tagging in the English Channel: Year One
Fishing News, Jul. 2009. Brown crab fishery part of shared research idea.
Fishing News, Sep. 2009. Crab-tagging in the English Channel: An update, after two years.
Fishing News International, Aug. 2009. Fishermen and scientists share research ideas, ways to jointly manage fishing.
GAP1, www.gap1.eu.
Hunter, E. 2010. Migration of edible crabs in the English Channel. Oral presentation to Animal Behaviour Conference (Prof. R. N. Hughes) School of Biological Sciences, University of Wales, Bangor, March 10.
Hunter, E., Riley, A., Eaton, D., Stewart, C., Lawler, A., McIntyre, R., Leocadio, A. & Smith, M. 2010. Electronic data storage tags reveal the behaviour of free ranging edible crab, Cancer pagurus L., in UK waters. Seventh International Crustacean Congress poster presentation. ICC7.
Hunter E., Stewart C., Eaton D., Lawler, A. & Smith M., 2011. A crab’s eye view: electronic data storage tags reveal migration and behaviour patterns of the edible crab, Cancer pagurus L. Oral presentation to Bio-logging 4. Hobart, Tasmania. March 2011.
130