Orkney Sustainable Fisheries Ltd. Report No. 19
Orkney Shellfish Research
Project Orkney Brown Crab (Cancer pagurus) Tagging
Project
Matthew T. Coleman & Elisabete Rodrigues
18.05.2017
Coleman M., Rodrigues E., (2017). Orkney Brown Crab (Cancer pagurus) Tagging Project. Orkney
Shellfish Research Project. Orkney Sustainable Fisheries Ltd. No.19, Pp 21.
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Executive Summary This report outlines the results of the crab tagging work undertaken by Orkney Sustainable Fisheries Ltd
(OSF) as part of the Orkney Shellfish Research Project (OSRP) from 2013 – 2016. This report contains
data collected from this project and pre-existing crab tagging data collected previously from OSF
research. This report draws comparisons from previous tagging working undertaken by Marine Scotland
Science with results collected by OSF. This project outlines differences in stock migratory behaviours
presenting new findings previously unseen within Scottish brown crab stocks.
OSF in partnership with the Orkney inshore fishery sector undertook crab tagging from 2013 – 2016.
Crab tagging involved attaching unique numbered cable tie tags to soft shelled or recently moulted
individuals. Biological information relating to an individual’s size, sex and mating success was recorded
with individuals released and subsequent recaptures reported by Orkney fishermen and beyond. A broad
size-range of individuals were tagged over the duration of the project, looking to identify differences in
stock movement behaviour relating to size. Differences in crab tagging objectives between years resulted
in differences in observed tagged sex ratios, with a far greater number of male brown crab tagged in the
first year of the project (2013) compared to the final years of the project.
At the conclusion of the OSRP a total 6,954 brown crab were tagged, of these 27% were male and 73%
female. The size of tagged male brown crab released ranged from125 -217mm carapace width (CW) with
an average of 150mm CW. Tagged female brown crab ranged from 107 -213mm CW with an average
CW of 160mm. As of March 2017, of all females released 4.25% (n=66) were recaptured, with 6.8%
(n=77) of males recaptured. Days at liberty ranged from 2 – 445 days, with distances travelled seen to
vary between sexes. Of those females recaptured distances travelled from point of releases and recapture
ranged from 0.9 – 258km. Whilst in the case of males, distances travelled ranged from 0.1 – 67km.
Preliminary interpretation of the results indicated that a number of long distance migrations were recorded
with a number of individuals moving in a westward direction, with two cases of individuals travelling a
minimum distance 236 and 258km respectively, with other individuals equally travelling 87– 178km prior
to recapture.
The interpretation of these results indicates the occurrence of a number of differing behavioural patterns
exhibited by brown crab tagged within Orkney. Two behavioural patterns were exhibited within females
with both localised movement patterns and long distance migratory behaviours observed. In the case of
male brown crabs, observed movement patterns and associated behaviours coincide with existing
literature, with males being predominantly inshore, undertaking little movement. The identified
behaviours provide further insight into potential brooding behaviours and locations within the Orkney
fishery.
The results of this study indicates the occurrence of movement of Scottish crabs stock around the west
coast transcending numerous stock assessment boundaries and both inshore and offshore fisheries. This is
the first study to of long distance movements within Scottish brown crab stocks.
Recommendations surrounding future work are discussed with the implementation of a Scottish-wide
tagging study, designed to capture inter-stock movements all around Scotland’s coasts. Additional
research priorities are highlighted, including the use of data storage tags to identify brown crab brooding
location.
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Contents Executive Summary ....................................................................................................................................... I
1. Project Background ............................................................................................................................... 1
2. Overview ............................................................................................................................................... 1
3. Rational ................................................................................................................................................. 2
4. Methodology ......................................................................................................................................... 2
2013 Data .................................................................................................................................................. 4
Distribution of tagging activity ................................................................................................................. 4
5. Results ................................................................................................................................................... 5
5.1. Size Frequency & Sex Ratios........................................................................................................ 5
5.2. Annual Size frequency .................................................................................................................. 5
5.3. Overall Sex Composition .............................................................................................................. 6
5.4. Regional Sex Composition............................................................................................................ 6
5.5. Sperm Plugs .................................................................................................................................. 7
5.6. Recapture Rates and Timing ......................................................................................................... 8
5.6.1. 2010/11 ................................................................................................................................. 8
5.6.2. 2012 ....................................................................................................................................... 8
5.6.3. 2015 ....................................................................................................................................... 9
5.6.4. 2016 ....................................................................................................................................... 9
5.7. Tagging Results ............................................................................................................................ 9
5.8. Marine Scotland Tagging Study 2010 ........................................................................................ 12
5.9. Marine Scotland & OSF Comparison ......................................................................................... 14
5.9.1. Female Movements ................................................................................................................. 14
Inshore Comparison ............................................................................................................................ 14
Offshore Comparison .......................................................................................................................... 15
5.9.2. Male Movement ...................................................................................................................... 16
Inshore Comparison ............................................................................................................................ 16
4.5. Discussion ............................................................................................................................................ 17
Migration Routes .................................................................................................................................... 18
Brooding regions ..................................................................................................................................... 18
4.6. Conclusion ........................................................................................................................................... 19
References: .................................................................................................................................................. 20
Appendix ....................................................................................................................................................... 1
III
List of Tables
Table 1 Total number of Brown crab individuals tagged per Year per sex from OSF tagging exercises
(2010-2016)................................................................................................................................................... 2
Table 2 Average size per sex per year of tagged individuals released (2010-2016) .................................... 5
Table 3 Percentage and Number of tagged crab recaptured per year of those released. ............................ 8
List of Figures
Figure 1 Procedure of Sexing brown crab, location of carapace measurements and position of cable tie
tag on a tagged individual prior to release ................................................................................................... 3
Figure 2 Identification of visible sperm plugs within a female brown crab (Edwards, 1979) ..................... 3
Figure 3 Distribution of OSF tagging release sites around Orkney (2010 -2016) ...................................... 4
Figure 4 Overall size frequency of all tagged individuals by OSF (2010 – 2016) ....................................... 5
Figure 5 Annual size frequency of tagged individuals (2010 -2016) ........................................................... 6
Figure 7 Overall sex composition of tagged crab per ICES rectangle (2010 – 2016) ................................. 7
Figure 8 Size frequency of tagged females recorded with sperm plug (2016) ............................................. 8
Figure 10 Long distance female brown crab movements from initial release points and recaptures from
around North West Scottish Coast (2010-2016) ......................................................................................... 10
Figure 9 Inshore movements of tagged female crabs from initial release and subsequent recapture (2010
-2016) .......................................................................................................................................................... 11
Figure 11 Inshore movements of tagged male brown crab from initial release site and subsequent
recapture (2010 -2016) ............................................................................................................................... 11
Figure 12 A) Offshore movement and recaptures of tagged female brown crab within the windsock
tagging area. B) Movement of tagged female brown crab by Orkney inshore fleet (Jones et al, 2010) ..... 12
Figure 13 A) Offshore movement and recaptures of tagged male brown crab within the windsock tagging
area. B) Movement of tagged male brown crab by Orkney inshore fleet (Jones et al, 2010) ..................... 13
Figure 14Comparison of recorded movements of tagged female brown crab recaptures within the Orkney
inshore fishery. ............................................................................................................................................ 14
Figure 15 Comparison of recorded movements of tagged female brown crab within both Marine Scotland
offshore tagging recaptures and Orkney Sustainable Fisheries long distance recaptures. ....................... 15
Figure 16 Comparison of inshore tagged male brown crab from Marine Scotland and Orkney
Sustainable fisheries tagging experiments .................................................................................................. 17
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Orkney Shellfish Research Project
Orkney Brown Crab (Cancer pagurus) Tagging Project
1. Project Background The Orkney shellfish research project (OSRP) has come to the end a four year project (2013 – 2016). The
project encompassed a number of objectives aimed to address the needs of two important sectors: Inshore
fisheries and marine renewables.
Specific objectives of the project relating to the fishing sector were:
• To assess the status of Orkney crustacean stocks in relation to sustainability criteria;
• To provide monitoring data and biological understanding to support the development and
implementation of a harvest strategy and management systems for sustainable fishing in the context of
marine spatial planning for renewable energy and other activities in Pentland Firth and Orkney waters;
• To satisfy shellfish consumers of Orkney’s commitment to sustainable fishing.
Objectives relating to the marine renewable energy industry:
• To determine key areas providing value to the Orkney creel fishery in terms of catch rates and the
magnitude, composition and quality of catches;
• To describe spatial patterns of space-use by the Orkney creel fishery in terms of fishing effort,
navigation, relocation of gear during bad weather and seasonal and inter-annual patterns of variability;
• To determine locations of critical habitat for crustacean species targeted by the Orkney creel
fishery, particularly in terms of spawning areas and movement and migration patterns of brown crab.
This report outlines the results collected over the past 4 years specifically relating to brown crab tagging
activities within the Orkney inshore fishery and the outputs associated with the analysis of data collected.
This programme was instigated to determine locations of critical habitat for crustacean species targeted by
the Orkney creel fishery, particularly in terms of spawning areas and movement and migration patterns of
brown crab.
2. Overview Extensive knowledge gaps relating to key biological processes and traits still exist within a number of
commercial important species, brown crab Cancer pagurus being one of them. This species receives
national management advice relating to current and historical trends in its exploitation (Marine Scotland
Science, 2016) however is still listed as a data limited stock. Under current EU directives member states
are required to manage all fish stocks at maximum sustainable yield (MSY) or equivalent by 2020,
however to move toward successful sustainable management, further information is required surrounding
key biological processes such as reproductive cycle and stock movements. Extensive research was
conducted in the early 1900s (Pearson, 1908) and late 1970 (Bennet & Brown, 1976; Edwards, 1979)
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investigating movement and reproductive behaviour of brown crab around the British Isles. This research
has underpinned the understanding of brown crab within UK waters and northern Europe as a whole for
the past century. Recent research however has started to shed further light onto biological processes such
as reproduction (Ungfors , 2007; Hunter et al, 2013; Haig et al 2016), stock genetics (Ungfors et al, 2009;
Moran, 2009) and migratory behaviour within this species (Ungfors & Nilsson, 2007). Even with this
research however extensive knowledge gaps still exist within this species and fishery.
3. Rational The role of this project was to gather information on the movement patterns of crabs within the Orkney
inshore fishery and beyond. The brown crab fishery is an important keystone specie of Orkney’s inshore
fishery with value at first sale of £3.6million in 2015 (Scottish Government, 2016). It is therefore very
important to understand the biological characteristics of this socio-economic important stock.
Previous tagging experiments have been conducted in Orkney in 2008 -2010 (Jones et al, 2010). This
study investigated the overlap between the inshore and emerging offshore fishery, with results indicating
possible stock movements between inshore and offshore areas through annual migration events. To build
upon the findings of this project Orkney Sustainable Fisheries Ltd. undertook a series of tagging projects
investigating the interaction between inshore and offshore fishery.
Brown crab tagging was conducted by OSF from 2013 – 2016 with participation of the Orkney inshore
fleet as a component of the OSRP. Over the duration of the OSRP a total of 6,954 crabs were tagged
(Table 1). An additional 747 individuals were tagged under a previous OSF crab tagging project (Lamb,
2012), totalling 7,294 tagged individuals (Table 1). No tagging was undertaken within 2014 due to staff
limitations.
Table 1 Total number of brown crab individuals tagged per year per sex from OSF tagging projects (2010-2016)
*See Section “2013 Data”
4. Methodology
Crabs were caught using baited creel deployed by commercial inshore fishers operating within the Orkney
assessment area. Creels were typically hauled within and out to the 12 nautical mile limit, with gear
deployment ranging from 15 – 80m depths. Tagging was conducted typically from August – December,
coinciding with the peak moulting period. A total of 9 vessels participated in the tagging project, either
through the OSRP observer programme or undertaking tagging activities independently.
2010/ 2011 2012 2013 2015 2016
Male 423 178 149 285
Female 324 45 394 770
Sub Total 747 223 5,355* 543 1,056
Total 7,924
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The selection of crabs was restricted to those that have recently moulted or “white crabs1” and those that
would otherwise be discarded by the fishery. The selection of these individuals meant that fishermen were
not required to sacrifice a proportion of their landed catch, nor was additional payment required to secure
individuals. Selective grading of suitable individuals to a particular criterion occurred prior to tagging;
this criterion restricted tagging to individuals that displayed no signs of damage, either through handling
or through damage resulting from other crustaceans within creels. Grading was done to ensure only
healthy crabs were selected.
Crabs were tagged using uniquely numbered claw tags (coloured plastic cable tie) attached to either claw
(Fig 1). Each tag in addition to its unique identifier was printed with a telephone number to enable fishers
to report subsequent recaptures. At the time of initial capture individuals were sexed, sized (Fig 1) and
sperm plug presence was recorded (Fig 2) allowing potential differences in migratory behaviour to be
monitored. Due to the nature of tags and morphological differences in cheplids, females <150mm were
not tagged due to the tag number becoming illegible, males were tagged from ≥130mm.
Figure 1 Procedure of Sexing brown crab, location of carapace measurements and position of cable tie tag on a tagged
individual prior to release
Tagging preference during 2015/16 was directed towards tagging predominantly female crabs. Male
brown crabs undertake limited inshore movements (<6km) and migratory behaviours, this behavioural
pattern being extensively documented throughout its natural range (Ungfors & Nilsson, 2007; Jones et al,
2008; Edwards. 1979). Small quantities of male brown crab however were still tagged to provide a
relative example of annual movement patterns within this inshore population.
Figure 2 Identification of visible sperm plugs within a female brown crab (Edwards, 1979)
1 White crab describes a crba that has recently moulted, when the shell is soft and the underside is white instead of
cream in colour.
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2013 Data
Access and use of 2013 data was severely limited due to data corruption. This has limited the inclusion of
this year’s data within the final reports analysis and subsequent interpretation of results. In some cases
fragments of data was extracted from previous OSF reports through the use of GIS georeferencing and
data mining. Subsequent safe guards relating to data security have now been in place within OSF since
2014 to ensure future data security.
Distribution of tagging activity
Distribution of tagging events over the duration of the project illustrates key areas in which large amounts
of tagging occurred (Fig 3). Primary tagging areas throughout all years are highlighted as being off the
West Mainland and Northern Hoy. In comparison small amounts of tagging occurred within and around
the Northern Isles, predominantly within 2010, 2015 and 2016. Limited tagging occurred along the East
Mainland, with tagging activities only occurring within this area in 2011.
Figure 3 Distribution of OSF tagging release sites around Orkney (2010 -2016)
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5. Results
5.1.Size Frequency & Sex Ratios
Overall size frequency of tagged male crabs ranged from 125 – 217mm CW, with an average CW of 150
±16mm (Fig 4), female crabs ranged from 107 – 213mm with an average CW of 160 ±16mm (Fig 4).
Annually male crab average size remained constant, predominately between 140 -150mm CW (Table 2),
in comparison average tagged females were predominantly between 150-160mm CW range (Table 2).
Figure 4 Overall size frequency of all tagged individuals by OSF (2010 – 2016)
5.2.Annual Size frequency
Annual size frequency was indicative of the tagging priorities of each year and the types of vessel from
which tagging occurred. In earlier years (2010 – 2012) the majority of tagged crabs were male, with peak
frequency occurring typically around the MLS (140mm CW; Fig 5). In subsequent years (2015/2016) a
broad size range of individuals were tagged (Fig 5, Fig 6), with the largest average size for both males
and females tagged during this time period (Table 2)
Table 2 Average size per sex per year of tagged individuals released (2010-2016)
Male Female
Combined 150 ±16 160 (±16)
2016 163 (±15) 164 (±11)
2015 148 (±15) 158 (±15)
2014 - -
2013 146 142
2012 144 158
2011 142 141
2010 149 160
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Figure 5 Annual size frequency of tagged individuals (2010 -2016)
5.3.Overall Sex Composition
Sex composition was seen to fluctuate between years and ICES rectangles. In earlier years of the project
(2010-2013) far greater numbers of male crab were tagged in relation to females, in the case of 2010 male
brown crab made up 62% of total crab tagged whilst females represented only 38% (Table 1). These
differences in tagging behaviour can be attributed to differing OSF staff tagging priorities at the time.
These differing methodologies over years present contrasting behavioural patterns within the stock. To
adjust for these differences in tagging strategy, greater onus was placed upon tagging female crabs in
2015/2016, resulting in females making up 72% of total tagged crab in both years. The results of this
present a holistic image of the movements of both female and brown crab around and further afield from
the Orkney Islands and its inshore fishery.
5.4.Regional Sex Composition
Regional sex ratios were investigated by combing all years per ICES rectangle in which releases occurred
(Fig 7). West coast releases (ICES 46E6 & 47E6) were dominated by female brown crab, comprising
66% of tagging releases, whilst males in both cases represented 34% respectively. In contrast, east coast
male crab represent a greater proportion (47E6: 41%) or dominate compositions entirely (46E7: 71%).
Differences in catch compositions can be attributed to a combination of environmental conditions and
fishery targeting behaviours and therefore shouldn’t be interpreted as representative of the sex ratio for
each of these regions.
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Figure 6 Overall sex composition of tagged crab per ICES rectangle (2010 – 2016)
5.5.Sperm Plugs
The reproductive stage of females was recorded during 2016 tagging. This involved recording the
presence or absences of sperm plugs within the oviducts (Fig 2). Sperm plugs are clearly visible making
their presence easily detectable. The presence or absence of sperm plug relates to potential behavioural
difference between individuals that have or haven’t successfully copulated. Such differences could be
demonstrated with mated/unmated individuals undertaking differing migratory behaviours. Existing
hypothesized brooding behavioural activities hinge upon individuals that have successfully copulated and
subsequently remain inshore within unknown brood locations, with non-reproducing individuals
undertaking large migrations. It is therefore essential that any future tagging studies undertaken record
the copulatory state (sperm plug presence/absence) of individuals.
Of those females tagged 16%were recorded as having sperm plugs present and ranged from 150 – 205
mm CW with the mean size being 177mm CW (Fig 8). In relation to previous studies undertaken by OSF
which also documented sperm plug presence, individuals recorded within 2016 tagging were far larger
than those recorded previously (83-148mm CW: Haig et al, 2016). However the failure to detect
individuals with sperm plugs at smaller size can be attributed to the discarding of these individuals due to
them being morphometrically unsuitable for tagging.
8
Figure 7 Size frequency of tagged females recorded with sperm plug (2016)
5.6.Recapture Rates and Timing
As of end of March 2017 4.25% (n=66) of female brown crab have been recaptured and 6.8% (n=77)
male crabs from 2010-2017. Recapture rates varied between years (Table 3). The reasons behind this
variability is unclear, however timing of tagging activities would have a significant effect on the recapture
rates of tagged individuals and tagging composition for each year on the subsequent recaptures.
Table 3 Percentage and number of tagged crab recaptured per year of those released.
2010/2011 2012 2015 2016
Male 3.5% (15) 22.4% (40) 6% (9) 2% (7)
Female 4.3% (14) 4.4% (2) 11.1% (44) 0.6% (5)
5.6.1. 2010/11
Recapture rates within 2010/11 demonstrate a relative, even recapture rate between sexes, with 99%
recaptures occurring within 30 days of initial release, with one individual at liberty for 56 days (Appendix
2). Average distance travelled for all individuals was <1km from initial release site including the
individuals at liberty for 56 days. One individual however was recorded moving a linear distance of 28km
in 24 days at liberty. This movement however remained within inshore Orkney waters.
5.6.2. 2012
Recapture rates within 2012 were more variable, with 38% recaptures occurring within 30days of initial
release (Appendix 3). During this time-period distance travelled ranged from 0.34 – 10km, with an
average of 1.7km. In the case of those individuals with times at liberty >30days limited movements were
9
documented, with distance ranging from 0.12 = 4.8km, and an average of 3km. Low average distances
travelled by individuals at liberty >30 days can be attributed to the greater number of male crabs
recaptured, with limited stock movements known to occur. Of the two females recaptured, recapture
occurred within 30 days, providing limited opportunity for individuals to undertake any large behavioural
movements.
5.6.3. 2015
Recapture rates within 2015 differed from previous years with 75% (n=33) of recaptures occurring within
the initial 30 days post release (Appendix 4). Of these recaptured during this period 91% (n=30) were
female with 9% (n=3) male. Distance travelled ranged from 0.1km – 16km, with an average of 2.7km. Of
the remaining individuals at liberty >30days, 29% (n=5) were male and 71% (n=12) were female. Marked
differences are observed in the movement patterns of the two sexes over these larger times at liberty.
Male time at liberty ranged from 47 – 445 days, with corresponding movements ranging from 2.8 - 67km,
with an average of 18km (Appendix 4). In contrast female time at liberty ranged from 35 – 378 days, with
an average of 178 days. Distance travelled ranged from 8.9 – 258km, with an average of 97km (Appendix
4). Of those females at liberty >30days, two were recaptured twice with subsequent re-releases. Two
individuals demonstrated contrasting movement patterns. Tag No.24236 undertook significant offshore
movements from its initial inshore release sites, moving 53km in a westerly direction. In contrast Tag No.
24103 moved in a north easterly direction, moving into inshore waters.
5.6.4. 2016
Recapture rates as of March 2017, record 25% (n=3) of recaptures occurred within the first 30 days post
release (Appendix 5). Of these 75% (n=2) were male and 25% (n=1) were female. Distance travelled for
males was 8.7 and 9.7km, whilst the female moved a distance of 22.2km. Those recaptured 30 days post
release comprised 75% (n=9) of those recaptured in 2016 44% (n=4) were female and 56% (n=5) male.
Females at liberty ranged from 51 – 136 days, with an average of 94 days. Distance travelled ranged from
7.6 -160km, with an average of 50.9km. Male’s days at liberty ranged 45 -98 days, with distances
travelled ranging from 0.7 – 36.9km, an average of 10.7km.
5.7.Tagging Results
Of those individuals tagged over the course of the project clear differences in behavioural pattern
emerged between that of male and female proportions of the stock.
Female crab movements were seen to vary considerably between individuals, with some undertaking
predominantly inshore limited movements between recaptures (Appendix 1; Fig 10). These limited
inshore movements are exhibited by Tag number 24103, measuring 158mm CW, moving a minimum
distance of 8.9km within 98 days at liberty. Similar behaviours are exhibited by other individuals; tag
number 1154 measuring 176mm CW travelled 9.254km during 91 days a liberty; tag number 8514
travelling a minimum distance of 7lkm during 651 days at liberty (fig 10).
However, some females travelled considerable distances > 150km from initial release (fig 9). With
recaptured long ranging individuals accounting for18% (n=12) of all recaptured females. Of these, two
females were recorded moving >200km minimum linear distance from first release. Tag number 25753
measuring 175mm CW travelled a minimum distance of236km in 336 days. Tag Number 24268
measuring 186mm CW travelled a minimum distance of 258km in 378 days. These two individuals
correspond with existing literature on large movements undertaken by brown crab, similar distances
10
recorded by individuals originally released off the coast of Ireland being recaptured within the French
fishery (Fahy & Carroll, 2008).
In comparison male crab movements are considerably different from those of female. The majority of
male crab recaptures occurred within 6km of the original release site and predominantly within the 6nm
inshore Orkney limit (Fig 11). However a number of individuals remained at liberty for large periods of
time. These individuals undertook limited movements from point of releases, opposite to that of the
behaviour exhibited by a number of females over similar time-frames (Appendix 1). This behaviour is
exhibited within tag number 22886, a male measuring 137mm who was at liberty for 344 days moving a
minimum distance of 3.8km from first release. Tag number 22520 measured 168mm CW and was at
liberty for 445 days, moving a minimum of 12km from initial release point, remaining inshore and
moving solely across rocky/hard reef substrate. Similar examples were seen with tag number 23333 and
tag 24125. These two male crabs measured 170mm CW and 157mm CW and travelled a minimum
distance of 4km and 2.8km over a total of 243 and 251 days.
Conversely a small number of males undertook large scale movements prior to recapture, accounting for
3% (n=2) of males recaptured. Of these males, tag number 10189was at liberty for 98 days in which it
travelled a minimum distance of 36.9km. Tag number 23311 was at liberty for 47 days and travelled a
linear distance of 67.5km (Appendix 1; Fig 11).
Figure 8 Long distance female brown crab movements from initial release points and recaptures from around North West Scottish Coast
(2010-2016)
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Figure 10 Inshore movements of tagged male brown crab from initial release site and subsequent recapture (2010 -2016)
Figure 9 Inshore movements of tagged female crabs from initial release and subsequent
recapture (2010 -2016)
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5.8. Marine Scotland Tagging Study 2010
Previous tagging experiments have been conducted by Marine Scotland Science (MS; Jones et al, 2010)
within Orkney waters and further offshore within the neighbouring Sule stock assessment area also
referred to as the “Windsock”. This study aimed to investigate the migratory behaviours and stock
movement patterns between the emerging offshore brown crab fisheries and localised inshore fishery. Of
the results presented, proportions of the offshore female stock appear to move inshore indicating possible
immigration from offshore areas into the inshore fishery (Fig 12A), whilst female inshore stock indicated
the possibility of emigration in a westward direction (fig 12B). Conversely male movement patterns
within both tagging offshore (fig 13A) and inshore areas movement patterns are limited (Fig 13B), with
individuals exhibiting localised movement patterns.
Figure 11 A) Offshore movement and recaptures of tagged female brown crab within the windsock tagging area. B) Movement of
tagged female brown crab by Orkney inshore fleet (Jones et al, 2010)
A
B
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Figure 12 A) Offshore movement and recaptures of tagged male brown crab within the windsock tagging area. B) Movement of
tagged male brown crab by Orkney inshore fleet (Jones et al, 2010)
A
B
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5.9. Marine Scotland & OSF Comparison
Aggregated data from both OSF tagging activities and MS (Jones et al, 2010) provide the opportunity to
compare the results of both studies simultaneously, allowing difference in behaviours to be investigated
between the different components of the Orkney brown crab stock. In the case of MS data, raw data
points relating to crab releases and recaptures were unavailable, resulting in data being extracted through
the use of ArcGIS georeferencing. Georeferencing allowed the merging of the two different published
data sets through the use of spatial configuration In this case MS maps contained spatial qualifiers in the
form of GPS grids.
5.9.1. Female Movements
Inshore Comparison
Inshore movements of brown crab were similar within both studies (fig 14). Of those individuals
recaptured a small number demonstrated large inshore movements in westward direction, whilst a high
proportion demonstrated smaller localised movements between recapture and release. A high
concentration of capture and release events is located off the North West coast of Hoy, highlighting its
potential importance as a mating or lekking region within Orkney. Similar observations are highlighted
within identified commercial species hotspots and succorfish data (See report: Succorfish Report, 2016),
demonstrating the importance of this area for commercial fishers also.
Figure 13Comparison of recorded movements of tagged female brown crab recaptures within the Orkney inshore fishery.
15
Offshore Comparison
Comparison between the MS offshore tagging exercise and long distance migration patterns documented
by the OSRP illustrates potential differences between stock movements (fig 15). In the case of the MS
tagging results, offshore crab illustrates a predominately localised movement pattern within the tagging
area. This behaviour could therefore be interpreted as representative of the crab stock movements
occurring outside of 12nm limit. However a number of individuals tagged within that project exhibited
movement pattern
towards inshore areas, demonstrate the potential role that the offshore stock has in the emigration of
individuals to inshore stock and fishery. Conversely in the case of the OSF project, recently moulted
individuals demonstrated little to no movement patterns towards offshore locations, remaining
predominantly inshore and moving in a south west direction. The lack of movement from inshore areas
Figure 14 Comparison of recorded movements of tagged female brown crab within both Marine Scotland offshore tagging recaptures and Orkney
Sustainable Fisheries long distance recaptures.
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back to offshore areas presents numerous questions relating to the movement cycle of brown crab stocks,
primarily the overall movement pattern between offshore and inshore areas. Offshore areas cannot be
deemed to experience localised recruitment due to the lack of suitable larval settlement habitat within
these areas therefore the necessity of inshore areas for recruitment still arises. Additional recruitment
areas separate to the Scottish mainland and islands could occur, with settlement and recruitment of
individuals from Sule Stack and North Rona.
These two studies provide insight into the movement patterns of the female portion of the stock, however
both studies provide a limited view of the movement pattern overall. Therefore the implementation of
other tagging activities would provide further insight into emigration and immigration behaviour cycles of
the species at a Scotland wide level.
5.9.2. Male Movement
Inshore Comparison
Inshore movements of male brown crabs from both MSS and OSRP tagging studies illustrate limited
localised moment within inshore areas. These combined movement patterns mirror that of other
behavioural movements exhibited by male brown crab in other regions in northern Europe (Edwards,
1979; Ungfors & Nilsson, 2007). In both the MSS and OSF studies two long distance migrations are
documented, indicating that large localised movements within inshore areas do occur, with the
movements potentially due to high crab density within localised areas or a behavioural responses of male
crab following migrating females.
17
Figure 15 Comparison of inshore tagged male brown crab from Marine Scotland and Orkney Sustainable fisheries tagging
experiments
4.5. Discussion
Crab tagging results illustrate the occurrence of annual migratory behaviour within brown crab around the
North West Scottish coast. Behavioural differences are observed between the sexes, with the potential
indication of differing behavioural movements depending on the condition and/or reproductive stage of
individuals. General trends in female’s movement highlights a population that undertake long westerly
migration moving between stock assessment areas and fisheries. In contrast males demonstrate a localised
distribution pattern undertaking limited movements within inshore areas, however exceptions to this
behavioural pattern have been recorded.
Reasons relating to the movements of brown crab along the north west of Scotland are currently largely
speculative, the condition of tagged females relating to sperm plug presence was recorded only in 2016.
The addition of this information provides the opportunity to untangle the behavioural response of
different tagged individuals and identify potential triggers causing these large scale migrations potentially
associated with reproduction.
This additional information would produce a valuable insight into the reproductive cycle of brown crab
with key components of this process significantly under-researched and large biological knowledge gaps
within its cycle. It is currently hypothesised that brown crab undertakes a broad iteroparous reproductive
cycle (Haig et al, 2016); meaning that they can undertake multiple reproductive cycles over their life
time. Loose annual reproductive cycles have been hypothesised (Pearson, 1908), with these based upon
18
early research conducted within the 1900s. Little progress into key biological progress has been
established since then. The observations and movement documented here coincide within the behavioural
patterns observed within newer studies conducted around the British coast with clear seasonal patterns
with inshore and offshore migrations. Further insight into female brown crab brooding behaviour has
been identified off the south west coast by Hunter et al ( 2013). In this case ovigerous female brown crab
were fitted with data storage tags providing the capabilities to track direction and associated behavioural
changes over the course of brooding, spawning and subsequent returning inshore emigrations . The extent
and direction of tagged females can be indicative of localised environmental patterns, such as water
temperature and primary tidal flow. Further understanding of reproductive biology could be inferred from
existing reproductive strategies of other decapoda (H.americanus - Cobb & Phillips,2012), with the
assumption of a biannual spawning cycle and decreases in reproductive frequency relating to size/age but
compensated within increase in overall fecundity. This proposed reproductive strategy also ties into
behavioural observation of brown crab, specifically the apparent differentiation between two components
of stock, perhaps those that undertake large migrations and that of individuals that moult and
subsequently remain inshore, abstaining from reproduction. This behaviour separation of the stocks has
subsequently been captured through tag returns within this study but also within other regions within
northern Europe (Ungfors et al, 2006; Fahy & Carrol, 2008)
Migration Routes
The roles of environmental drivers behind crab migration that occur in the north west of Scotland still
remain unknown. Observed movement patterns within this study illustrate clear interaction occurring
across multiple assessment areas and within different components of the Scottish creel fisheries (both
inshore and offshore fisheries). These migrations tie into existing knowledge surrounding crab behaviour
with migrations occurring against primary currents within the surrounding region. The uses of primary
currents are hypothesised to be used within larvae transport (Sinclair, 1988), with larvae carried back on
regional primary currents, providing the opportunity for both localised and long distance recruitment.
Similar migratory behaviours and larval transport has been observed within other areas of the UK (Eaton
et al, 2003), with female migrations documented along the Northumberland coast moving from a south to
a northern orientation post moult (Edwards, 1979). Similar migrations were observed and documented
within populations off the coast of Norway and Sweden (Ungfors et al, 2007).
Brooding regions
The area in which the brooding process occurs within Scottish brown crab or stocks is still unknown. The
result from this study highlights the possible occurrence of these brooding sites based on the migratory
behaviours observed. Of the currently reported recaptures a number of these have been in the water
surrounding the Western Isles.
Based on existing predictive habitat mapping and the observed migratory behaviours, the west coast of
Lewis provides a potential area for a brooding site within the Scottish crab stock along the North West.
This area provides large ideal habitat extents of sand/muddy sand and coarse sediment and is preferable
burrowing and brooding habitat for brown crab surrounded by large expanses of hard reef substrate
(Appendix 6). Ocean currents additionally provide ideal temperature conditions with average SST around
the Western Isles 7.6oc (NOAA), with 7 -8
oc necessary for larval development and spawning (Eaton et al,
2003).
19
Alternatively, brooding locations could be localised and far closer inshore, with the interpretation that
localised female inshore movements in fact form the Orkney spawning stock. Such an interpretation is
strengthened by the implausibility of larval transport from spawning locations situated off the Isle of
Lewis reaching Orkney. Based on larval biology and known diurnal movement behaviours, tidal regimes
and larval development time scales, such recruitment appears implausible. Similar stock differentiations
are exhibited within crab populations along the north east English coast. With localised recruitment from
brooding locations close inshore and restricted localised larval transport and settlement, long distance
travel only occurs through emigration and immigration of adults from neighbouring regions (Eaton et al,
2003). This therefore presents the idea that large scale movements observed within this tagging
experiment are in fact emigration behaviours, with individuals naturally emigrating between regions and
assessment areas.
However the identification of these brooding areas is purely speculative and further tagging studies, both
national tagging studies and regional data storage tagging (DST) are recommended. The use of DST
tagging experiments similar to that of Hunter et al (2013) would allow the possibility for geolocation of
tagged individuals providing fine scale movements, something that is lost from typical A to B tagging
studies. This could potentially identify the location of brood stock areas.
4.6. Conclusion The results of this study and that of Marine Scotland present the foundation of crab movements around
the northern west coast of Scotland. It highlights a number of additional avenues of research including the
development and implication of national tagging experiments to understand the regional and broad scale
movement patterns of the brown crab stock around Scotland. Additionally it highlights key knowledge
gaps surrounding this species, specifically spawning behaviour and how to address these knowledge gaps
(potentially through use of DST). OSF is currently in the process of implementing and organising a
national tagging project aiming to address some of these knowledge gaps and in partnership with Heriot
Watt University is in the initial stages of developing a low cost open source DST.
20
References:
Bennett, D. and Brown, C. (1976). The crab fishery of south-west England. Laboratory Leaflet., Fisheries
Lab. Lowestoft, 33, p.11.
Bruce, M. (2009). Population genetic structure of brown crab (Cancer pagurus) in Irish waters. Master of
Science in Molecular Ecology. Galway-Mayo Institute of Technology.
Cobb, J. and Phillips, B. (2012). ˜Theœ Biology and Management of Lobsters. Burlington: Elsevier
Science.
Eaton, D., Brown, J., Addison, J., Milligan, S. and Fernand, L. (2003). Edible crab (Cancer pagurus)
larvae surveys off the east coast of England: implications for stock structure. Fisheries Research, 65(1-3),
pp.191-199.
Edwards, E. (1979). The edible crab and its fishery in british waters. Surrey: Fishing news books.
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. The Irish Naturalists' Journal, 29, 119-121.
Haig, J., Bakke, S., Bell, M., Bloor, I., Cohen, M., Coleman, M., Dignan, S., Kaiser, M., Pantin, J.,
Roach, M., Salomonsen, H. and Tully, O. (2016). Reproductive traits and factors affecting the size at
maturity of Cancer pagurus across Northern Europe. ICES Journal of Marine Science: Journal du Conseil,
73(10), pp.2572-2585.
Hunter E, Eaton D, Stewart C, Lawler A, Smith MT (2013) Edible Crabs ‘‘Go West’’: Migrations and
Incubation Cycle of Cancer pagurus Revealed by Electronic Tags. PLoS ONE 8(5): e63991.
doi:10.1371/journal.pone.006399.
Jones G, Gibson P, Dobby H, McLay A, (2010). Brown Crab (Caner pagurus) Migrations off the
Northern Scottish Coast. Scottish Industry Science Partnership Report 02/10. Marine Scotland Science.
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Lamb, S. (2011). 2010/2011 Project Review. Orkney Sustainable Fisheries Ltd.
Mesquita C, Dobby H, McLay A (2016) Crab and Lobster Fisheries in Scotland: Results of Stock
Assessments 2009 -2012. Scottish Marine And Freshwater Science Vol 07 No. 09. Marine Scotland
Science, ISSN:2043-7722.
Pearson, J. (1908). Cancer (the edible crab). Liverpool Marine Biology Committee Memoirs, 16, p.263.
Ungfors, A. (2006). Sexual maturity of the edible crab (Cancer pagurus) in the Skagerrak and the
Kattegat, based on reproductive and morphometric characters. ICES Journal of Marine Science, 64(2),
pp.318-327.
Ungfors, A., Hallbäck, H. and Nilsson, P. (2007). Movement of adult edible crab (Cancer pagurus L.) at
the Swedish West Coast by mark-recapture and acoustic tracking. Fisheries Research, 84(3), pp.345-357.
Ungfors, A., McKeown, N., Shaw, P. and Andre, C. (2009). Lack of spatial genetic variation in the edible
crab (Cancer pagurus) in the Kattegat-Skagerrak area. ICES Journal of Marine Science, 66(3), pp.462-
469.
21
Ungfors, A., McKeown, N.J., Shaw, P.W. and André, C., 2009. Lack of spatial genetic variation in the
edible crab (Cancer pagurus) in the Kattegat–Skagerrak area. ICES Journal of Marine Science: Journal
du Conseil, 66(3), pp.462-469.
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1
Appendix
Appendix 1 Average distance travelled by recaptured brown crab individuals in relation to days at liberty (2010 -2016
Appendix 2 Details of recaptures, distance between release and recapture sites, bearing and day between releases and recapture
of crabs tagged in 2010& 2011. Dashed lines separates days at liberty >30days
Year Tag_Number Sex CW Sperm
Plug
Days at
Liberty
Distance
Travelled (km) Direction
2010 15327 M 164 NA 5 0.24
2010 15108 M 179 NA 6 0.36
2010 15397 M 172 NA 13 0.71
2010 14494 M 158 NA 19 0.50
2010 14611 M 188 NA 19 0.65
2010 14416 M 157 NA 20 0.54
2010 14454 M 163 NA 20 0.54
2010 15108 M 163 NA 20 0.58
2010 15327 M 205 NA 21 0.18
2010 15344 M 217 NA 21 0.59
2010 15402 M 217 NA 24 28.17
2010 14465 M 164 NA 26 0.92
2010 14693 M 166 NA 26 0.63
2010 15123 M 164 NA NA 1.33
2011 1881 F 152 NO 56 8.75
0
50
100
150
200
250
300
2 4 6 9
12
15
19
21
24
27
31
34
36
42
45
49
52
63
68
72
86
10
6
13
6
15
6
21
1
25
1
33
7
35
1
44
5
Dis
tance T
ravelle
d (
Km
)
Days at Liberty
Female
Male
2
Appendix 3 Details of recaptures, distance between release and recapture sites, bearing and day between releases and
recapture of crabs tagged in 2012. Dashed lines separates days at liberty >30days
Year Tag_Number Sex CW Sperm
Plug
Days at
Liberty
Distance
Travelled (km) Direction
2012 225 M 145 NA 4 5.26 W
2012 228 M 140 NA 4 0.72 NW
2012 177 M 136 NA 7 10.11 N
2012 182 M 138 NA 7 0.53 W
2012 293 M 133 NA 7 0.88 NW
2012 187 M 130 NA 9 0.57 N
2012 291 M 151 NA 9 0.65 SW
2012 208 M 133 NA 13 0.92 SW
2012 247 M 148 NA 13 0.90 W
2012 293 M 133 NA 15 0.69 S
2012 225 M 145 NA 17 0.55 E
2012 111 M 154 NA 26 1.01 N
2012 228 M 140 NA 27 0.34 S
2012 245 M 139 NA 33 0.79 W
2012 187 M 130 NA 34 12.48 S
2012 283 M 135 NA 35 0.15 NW
2012 160 M 137 NA 36 0.37 S
2012 165 M 147 NA 36 4.81 NW
2012 170 M 140 NA 36 0.56 W
2012 225 M 145 NA 39 0.86 SW
2012 147 M 140 NA 42 18.39 W
2012 171 M 143 NA 44 2.01 S
2012 1329 M 150 NA 58 1.2 S
2012 112 M 139 NA 63 0.47 N
2012 141 M 154 NA 63 0.32 N
2012 156 M 142 NA 63 0.47 S
2012 167 M 151 NA 63 0.12 S
2012 134 M 133 NA 64 2.53 SW
2012 135 M 141 NA 64 3.12 W
2012 116 M 140 NA 68 1.07 N
2012 186 M 147 NA 69 1.18 S
2012 111 M 154 NA 72 0.16 SE
2012 173 M 166 NA 72 8.13 SW
2012 166 M 160 NA 73 1.93 S
2012 132 M 136 NA 98 0.24 S
2012 110 M 134 NA 114 3.66 SW
2012 125 M 142 NA 114 4.72 NW
2012 18 M 137 NA 138 3.64 E
2012 214 F 138 NA 17 2.38 S
2012 233 F 161 NA 17 0.97 W
3
Appendix 4 Details of recaptures, distance between release and recapture sites, bearing and day between releases and
recapture of crabs tagged in 2015. Dashed lines separates days at liberty >30days
Year Tag
Number Sex CW
Sperm
Plug
Days at
Liberty
Distance Travelled
(km) Direction
2015 22518 M 140 NA 4 3.79 W
2015 24179 M 142 NA 6 0.1 W
2015 317 M 139 NA 19 6.86 N
2015 23311 M 147 NA 47 67.56 SW
2015 23333 M 170 NA 243 4.04 E
2015 24125 M 157 NA 251 2.87 N
2015 22886 M 137 NA 344 3.88 SE
2015 22520 M 168 NA 445 12.72 S
2015 23312 M 145 NA NA 16.37 NE
2015 22511 F 138 NA 2 3.5 SW
2015 385 F 149 NA 3 1.35 W
2015 322 F 150 NA 3 1.28 SW
2015 338 F 152 NA 3 1.28 S
2015 358 F 158 NA 3 1.28 W
2015 301 F 175 NA 3 1.35 W
2015 22871 F 158 NA 5 0.34 S
2015 22852 F 163 NA 5 0.51 NW
2015 22508 F 154 NA 6 0.1 NW
2015 22521 F 148 NA 9 2.24 W
2015 24207 F 154 NA 9 10 S
2015 24236 F 158 NA 9 10.67 SW
2015 24265 F 163 NA 9 1.38 SW
2015 24266 F 165 NA 9 1.12 SW
2015 24272 F 182 NA 9 2.82 W
2015 24259 F 188 NA 9 1.137 W
2015 22802 F 135 NA 10 0.54 W
2015 380 F 146 NA 10 1.7 SW
2015 303 F 147 NA 10 1.681 W
2015 390 F 151 NA 10 1.92 SW
2015 311 F 152 NA 10 1.41 W
2015 309 F 161 NA 10 1.68 SW
2015 387 F 167 NA 10 1.7 SW
2015 392 F 170 NA 10 1.92 SW
2015 22507 F 183 NA 12 4.01 W
2015 24103 F 158 NA 19 0.8 W
2015 22509 F 161 NA 21 16.41 NW
2015 25728 F 170 NA 22 0.125 NW
2015 24293 F 193 NA 28 2.92 NW
2015 24285 F 158 NA 31 1.88 NE
4
2015 24277 F 144 NA 35 1.44 NW
2015 383 F 165 NA 45 1.88 NW
2015 24236 F 158 NA 52 53.05 W
2015 23300 F 129 NA 86 65.05 SW
2015 23348 F 168 NA 86 44.32 W
2015 25721 F 190 NA 106 178.25 SW
2015 24168 F 160 NA 156 178.25 W
2015 24233 F 147 NA 161 91.25 SW
2015 24246 F 168 NA 161 87.45 SW
2015 24103 F 158 NA 212 8.98 NE
2015 25753 F 175 NA 336 236.6 SW
2015 23375 F 172 NA 337 64.58 SW
2015 24225 F 159 NA 351 93.86 SW
2015 24268 F 186 NA 378 258.12 SW
2015 315 F 147 NA NA 87.75 SW
2015 24242 F 154 NA NA 2.43 W
Appendix 5. Details of recaptures, distance between release and recapture sites, bearing and day between releases and
recapture of crabs tagged in 2016. Dashed lines separates days at liberty >30days
Year Tag Number Sex CW Sperm
Plug
Days at
Liberty
Distance Travelled
(km) Direction
2016 11502 M 158 NA 12 8.79 N
2016 8577 M 164 NA 21 9.74 W
2016 9291 M 176 NA 45 0.71 S
2016 9270 M 172 NA 49 0.99 SE
2016 9227 M 172 NA 52 11.57 N
2016 11577 M 165 NA 70 3.51 E
2016 10189 M 160 NA 98 36.92 S
2016 8542 F 158 NA 12 22.27 SW
2016 8514 F 170 NA 51 7.643 S
2016 11546 F 176 NO 91 9.54 N
2016 10126 F 178 NO 100 26.13 W
2016 8685 F NA NA 119 24.85 E
2016 9581 F 173 NA 136 160.41 SW
5
Appendix 6 Identification of potential brooding sites based on habitat predictive mapping and brown crab tag recaptures situated
of the Isle of Lewis.
Appendix 7. Identification of potential brooding sites based local bathymetry and brown crab tag recaptures situated of the Isle
of Lewis.
6
