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  • 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.

  • I

    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.

  • II

    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

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