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Scottish Marine and Freshwater Science Reports

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Determination of the distribution of the resident inshore and offshore migratory cod populations around Shetland (IVa) and westwards into VIa

Scottish Marine and Freshwater Science Vol 7 No 28

The current genetic analysis alludes to finer scale structuring of Atlantic cod stocks in the IVa and VIa stock units than had previously been reported by Heath et al. (2014). Consistent with previous studies of maturation, cod from Viking sampled in 2014 matured at a later age and larger size than other areas, providing a phenotypic population marker. During spawning time there was no indication that the Viking group extended beyond the > 100 m waters of the northern North Sea. Indeed, the new genetic and maturity evidence suggests that Shetland coastal cod (ShIE) appear to extend into waters > 100 m east of Shetland. The possible separation of cod from Scottish inshore waters from those offshore is also reminiscent of the inshore-offshore division seen in the northern North Sea. There is some indication of mixing of populations outside the breeding season in the genetic analysis as well as the observation of large immature cod present in west coast samples. The present study has considerably expanded our understanding of the Viking cod from northern IVa and when combined with the studies by Poulsen et al. (2011) and Heath et al. (2014), provides a good indication of population extent at spawning time and suggests a split around 0030 W.

Doyle, A., Davie, A., Wright, P., Coull, K. and Angus, C. (2016) Determination of the distribution of the resident inshore and offshore migratory cod populations around Shetland (IVa) and westwards into VIa. Scottish Marine and Freshwater Science Vol 7 No 28, 32pp. DOI: 10.7489/1882-1
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Spatial / Geographical Coverage Location
Temporal Coverage
2013-01-01 to 2016-12-01
UK Open Government Licence (OGL)
Alice Doyle
Data Dictionary

The aims of this project were to determine the western extent of the offshore cod sub-population that inhabits the waters around Shetland as juveniles, and to determine the separation at maturity between coastal and offshore populations of cod during spring and autumn. Methods Sample Analysis Biological material (gonad samples, otolith and gill clipping for genetics), along with other biological measures (length, sex & macroscopic maturation stage) were collected from six areas; Shetland east coast - inshore and offshore (ShIE, Viking), Shetland west coast – inshore and offshore (ShIW, ShOW), and Scottish west coast – inshore and offshore (ScIW, ScOW) (Figures 1 & 2). ShIW corresponds to the coastal cod group known to show high site fidelity to the western waters of Shetland (Neat et al., 2006). Samples were taken during the autumn when mixing among populations may occur, and again in February and March during the spawning season. The requested sampling protocol for samplers is given in Appendix 1. The NAFC Marine Centre collected samples from the east and west of Shetland whilst SFF collected samples from VIa. Due to poor weather conditions NAFC were unable to collect many samples from offshore areas but fortunately, additional material was provided by MSS from commercial and research vessels so that all six areas had at least the minimum sample requirements for analyses. In total 1524 cod were obtained (721 from November - December, 803 from February – March; see Figure 1 & 2 for breakdown). For samples from research vessel catches, weight measurements were also obtained. The samples obtained complement past sampling programmes (e.g. see Heath et al., 2014) and considerably expand the westward extent of past sampling. All biological samples have been relocated to the MSS Marine Laboratory, Aberdeen. To ensure optimal quality of samples, all gill clippings received were transplanted (and occasionally sub-sampled) into fresh vials of ethanol to minimise degradation. Similarly, ovary samples were topped up with fresh NBF to ensure maximal fixation.

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