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.