Workshop on stock identification of North Sea cod (WKNSCodID)

The Workshop on Stock Identification of North Sea Cod (WKNSCodID) reviewed information on population structure of Atlantic cod (Gadus morhua) in the North Sea and adjacent waters to recommend the most plausible scenario of population structure for stock assessment and fishery management advice. The review considered geographic variation and movements of cod life stages inferred from genetic analyses, scientific surveys, fishery data, tagging, life-history, distribution of eggs and larvae, otolith microchemistry and shape, and parasites. Based on the review, several population structure scenarios were hypothesized (including the scenario assumed in the current advisory unit), and plausibility of each scenario was evaluated. Practical implications of each scenario, including the derivation of a catch time-series, were considered to form recommendations for benchmark stock assessment workshops. Since 1996, cod were assessed as a single stock in th North Sea (Subarea 4), Skagerrak (Subdivision 20), and the Eastern English Channel (Division 7.d). Adjacent advisory units are Kattegat cod (Subdivision 21), Norwegian coastal cod (subareas 1–2), Faroe Plateau cod (Subdivision 5.b.1), cod West of Scotland (Subdivision 6.a), and cod in the Western English Channel and southern Celtic Sea (subdivisions 7.e–k). A large body of scientific information is available for identification of cod population structure in the North Sea and adjacent areas, and a diverse group of experts participated in the workshop to help meet WKNSCodID’s objectives. WKNSCodID concluded that North Sea cod appear to be isolated from the cod population on the Faroe Plateau (Subdivision 5.b.1) and Norwegian Coastal Cod (subareas 1–2). Significant and persistent patterns of genetic variation indicate reproductively-isolated populations of Viking cod and Dogger cod that have some spatial overlap and mixing after spawning. The Skagerrak and northern Kattegat appear to be a nursing ground for Viking and Dogger cod, with most cod in the Skagerrak being Viking cod. These genetically different groups have different rates of maturity and growth. Trends in biomass and recruitment are strongly correlated among subareas of the North Sea, but subarea trends diverged in the last decade, with no apparent rebuilding in the southern North Sea. The common trends in biomass and recruitment among subareas suggest that there is some mixing of populations after spawning in some areas of the North Sea, and common environmental factors throughout the region...