Combination of genetics and spatial modelling highlights the sensitivity of cod (Gadus morhua) population diversity in the North Sea to distributions of fishing

Conserving genetic diversity in animal populations is important for sustaining their ability to respond to environmental change. However, the ‘between-population’ component of genetic diversity (biocomplexity) is threatened in many exploited populations, particularly marine fish, where harvest management regions may be larger than the spatial extent of genetically distinct sub-populations. Using single nucleotide polymorphism (SNP) data we delineated the geographic limits of three population units of Atlantic cod (Gadus morhua) in northwest European waters. Two of the populations co-habit the North Sea, and trawl survey data showed differing trends in their abundances. We developed a spatial model of these units to simulate population dynamics under spatial patterns of harvesting, Competition between units during the pelagic juvenile stages in the model led to suppression of the more localised northern North Sea (Viking) unit by the more widespread (Dogger) unit, and its premature extinction under some spatial patterns of fishing. Fishery catch limits for cod are set at the scale of the whole North Sea without regard to such sub-population dynamics. Our model offers a method to quantify adjustments to regional fishing mortality rates to strike a balance between maximising sustainable yield and conserving vulnerable populations