Seal population recovery and conservation can be reconciled with viable commercial fisheries in the North Sea, although both may be threatened by climate change

Seals are key predators in the North Sea, with the potential to exert top-down control on commercial fish populations and reshape energy flows throughout the foodweb. Grey seal numbers have increased considerably, leading to the worry that they might adversely impact the North Sea fishery. We use the StrathE2E2 model to investigate the joint effects of climate change and seal populations on an idealized 12-fleet North Sea fishery. StrathE2E2 is an end-to-end model of intermediate complexity which has been designed as a big-picture model of the ecosystem from nutrients to top predators, with the ability to look at bottom-up effects in a rigorous manner alongside the top-down impacts of fishing. We consider effects on the seal population for the reference climate period of today and for RCP8.5 around 2050. We do this through Management Strategy evaluation (MSE), which is a powerful way of simulation-testing management procedures and can allow the incorporation of ecosystem considerations via a suitable choice of operating model. Our MSE considers a range of possible management options using (a) common rules for the fleets and (b) emergent fleet dynamics based on regulated catch-maximization fleet behaviour. Hitherto most MSEs have considered a single stock, although there have been some multispecies studies. Other novel features of the study include the use of multi-functional-group harvest control rules for the fleet segments and application of an integrated risk measure of the impact of functional group depletion across the foodweb. We find that (a) there is a modest cost associated with larger seal populations, although healthy seal populations can co-exist with viable commercial fisheries at the ecosystem level; (b) climate change is expected to result in decreased foodweb productivity, leading to reduced fisheries yields and lower seal biomass, other things being equal. We find that the optimum mix of fleet segments is sensitive to minimum required biomasses for seals, indicating the importance of stakeholder participation in objective-setting. Our methods can readily be applied to other ecosystems where suitable modelling tools are available, and can help to inform fisheries management plans and associated tradeoffs.