Cod at drift in the North Sea

Abstract

There has been a large-scale geographical re-distribution of the North Sea cod stock over the past century, and recent surveys indicate a north-eastern modal distribution. Here we assess the consequences of the contemporary distribution of North Sea cod (Gadus morhua) spawning biomass to inter-ocean recruitment potential. By simulations of drifting cod eggs and larvae spawned in the northern North Sea over 16 spawning seasons (in the period 1995–2016), we show that a large portion of the North Sea produced pelagic juveniles most likely settle along the Norwegian Sea shelf. For example during the early 2000s when the North Sea cod spawning biomass was at its lowest, 20% to 27% of larvae produced in the northern North Sea most likely settled along the Norwegian Sea shelf, while as few as 8% and 10% were retained within the North Sea in some years. We hypothesise the spillover of North Sea cod into nursery habitat along the Norwegian north-western coast to be beneficial to the stock, as larvae would encounter far higher abundances of their favoured prey, the copepod Calanus finmarchicus. Looking back at a century of overfishing, warming, and variable nursery conditions for cod in the North Sea, getting entrained in the Norwegian coastal current seems like a viable “back-door exit” strategy, allowing the north-eastern spawning cod to thrive even in seemingly adverse climatic periods.