Climate-induced shifts in cod spawning phenology across the North Atlantic.

Spawning underpins fish stock regeneration. Here, we investigate climate effects on spawning phenology in Atlantic cod (Gadus morhua), a severely exploited species whose stocks are now also affected by climate change. For the first time, we model daily cod egg occurrences as a spawning proxy across the North Atlantic. With a concordance of 84% between our model and basin-scale observations, our results predict different climate change outcomes for western (eastern Newfoundland) and eastern (North Sea) stocks. In eastern Newfoundland where cod is at the centre of its thermal niche, a phenological shift in spawning was observed. In the warmer North Sea, such a phenological adjustment did not occur, leading to a substantial reduction in spawning. Our model explains fluctuations in the North Sea cod stock better than those in the Eastern Newfoundland stock, where unaccounted anthropogenic (overfishing) and environmental (trophodynamics) factors may have influenced cod abundance over the considered time series. Our results provide an improved understanding of how future climate change and other drivers interact in driving stock dynamics.