Unmasking contrasting regime shift dynamics across three substocks of Atlantic cod in the North Sea

Abstract The Atlantic cod (Gadus morhua) in the North Sea has experienced a regime shift and is now considered to be in a persistent depleted state. Recent scientific evidence confirmed the presence of multiple populations of Atlantic cod in the North Sea through genetic evidence. Here, we investigated if the regime shift detected for North Sea cod manifested differently across areas [Southern, Northwestern-, and Viking (North–East)], revealing different dynamics and recovery potential for the corresponding populations. Methods from driver–state plots to stochastic cusp models were applied to detect the regime shifts. We found that cod in the Southern North Sea has undergone a regime shift and is now in a persistent depleted state. The Viking population is in a high resilience state, and it is recovering. Finally, the Northwestern North Sea population did not show signs of a regime shift and is showing a rising biomass level. Several abiotic and biotic drivers (e.g. fishing and warming) contribute to these differences. Our study provides useful information for the restructuring of management units of cod in the North Sea and highlights the importance of looking at regime shifts spatially.