Evidence of spatial regime shifts: contrasting spatial dynamics of three Atlantic cod (Gadus morhua) populations in the North Sea.

Recent scientific assessments have started to consider the presence of multiple populations of Atlantic cod (Gadus morhua) in the North Sea due to genetic evidence. The Atlantic cod stock as a whole has experienced a regime shift in the North Sea and is now considered to be in a persistent depleted state. Here we investigated if the regime shift detected for North Sea cod manifested differently across areas (South, North-West and Viking (North-East)), revealing different dynamics and recovery potential for the corresponding populations. Methods from change point analyses 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 low state but with high resilience, making its dynamics more predictable and thus easier to manage. Finally, the north-western population did not show signs of a regime shift and is in a high biomass level. Several abiotic and biotic drivers (e.g. fishing, 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.