Distinct dynamic stability patterns among three Atlantic cod subpopulations in the North Sea

Atlantic cod (Gadus morhua) in the North Sea and adjacent waters has been extensively studied due to its continued population decline despite reduced fishing pressure. Recent evidence reveals that it forms a metapopulation consisting of three subpopulations-- Northwestern, Southern, and Viking--highlighting the need to understand their distinct dynamics to support spatially targeted fisheries management. We applied a dynamical systems approach to quantify two key properties for each subpopulation from 1983 to 2023: dynamic stability, indicating whether subpopulations tend to return to or move away from their state each year, and the relative sensitivity of age groups, reflecting the magnitude of change in each groups abundance in response to perturbations in the age-structured interaction network. We found distinct dynamic stability patterns among the three subpopulations. The Northwestern subpopulation fluctuated between stable and unstable states before stabilizing at low abundance. The Southern subpopulation shifted from high-abundance, stable states toward low abundance at a transient with an uncertain future. The Viking subpopulation remained stable with relatively constant abundance over time. Throughout the study period, the age-1 group remained highly sensitive to perturbations in both the Southern and Viking subpopulations, whereas no single age group showed dominant sensitivity in the Northwestern subpopulation. Fishing mortality causally influenced the abundance and/or stability of the Northwestern and Southern subpopulations, but not the Viking subpopulation. In contrast, sea bottom temperature did not affect the abundance or stability of any of the subpopulations. Our findings reveal distinct patterns in abundance, dynamic stability, relative sensitivity of age groups, and responses to external drivers across the three subpopulations, highlighting the importance of accounting for the complex population structure of Atlantic cod in future research, assessments, and management efforts.