Marine ecosystems provide essential functions and services, including nutrient cycling, biodiversity support, and food resources. However, these services are increasingly threatened by disturbances such as climate change and overfishing. These threats highlight the urgent need for management strategies that promote the stability and resilience of marine ecosystems. The resilience of a fish population is commonly assessed by measuring how far it is from, or how quickly it returns to, fixed biological reference points—indicators of its potential to recover. However, not all populations behave in ways that align with fixed benchmarks. Some marine fish populations display state-dependent dynamics, where their population states change continuously over time rather than fluctuating around a stable equilibrium. In such cases, resilience is better understood relative to a population’s current state rather than static reference points. Monitoring how resilience evolves in these dynamic systems can provide valuable insights into the underlying mechanisms of population recovery.
A well-known example is the Atlantic cod population in the North Sea, which has experienced prolonged recovery delays. As a key species in one of the world’s oldest fisheries, North Sea cod have played a major role in shaping the history, economy, and culture of Western Europe since the 16th century. Today, this population faces significant challenges, including rising ocean temperatures, more frequent and intense extreme weather events, and the risk of irreversible regime shifts in the North Sea ecosystem. Over the past few decades, the cod population has experienced several abrupt declines, with the most dramatic occurring around the year 2000, primarily due to a sharp decrease in juvenile survival. That same year also marked the introduction of fishing reforms under the European Union’s Common Fisheries Policy, which led to reduced fishing pressure. Yet despite these efforts, the population remains at low levels. This raises a critical question: Can changes in the system’s dynamic stability and age group interactions over time help explain the persistent struggles of the North Sea cod population?
Adding further complexity, recent evidence shows that Atlantic cod in the North Sea and adjacent waters form a meta-population—a larger population made up of three reproductively isolated subpopulations: the northwestern, southern, and viking subpopulation. Recent research has revealed that these subpopulations differ in genetic makeup, physical characteristics, life history traits, physiology, and rates of maturity and growth. Since 2024, stock assessments have been conducted separately for each subpopulation. These findings highlight the importance of studying cod dynamics at the subpopulation level to support more targeted and effective fisheries management. For instance, recent studies show that the three subpopulations respond differently to environmental pressures such as fishing and temperature changes, and they experience regime shifts at different times. However, key questions remain unanswered: How has the dynamic stability of each subpopulation changed over time? What factors have driven these changes?
In this project, we use a dynamical systems approach to assess the stability of Atlantic cod populations across time and space in the North Sea and adjacent waters. Our three main objectives are: 1) To track changes in population dynamical stability over time and across geographic regions; 2) To understand how environmental and human-driven factors influence the stability of the three cod subpopulations; and 3) To provide scientific support for fisheries management and broader research on marine ecosystem stability.
