Non-linear and unstable stock-recruitment dynamics in a depleted top-predator population: the case of Western Baltic cod.

Atlantic cod is traditionally an important fishery target worldwide and has in addition to it’s ecological importance important cultural, social and economic and ecological functions. However, worldwide most of the population of this top-predator collapsed around the 1990’s are still largely depleted. Because of its importance, the management of Atlantic cod has been a priority for many countries over the last 30 years.

In this study, we investigated the dynamics of the cod population applying a range of techniques to detect non-linear and unstable dynamics in the stock-recruitment relationship, such as change point and wavelet analysis. Our results show that Western Baltic cod recruitment is driven by both spawning stock biomass and temperature but that the relationship between recruitment and these variables is not consistent over time. Subsequently, we applied the stochastic cusp model to understand how temperature can interact with the stock-recruitment relationship. We found that the recent temperature increase in the Western Baltic Sea induced discontinuous dynamics of the stock-recruitment relationship which resulted in a stable low productivity regime apparently keeping the stock in a low biomass state. Our results have important implications for management and recovery efforts for this depleted top-predator population and confirm the need for ecosystem-based management.