Closing certain essential fish habitats to fishing could be a win-win for fish stocks and their fisheries – Insights from the western Baltic cod fishery

The management of the western Baltic cod relies on a combination of regulation tools, including a seasonal spawning fishing closure. The complex population dynamics of the stock have nevertheless been posing considerable challenges to design closures, and the actual benefits of the seasonal spawning closure are still unclear. Often, only biological indicators have been used to evaluate the effectiveness of the closure, without considering socio-economic effects on the fishery. In this study, we applied a Species Distribution Model (SDM) integrating commercial fishery and research survey data on a 15-year time series to design multiple alternative spatial closures, all based on identifying persistent essential fish habitats (i.e., nursery, spawning, and feeding grounds). We further used the spatial-explicit Management Strategy Evaluation (MSE) tool DISPLACE to contrast the outcomes of these fishing closures, and identify which provided the optimal balance between socio-economic and biological demands and sustainability. Our results indicated that all closures benefitted the fisheries and stock. Although fishermen redirected their fishing effort to some extent to other stocks, increased profits were largely driven by increased cod catches as a consequence of reconstructed stock structure and enhanced spawning biomass. We conclude that the benefits of the closures are more linked to their size, than their actual purpose (i.e., protecting nursery, spawning, or feeding grounds).