Designing spawning closures can be complicated: Experience from cod in the Baltic Sea

Fisheries management measures often include spatio-temporal closures during the spawning period of the fish with an overarching aim of improving the stock status. The different mechanisms how a spawning closure potentially can influence the stock are often not explicitly considered when designing such closures. In this paper, we review and synthesize the available data and knowledge on potential effects of the implemented spawning closures on cod in the Baltic Sea. The Baltic cod example represents a relatively data rich case, which allows demonstrating how a closure might affect different parameters of stock status via different mechanisms, including potential unintended negative effects. We conclude that designing relatively small area closures appropriately is highly complex and data demanding, and may involve tradeoffs between positive and negative impacts on the stock. Seasonal closures covering most of the stock distribution during the spawning time are more robust to data limitations, and less likely to be counterproductive if suboptimally designed.