Stock Rebuilding in a Mixed‐Population Fishery Context: The Case of North Sea and Western Baltic Herring

ABSTRACT Managing and giving advice for mixed‐population fisheries of the same species can be challenging due to difficulties in predicting stock mixing, technical interactions, and in ensuring precautionary stock dynamics. Most fish species consist of several populations all of which need to be sustainably managed to preserve biodiversity and resilience to environmental changes, and to sustain productive fisheries. Mixing of populations of the same species with different productivity represents additional challenges for assessment and management. In such cases, there is a need to develop assessment and advice methods to consider mixed stocks and advise managers on fishing opportunities. In this study, we develop a framework including multifleet multi‐stock assessment models and forecasts where future stock dynamics depend on stock mixing, fleet‐specific fishing opportunities, and management in place. This framework is applied to the large North Sea herring stock and the overexploited western Baltic herring stock, which in part of their distribution areas are found to be mixed in both catches and surveys. The framework is used to investigate the trade‐offs between stock rebuilding and fishing opportunities in a mixed fishery. The study shows that spatial management implying transfer of quotas away from the areas where the stocks mix and towards the stock in good condition could help stock rebuilding without reducing overall fishing opportunities, however with a slower rebuilding rate compared to a more drastic fishery closure. The implementation of quota transfer should however consider the spatial relocation of fishing opportunities to avoid catch advice overshoot and inflated quotas.