Performance Of Stock Assessments For Mixed-Population Fisheries: The Illustrative Case Of Atlantic Bluefin Tuna

Accounting for movement and mixing in stock assessment is important for managing sustainable fisheries, particularly for highly migratory species. However, many fisheries management approaches continue to use single-stock, single-area models to assess mixed-population stocks that are known to have complex movement dynamics. We evaluated a single-stock, single-area stock assessment model's performance on fishery pseudodata generated using a spatially complex operating model that incorporates movement and mixing of simulated Atlantic bluefin tuna-like populations. Structural model misspecification produced positively biased perceptions of size and productivity of the smaller western population, based on supplement by the larger eastern population, and negatively biased perceptions of the size and productivity of the eastern population due to net movement of fish out of the eastern stock area. This bias could lead to unintended overexploitation of the smaller western population and potential for foregone yield of the larger eastern population. Our findings provide a greater understanding of the effects of movement and mixing on single-stock, single-area model-based management approaches and emphasize the importance of explicitly considering these dynamics in ensuring the sustainability of highly migratory species like Atlantic bluefin tuna.