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Research publication: Evaluation of MSY-based management strategies to variations in carry capacity or migration of Atlantic Bluefin tuna

Fromentin and Kell (eTIP result: ) showed that identifying the underlying process that generates long-term variations in bluefin tuna catch is of key importance, because distinct processes lead to different perceptions of stock status.

In this study, the performances of stock assessment methods with respect to:
- Their ability to provide good estimates of MSY, FMSY and BMSY, and
- Assessing stock status and exploitation level relative to these MSY targets were evaluated.

In addition, the robustness of the current ICCAT management strategy (based upon MSY) to uncertainty about the true population dynamics and historical exploitation levels was evaluated and contrasted with a simpler management strategy. Reference points appear to be more robust to dynamic uncertainty than the estimates of absolute values and trends in F and SSB, but they perform better when fluctuations in catch result from changes in carrying capacity than in migration. F-based reference points were less biased and more precise than biomass- and/or yield-based reference points and F0.1 appeared to be the best proxy for FMSY. However, F0.1 cannot indicate past and current levels of exploitation relative to FMSY when there is uncertainty about the dynamics. The F0.1 management strategy based on ADAPT-VPA stock size estimates finally only performed slightly better than a strategy based on a simple size limit in term of SSB. However, the former leads to much lower catch levels than the latter.

We conclude that the performances and robustness of distinct management strategies strongly depend on biological processes (i.e. the underlying dynamics) and when they are implemented relative to the stock size cycle, but also on concrete objectives, such as fleet composition, gear selectivity, economic constraints. Such an investigation can only be resolved through simulation studies.

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