A paradigm shift: using catch and abundance indices to assess the impact of tuna purse seiner FAD and FSC fishing on the stock status of little tunny in the Northeast Atlantic Ocean

Abstract

The Atlantic Ocean’s little tunny is a priority for research by the International Commission for the Conservation of Atlantic Tunas (ICCAT). Limited studies exist on Northeast Atlantic Ocean stock dynamics, and conventional catch and abundance assessments are lacking. This study assesses the impacts of tuna purse seiner fish aggregating device (FAD) and free-swimming school (FSC) fishing and proposes models for assessment using ICCAT data aggregated into different time blocks. Applying JABBA (Just Another Bayesian Biomass Assessment) and CMSY++, model diagnostics indicate that the 2011–2021 time series arrangement offers more reliable stock status estimates. Both models show overfished status (biomass/biomass max sustainable yield [B/B_MSY] < 0.5) from overfishing (fishing mortality/fishing mortality max sustainable yield [F/F_MSY] > 1.5) by the FAD and FSC techniques. FAD catches have leveled with the stock's average surplus production (ASP), while FSC catches are closer to ASP. JABBA projections suggest B_MSY for the stock in 2023–2024, with FSC and FAD catch limits of 3000–4000 tonnes. By implication, the stock may collapse from tuna purse seiner's FAD and FSC fishing unless catch limits are implemented. JABBA is most effective for little tunny with shorter catch and catch per unit effort (CPUE) data, while CMSY++ indicates overfished stock status with and without abundance indices. Both applied models are suitable for little tunny stock status prediction, but considering the abundance indices used in this study, management advice should be applied cautiously.