Assessment of the stock status of blackfin tuna Thunnus atlanticus in the Southwest Atlantic Ocean: a length-based approach

This is the first assessment of blackfin tuna Thunnus atlanticus stock status. The aim of the present study was to estimate the status of its Southwestern stock using length-based models. A total of 1929 individuals was gathered in four years of length data (1998–2019). Two length-based models were applied, Length-Based Spawning Potential Ratio (LBSPR) and Stock Synthesis Data-limited tool (SS-DL) and five scenarios combining a set of growth parameters (asymptotic length-L∞, and growth constant-K) were built to accommodate different life histories available in the literature. The natural mortality (M) was estimated through four empirical methods. Given that the fishery only selects mature fish, length at selectivity were larger than the length at first maturity and high relative fishing mortality were found at most scenarios, even with high spawning potential ratio (SPR) values. The LBSPR estimated higher values of SPR, it presented a steeper decline than SS-DL between 1998 and 2019. In addition, growth parameters influence the estimation of the virginal stock, and intermediate M/K values seems to represent better the estimated length composition, given the low historical catch of the species. A declining pattern in SPR was observed over time with a reduction of 12% according to LBSPR and 7% decrease based on SS-DL methods. However, the overall evaluation of the Southwestern Atlantic stock of blackfin tuna in 2019 revealed an SPR range of 0.41 to 0.63, suggesting sustainable exploitation. Even with the limited data, we could have a proxy of stock status estimation by using length-based models which highlights the importance of such data. However, due to the high uncertainty of the results, better collection of catch, effort and length data should be considered. Additionally, SS-DL seems to estimate better SPR values when variability of growth parameters are placed for a given species.