Predicting sustainability of fish stock using a simple length-based approach: An example of little tunny in the Southeast Atlantic

Abstract

The stock status of little tunny ( Euthynnus alletteratus ) in the Southeast Atlantic remains poorly understood, posing a major challenge for effective fisheries management. This study applies the aggregated Length-Based Indicator (aLBI) framework to conduct a data-limited, length-based assessment, with a specific focus on purse-seine fisheries operating on Fish Aggregating Device (FAD) and free-school (FSC) fishing modes. Length-frequency data from more than 22,000 individuals were used to estimate key life-history parameters and Froese sustainability indicators, including the proportion of mature fish (Pmat), the proportion harvested at optimal length (Popt), and the proportion of mega-spawners (Pmaga). Results indicate that FAD-associated fishery is severely depletion, with a high probability that stock biomass is below the limit spawning biomass reference point (LSB25). In contrast, the FSC fishery appears to be in a substantially better condition exhibiting indicator consistent with a more sustainable exploitation pattern. Sensitivity analysis show that the assessment outcomes are robust to uncertainty in asymptotic length (Linf) but highly sensitive to estimates of length at maturity (Lmat), identifying this parameter as a critical research priority. The study represents the first application of the aLBI framework to assess the stock status of little tunny across the Atlantic Ocean and highlights the utility of length-based methods for informing management in data-limited fisheries. We conclude that urgent, gear-specific management measures are required to ensure stock sustainability, particularly through the regulation of fishing mortality in the FAD associated fishery.