Assessing the sustainability of frigate tuna (

The frigate tuna holds a predominant position in the neritic tuna landings of Sri Lanka. Despite the importance of frigate tuna in fisheries, a declining trend in annual landings was observed in recent years. However, scientific assessments of the stock status and sustainability of frigate tuna fishery are extremely scarce in Sri Lankan waters. Considering the facts of the tendency of frigate tuna to thrive close to the continental shelf while having short-distance migrations and the existence of a single stock in Sri Lankan waters, two length-based approaches—Length-Based Indicators (LBIs) and Length-Based Spawning Potential Ratio (LBSPR)—were applied using forked length data from the Indian Ocean Tuna Commission database (2018-2022) to address the research gaps. The von Bertalanffy growth parameters were estimated at L∞ = 64.8 cm (FL); K = 0.521 yr−1; mean M = 0.925 yr−1. All the LBIs indicate a declining trend over the study period. The LBIs for the conservation of immature and mature individuals have fallen below the reference level since 2020. Furthermore, LBSPR results also indicated a declining trend which fell below the limit reference point of SPR 20% since 2020. The modes of the length distribution in the catch and the length (FL) at 50% and 95% selectivity also declined throughout the study period. All the model outcomes indicate the unsustainability of the fishery and overexploitation of the frigate tuna resource in Sri Lankan waters. The growing use of ring nets, contributing approximately 75% of the annual frigate tuna catch, is likely a key driver of the fishery’s unsustainability. To promote a sustainable fishery, immediate reductions in fishing mortality are required, which should include a halt to new licences for ring net fishing. Collaboration with the IOTC and neighbouring countries is crucial for regional resource management.