Population structure, stock status, and management implications of longtail tuna (Thunnus tonggol) in the Natuna Sea, Indonesia

This The Natuna Sea, a strategic and biodiverse marine area, supports significant fishery resources. This study investigated the population dynamics of longtail tuna (Thunnus tonggol), a species of considerable ecological and economic importance, to provide a scientific basis for sustainable fisheries management in the region. Monthly fork length data were collected throughout 2021 from drift gillnet catches landed at Pemangkat Fishing Port, West Kalimantan, and were analyzed using the FiSAT II software package. Key population parameters were estimated as: asymptotic length (L?) = 114.45 cm, growth coefficient (K) = 0.36 year-1, and theoretical age at zero length (t?) = - 0.2865 years. Natural mortality (M) was 0.65 year-1, fishing mortality (F) was 0.58 year-1, and total mortality (Z) was 1.23 year-1. These values yielded a current exploitation rate (E) of 0.43. Recruitment was bimodal, occurring in March–April and August–September. The length at first capture (Lc) was estimated at 54.33 cm. The spawning potential ratio (SPR) was estimated to be 30%, a level considered above the minimum biological limit reference point for sustainability. The yield-per-recruit analysis demonstrates that the existing exploitation rate (E = 0.43) falls short of the rates necessary to attain maximum yield per recruit (Emax = 0.598) and the optimal yield per recruit (E0.1 = 0.507). These findings suggest that the T. tonggol stock in the Natuna Sea is currently in a developing exploitation phase, with potential for carefully managed increases in fishing effort while ensuring long-term sustainability.