Advances in Biology & Earth Sciences

Understanding ontogenetic size structure is essential for describing the demographic composition of fish populations interacting with fisheries. Yellowfin tuna is a widely distributed pelagic species, yet information on its landed size composition in Indonesian waters remains limited. This study examined the ontogenetic size composition of yellowfin tuna landed in West Sumatra waters based on fork length (FL) data collected at the Oceanic Fisheries Port of Bungus during 2019, 2020 and 2024. A total of 8.774 individuals were classified into three operational size categories: juvenile (FL < 30 cm), subadult (30 ≤ FL < 90 cm) and adult (FL ≥ 90 cm). Landings were strongly dominated by adults (97.91%), while subadults (2.04%) and juveniles (0.05%) were minimally represented. Generalized additive modelling indicated significant interannual differences in mean fork length, with a baseline mean of 126.28 cm in 2019, increasing by 5.47 cm in 2020 and 1.50 cm in 2024 (p < 0.01), alongside a significant non-linear seasonal effect (p < 0.001). Because the dataset derives exclusively from port-based landings, the observed structure reflects recorded landing composition rather than the complete population-level ontogenetic distribution. These findings provide a statistically supported baseline for monitoring landed size dynamics while emphasizing cautious ecological interpretation.