Reproductive Biology and Spatial Maturity Patterns of Skipjack Tuna (Katsuwonus pelamis) and Mackerel Scad (Decapterus macarellus) in Western Sumatra Waters (FMA 572), Eastern Indian Ocean

Understanding reproductive dynamics of pelagic fishes is essential for sustainable fisheries management in data-limited tropical systems. This study presents an integrated assessment of the reproductive biology of Katsuwonus pelamis and Decapterus macarellus in Fisheries Management Area 572 (western Indonesia). A total of 1529 individuals (787 individuals of K. pelamis and 742 individuals of D. macarellus) were examined using macroscopic staging with histological validation to characterize gonadal development, size at first maturity, fecundity, and spatial patterns of maturity distribution. Both species exhibited asynchronous ovarian development consistent with batch spawning and extended reproductive activity during the May–July 2025 survey. Estimated L50 = 42.3 cm FL and L95 = 52.5 cm FL for K. pelamis, and L50 = 21.9 cm FL and L95 = 29.0 cm FL for D. macarellus. Batch fecundity increased non-linearly with body size, indicating a disproportionate reproductive contribution of larger females. Mature individuals were recorded in all lunar phases and occurred more frequently in offshore fishing grounds (>12 nautical miles) than in coastal areas. These findings provide region-specific reproductive parameters and spatial maturity information that support size-based management and marine spatial planning in western Indonesian waters.