Reproductive Ecology and Spawning Migration of the Broadhead Sleeper Eleotris melanosoma (Eleotridae) in the Mekong Delta, Vietnam

Abstract

The reproductive ecology of Eleotris melanosoma was investigated in the Can Tho River, a tributary of the Bassac River, from July 2024 to November 2025. Spawning-season migration patterns, sex ratio variation across size classes, gonadosomatic index (GSI), and environmental parameters were examined to identify environmental triggers of migration and to elucidate key reproductive characteristics of the species. An overall sex ratio (male : female) of 0.81 : 1.0 was observed, and the ratio varied across size classes. In addition, significant size differences between males and females indicate sexual dimorphism. Downstream migration occurred during ebb tides associated with new and full moon phases in the early flooding season. Migrating individuals were frequently observed drifting alongside floating mats of water hyacinth; however, the ecological significance of this association remains uncertain. GSI values indicated that spawning activity extended from July to October. Generalized additive models identified sediment concentration as a significant positive predictor of GSI, whereas total dissolved solids showed no significant effect. Although the ultimate spawning habitat remains unclear, the documented seasonal migration pattern suggests potential vulnerability to hydrological alterations associated with dyke and sluice gate systems, as well as to climate-related environmental change. Implications for management and potential mitigation strategies are also discussed.