Distribution and Ecology of Brackish Water Fish in the Nun River, Bayelsa State

The Nun River is one of the major distributaries of the Niger Delta, forming extensive brackish water transition zones that support diverse assemblages of euryhaline and stenohaline fish species. This study examined the distribution, composition, and ecology of brackish water fish communities in a 20-kilometre stretch of the Nun River, Bayelsa State, South South Nigeria. Fish were sampled bimonthly over five months at six stations using gill nets, cast nets, and traps. Salinity, pH, dissolved oxygen, temperature, and turbidity were measured concurrently at each station. Fish species were identified and assigned to estuarine ecological guilds based on their salinity tolerance and life history. Dietary analysis was performed on dominant species through gut content examination. A total of 48 fish species were recorded, including 22 marine migrants, 14 freshwater-dependent species, and 12 true estuarine residents. Species richness peaked at intermediate salinity zones (5 to 15 ppt), consistent with estuarine mixing dynamics. Siganus rivulatus and Mugil cephalus were the most abundant marine visitors during dry season incursions. Trophic analysis revealed omnivorous diets dominated by detritus, aquatic invertebrates, and plant material. Stations affected by oil infrastructure showed significantly reduced species richness. Salinity was confirmed as the strongest environmental predictor of species composition through canonical correspondence analysis. The study recommends continuous monitoring of estuarine fish assemblages to detect hydrological and pollution-driven shifts in community structure. Keywords: brackish water fish, Nun River, estuarine ecology, Bayelsa State, salinity gradient.