Temporal Turnover Outweighs Spatial Differences in The Fish Assemblage of a Peri-Urban Tidal River During the Early Rainy Season: The Egbelu Reach of The New Calabar River, Niger Delta, Nigeria

Tidal rivers that pass through growing Niger Delta towns sustain artisanal fisheries, but surveys rarely collect fish and water quality data at the same places and times. We sampled three stations on the Egbelu reach of the New Calabar River, Rivers State, Nigeria, each month from May to July 2026. On every visit we measured eleven physico-chemical variables and caught fish with gill nets, cast nets and hooks. Salinity, conductivity and total dissolved solids differed among stations (p < 0.001), while only temperature and pH changed from month to month. Dissolved oxygen (6.50–6.80 mg L−1) and BOD5 (2.50–2.80 mg L−1) pointed to well oxygenated water with modest organic loading. We caught 1,533 fish belonging to 20 species and 11 families. Cichlidae made up 28.6% of individuals, Clupeidae 16.0% and Alestidae 11.4%, and Sarotherodon melanotheron was the most abundant species (12.5%). Every species turned up at every station, diversity was almost the same everywhere (H′ = 2.91–2.93; J′ = 0.97–0.98), and Bray–Curtis similarity was above 92%. Sampling month, on the other hand, explained 81% of the variation in assemblage composition (PERMANOVA, pseudo-F = 11.04, p = 0.028), against only 4% for station (p = 0.861). Mugil cephalus, Neochelon falcipinnis, Elops lacerta and other marine-associated species declined from May to July, as Brycinus spp., Hemichromis fasciatus and Chrysichthys nigrodigitatus became more common. In the early rainy season, then, the assemblage changed far more over time than across space, and monitoring of the reach should follow the seasonal cycle rather than add stations.