Distribution and community structure of riverine fish in two east-flowing rivers in Kenya

Abstract

This study investigated the distribution, composition and habitat associations of freshwater eels in the Athi–Galana–Sabaki (Athi) and Ramisi Rivers using fortnightly fyke-net sampling conducted between April 2021 and March 2022. A total of 304 eels were recorded, with the Ramisi contributing 76% of individuals and the Athi River 24%. The assemblage was dominated by Anguilla bengalensis (75% of all eels), followed by A. bicolor (12%), A. mossambica (8%) and A. marmorata (5%). A. bengalensis was widespread and most abundant upstream in both rivers, indicating broad habitat plasticity. In contrast, A. bicolor and A. mossambica were largely restricted to lower, more saline reaches, while A. marmorata occurred primarily upstream. Eel biomass exceeded that of non-eel fish at most sites and differed significantly among locations, with highest values recorded in the upper Ramisi River (ANOVA: F = 2.9, p = 0.04). A one-way analysis of similarity (ANOSIM) indicated significant spatial differences in eel distribution between rivers and reaches (Global R = 0.49, p < 0.05). Subsequent SIMPER analysis showed that these differences were primarily driven by variations in A. bengalensis (6%) and A. bicolor (1.12%). Multivariate analyses revealed significant spatial structuring of eel assemblages, with occurrence patterns strongly associated with conductivity, total dissolved solids, salinity, discharge, temperature, pH, and dissolved oxygen. These findings provide the first contemporary, reach-scale assessment of WIO eels in Kenyan rivers and highlight clear species-specific habitat associations along the river–estuary continuum. Conservation and fisheries management should prioritise maintaining longitudinal connectivity, protecting estuarine transition zones, sustaining water-quality gradients, and controlling invasive species to safeguard eel populations and their ecosystem functions.