Fish assemblages and size-spectra variation among rivers of Lake Victoria Basin, Kenya

Abstract

Riverine ecosystems are some of the most biologically diverse on earth, but are also among the most threatened by anthropogenic actions. Lack of local-scale studies monitoring changes in riverine populations makes it difficult to determine how communities respond to region-specific anthropogenic threats. In this study, we compared fish assemblage structure in six rivers in the Lake Victoria Basin (LVB), Kenya. Data collected during different periods between 2004 and 2018 were used to derive abundance size-spectra parameters and community diversity metrics as proxies for ecosystem condition and function. Species dominance in the rivers was high with only five species accounting for >72% of cumulative relative abundance while, 33 species contributed <10% of all individuals sampled. One-way analysis of similarity test (ANOSIM) indicated significant dissimilarity in fish composition between the rivers (Global R = 0.405, p = 0.0187) while, similarity percentage analysis (SIMPER) partitioned differences to variations in relative abundance of mainly two species (Labeobarbus altianalis, and Labeo victorianus). Eight species were restricted to single river basins suggesting presence of local endemism. Abundance size-spectra parameters (intercepts and slopes) used as proxies for food-web capacity and ecological efficiency fluctuated over time with periods of relative stability, indicating spatial (among rivers) and temporal (yearly) variations in ecological processes in the rivers. The food-web capacity and ecological efficiency indices showed basin-specific relationships with community metrics (dominance, richness, evenness, Shannon-Weiner and Simpson diversity indices) and catch per unit effort (CPUE), suggesting their potential use as indicators of ecosystem structure. The findings of this study show promise in the use of size-spectra parameters as proxies for riverine ecosystem structure and functioning under a gradient of perturbation.