Stream Fish Assemblages Along the Longitudinal Gradient of the Nyangui River.

Abstract

This study investigated the influence of longitudinal and environmental gradients on fish biodiversity in a central African context. The Nyagui River was used as a study site. Fish and microhabitat data were collected at fourteen stations in three periods of different flow. A total of twenty-four species were collected including Mesobola brevianalis that was not previously documented in the lower Zambezi system. A number of species that were expected to occur at low altitude were not collected during this sampling period. Species richness, diversity, and relative abundance increased from upstream to downstream in all sampling months and this was related to increasing catchment area. The number of species and their relative abundance was also correlated to habitat diversity that was calculated using the Shannon Diversity index and this relationship was explained by a power curve and was significant in October and November. Altitudinal difference in species composition was also exhibited. The most abundant species, accounting for >50% of the total catch, at altitude >1 400m were Barbus paludinosus, Micropterus salmoides and Clarias gariepinus while at lower altitude (<1 400m) Chiloglanis neumanni, Labeobarbus marequensis, Labeo cylindricus and Barbus trimaculatus were more abundant reflecting a response of fish to the physical constraints imposed by stream size and gradient. Based on Canonical Correspondence Analysis, the species at the upstream stations were associated with slow and shallow habitats with abundant aquatic vegetation that probably provided cover from predators. Species at the downstream stations were on the other hand associated with fast flowing riffles on coarse substrates, shallow pools and deep pools with rocks. The construction of Kunzvi Dam is likely to lead to the decline in species that are associated with flowing water both in the dam and downstream and an increase in the lacustrine adapted cichlids. It was concluded from this study that catchment area is a good predictor of species richness and their abundance and the number of species and their abundance is correlated to habitat heterogeneity at each station and this relationship is weak during the time of flooding. Species were also associated with specific habitats and any alteration in flow is likely to lead to change in species composition and abundance