Freshwater fishes of Lower Guinean forest streams: Aquaculture heavily impacts the structure and diversity of communities

Little is known about fish diversity in the coastal streams south-eastern Nigeria in this world-renowned biodiversity hotspot. In these ecosystems, the combination of seasonal changes in hydrology as well as the presence of coastal forests provides a greater biotope diversity, food and shelter for many fish species. Currently, however, deforestation, pollution and exotic species invasions impact the system's hydrology, water quality ultimately changing fish assemblage composition. In this paper, we describe the current status of fish diversity in the forested coastal streams of south-eastern Nigerian based on recent collections and data drawn from selected scientific publications. We found a total of 88 fish species from 27 families in 10 orders. Fish assemblages were generally characterised by a low evenness, with 90% of specimens belonging to over a quarter of the overall number of taxa, and strongly dominated by species of aquaculture interest, such as tilapiine cichlids. The studied stations had a high heterogeneity and non-comparable diversity profiles; stressing the role played by local conditions. Stations closer to the River Niger Delta differed significantly from the remaining large relatively homogeneous cluster. We found that the spatial turnover components of β-diversity were significant, and this was related to longitudinal distance, and not to species replacement by ecological vicariants. The observed species composition and the diversity patterns are consistent with a scenario whereby an originally high biodiversity is being eroded because of habitat degradation and the impact of alien species.