Fish Association Dynamics in Three Clearwater and-Blackwater River Systems in the Eastern Delta of Nigeria

Fish species associations in three rivers, Orashi, Sombriero and New Calabar on the Eastern delta of Nigeria were studied using the presence-absence method. A total of 154 species were reported of which 135 species occurred on the Orashi River; 79 species on the Sombriero River and 63 species on the New Calabar River. A hierarchical cluster analyses identified five association types; a ubiquitous group of the Orashi-Sombriero-New Calabar River cluster making up 31.8% of total species. Other associations identified were the Orashi-Sombriero at 0.65%; the Orashi-New Calabar at 8.4% and Sombriero – New Calabar, co-occurring at 5.2%; of total species. The restricted distributions were identified with 52.6% of fish associations confined to Orashi River while 1.3% were confined to Sombriero River. A Sorenson similarity index shows a higher degree of taxonomic homogenization between Sombriero and New Calabar (0.9) than between either Sombriero and Orashi (0.54) or New Calabar and Orashi Rivers (0.5). The findings in this study are consistent with the hypothesis that fish species diversity is mainly influenced by fine-scale environmental factors such as interconnectivity of rivers and streams. In this regard, the study results agree with interpretations from previous studies of fish assemblage structure where habitat diversity either on a longitudinal or spatial gradient influence fish diversity. The remarkable difference in fish taxonomic richness between the Orashi River and the other two river systems (New Calabar and Sombriero) is evidence that the exclusive interconnection of the catchment areas of the Orashi River and the Niger River during the wet season promotes habitat diversity. Consequently this promotes the mixing of fishes and the sharing of species resulting in the highest species richness being recorded on the Orashi River despite the fact that the three river systems exhibit Blackwater and Clearwater characteristics during the wet season and dry season respectively.