Abundance and Diversity of Fish Assemblages Along the River-Estuary Continuum in a Fluvially Dominated Southern African Coastal System

Estuaries are amongst the most productive ecosystems worldwide and provide nursery habitat for fish species of marine and freshwater origin; however, the distribution and community structure of fishes within freshwater-dominated coastal system is limited. This study presents an overview of the temporal and spatial patterns in the abundance, diversity and distribution of fishes along a river-estuarine continuum and compared them to other southern African estuaries. A total of 30 species belonging to 16 families were recorded in the Orange River Estuary (ORE) comprised mostly of freshwater (47%), marine (33%) and estuarine fishes (20%). Chelon richardsonii (marine estuarine-opportunist) and Gilchristella aestuaria (solely estuarine) were the numerically dominant species in the estuary with 63% and 20% of catches, respectively. The two species utilise a broad salinity range (0.l–34.9 PSU). Also, C. richardsonii dominated catches in the river section with 52% followed by Labeobarbus aeneus (freshwater straggler) and Pseudocrenilabrus philander (freshwater estuarine-opportunist) with 15% and 9%, respectively. Species diversity and catch-per-unit-effort (CPUE) revealed seasonal variation in fish assemblages during the high-flow and low-flow season but not inter-annually. This highlights the continuity in fish assemblages utilising the ORE. High catches (dominated by C. richardsonii) but low species diversity observed close to the mouth of the estuary agree with the biogeographical trend of decreasing species richness from east to west on the South African coast. In comparison with other estuaries, the ORE is freshwater dominated, whilst the Berg and Olifants estuaries are dominated by marine taxa but with similar richness values.