Search

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.

A continuous gradient of physical and biological processes exists within a river system from headwaters to mouth, according to the River Continuum Concept. In this study, the River Continuum Concept was used to compare patterns in fish assemblages and diversity trends to Remane predictions. The findings show that species diversity is low near the estuary's mouth and increases upstream. While the presence of several marine and estuarine species increased species diversity upstream in the river due to the dominance of freshwater fishes, it decreased downstream due to the presence of several marine and estuarine species. These patterns are consistent with the biogeographical trend of decreasing species richness along the South African coast from east to west. Based on the findings of this study, the River Continuum Concept ignores plasticity, which is evident in species that use transitional waters. As a result, the concept is insufficiently valid for the Orange River Estuarine Continuum, and a new concept known as the “River-Estuarine Continuum” has been proposed.

The Orange River is one of the longest rivers in southern Africa and one of the most regulated rivers in the world. Its estuary supports diverse marine and freshwater life, yet, ecosystem structure and functioning within the estuary are poorly understood. This study investigated trophic linkages and interactions among dominant fishes through carbon (δ13C) and nitrogen (δ15N) stable isotope (SI) ratios along a salinity gradient. The five dominant species utilizing the Orange River Estuary Continuum (OREC) included marine Chelon richardsonii, estuarine Gilchristella aestuaria, and freshwater species Labeobarbus aeneus, Pseudocrenilabrus philander and Mesobola brevianalis. Variations in SI ratios were associated with changes in body size, seasonality and salinity. Variability in seasons and the salinity continuum influenced SI ratios. Chelon richardsonii, P. philander and L. aeneus occupied the widest isotopic niche in the lower OREC, an indication of generalist feeding behaviour in these species. Mesobola brevianalis had the narrowest isotopic niche, perhaps suggesting more specialized feeding on zooplankton. Isotopic similarity was observed among the five co-occurring species, suggesting trophic overlap, particularly during the high-flow season when environmental variables, productivity and food availability are predominantly freshwater driven and uniform throughout the river estuary continuum. This study is the first to utilize δ13C and δ15N analyses to assess trophic dynamics of dominant species co-occurring in the estuary and estuarine-freshwater ecotone of the lower OREC.