Stable isotopes reveal trophic linkages among fish species utilizing the Orange River Estuary Continuum

Abstract 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 of these species. Mesobola brevianalis had the narrowest isotopic niche, perhaps suggesting more specialized feeding on zooplanktivores. Isotopic similarity was observed among the five co-occurring species, suggesting trophic overlap, particularly during the high-flow season. 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.