Diet of selected fish species in the freshwater-deprived St Lucia Estuary, South Africa, assessed using stable isotopes

Abstract

The St Lucia Estuary, Africa's largest estuarine lake, is the most important juvenile fish nursery on the south-east African coastline and is currently experiencing an unprecedented crisis. Prolonged freshwater deprivation has resulted in hypersaline conditions in the northern regions of the lake, as well as low water levels. The mixed model SIAR (Stable Isotope Analysis in R) was used to determine the likely contribution of each of the available carbon sources to the diets of selected fish species. This was complemented with ad-hoc gut content analysis of representative specimens. Two diverse regions of the lake system were investigated: Charters Creek, which has been severely affected by the desiccation process, and the Mouth, which is somewhat protected due to the freshwater input from the Mfolozi and Mpate rivers. The mysid Mesopodopsis africana was found to be important in the diet of a number of fish species, namely, Leiognathus equula, Ambassis ambassis and Gerres acinaces. The copepod Pseudodiaptomus stuhlmanni and the mysid Rhopalophthalmus tropicalis were most important in the diet of Gilchristella aestuaria. Mozambique tilapia (Oreochromis mossambicus) exhibited the most variable diet, the flexibility of which may aid survival during harsh conditions and could explain its current dominance throughout the system.