Nutritional Condition of Fish Larvae in South African Estuaries As a Proxy to Assess Their Mortality Under Different Environmental Conditions

Because of its abundance and its essential role as a mid-trophic level species in estuaries, the estuarine round-herring, Gilchristella aestuaria, is an excellent candidate to assess the ecological status of estuaries. One of the most widely used approaches to quantitatively evaluate how fish are influenced by their environment is the assessment of the nutritional condition of their larvae through the estimation of the RNA/DNA ratio. This technique is extensively utilized because of its high sensitivity to variations under critical environmental factors, and can be interpreted as a proxy for growth rate and mortality and, ultimately, recruitment success. By assessing and comparing the nutritional condition of G. aestuaria fish larvae from different estuaries in South Africa, this study intends to comprehend which environmental factors, such as food quality, prey availability and temperature, are the major conditioners of the mortality and recruitment of estuarine fish larvae. Preliminary results have shown that nor the availability of food neither the temperature are correlated with the nutritional condition of the estuarine round-herring larvae. We therefore hypothesize that the energetic value of the available prey in each estuary might play a crucial role in the nutritional condition of these larvae.