Post-flood dietary variation in the Mozambique tilapia Oreochromis mossambicus in the St Lucia Estuary, South Africa

Although originally endemic to southern Africa, the Mozambique tilapia Oreochromis mossambicus is now among the most widely distributed exotic fish species worldwide. It has become the dominant fish species in the St Lucia estuarine lake (South Africa) since the closure of the mouth in 2002 and is therefore a crucial component of the food webs throughout the system. Following a decade-long drought phase, the estuary has received a large amount of freshwater inflow since 2011, resulting in a salinity decrease throughout the system. We compared dietary composition of O. mossambicus among 3 sites across a salinity gradient between the hypersaline and diluted stage to determine whether environmental conditions influence the diet of this species. Stable isotope analysis of carbon and nitrogen were used in conjunction with gut content analysis to elucidate dietary composition. A wide range of dietary sources was found during the hypersaline stage, with all sources contributing similar proportions to the diet. However, during the diluted stage that currently prevails in the system, specific dietary sources such as sediment organic matter were more dominant in the diet. Trophic position and salinity showed a significant negative relationship, indicating the adaptability of this species to salinity changes. A high degree of variability in the stomach contents of these fish was identified, with clear differences among sites and between seasons. This is an indication of the trophic plasticity that this species exhibits, which aids its ability to adapt to different environmental conditions and dominate the fish community throughout the St Lucia estuarine system.