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.
Among the 155 recorded fish species in the St Lucia estuarine system, Ambassis ambassis is one of the most prominent. After a decade dominated by dry and hypersaline conditions, the St Lucia system has changed dramatically with the onset of a new wet phase in 2011. In response, A. ambassis has expanded its distribution throughout the system. Stable δ 15 N and δ 13 C isotope analysis was used in conjunction with gut content analysis to elucidate the diet of this species at 5 representative sampling localities. Zooplankton as well as terrestrial and aquatic insects were prevalent in the diet. Non-metric multidimensional scaling analysis revealed a considerable dietary overlap for the fish collected at the different localities. Seasonally, trophic position differed significantly, with the dry season showing consistently higher isotopic signatures at all sites. A significant relationship was identified between trophic position, salinity and temperature, indicating the potential effect of these variables on the diet. The ability of A. ambassis to link lower trophic level organisms, such as zooplankton, with larger piscivorous predators is thus indicative of the vital role this species occupies in the food webs of the St Lucia system.
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