Among the 155 species of fish recorded so far in the St Lucia estuarine lake, Oreochromis mossambicus and Ambassis ambassis are the two most prominent. Although originally endemic to southern Africa, O. mossambicus is now one of the most widely distributed exotic fish species worldwide. Together with A. ambassis, they have become the dominant fish species in the St Lucia estuarine lake since the closure of the mouth in 2002 and are, therefore, a crucial component of the food webs throughout the system. After a decade dominated by dry and hypersaline conditions, the St Lucia system has changed dramatically in terms of prevailing environmental conditions, as a result of higher than average rainfall at the end of 2011 and the onset of a new wet phase at the start of 2012. In response, A. ambassis, which prefers lower salinity regimes, has expanded its distribution range throughout the estuarine lake. Stable δ¹⁵N and δ¹³C isotope analysis was used in conjunction with gut content analysis to elucidate the diet of these species at sampling localities spanning the geographical range of the system and determine whether these species shift their diet in response to environmental or climatic shifts. From both studies it is evident that from a temporal and spatial scale these two species adopt similar, yet very different, dietary tactics. Oreochromis mossambicus was shown to adopt a generalist feeding strategy, opportunistically feeding on dietary items that are available thus allowing this species to alter its diet according to the environment that it inhabits. Trophic positioning of this species was found to be controlled by salinity in St Lucia as dietary composition differed greatly between sites. In contrast, Ambassis ambassis displayed a more specialist dietary composition, feeding predominantly on zooplankton. However, this species also opportunistically supplements its diet with additional sources when available. Trophic position of A. ambassis was higher in the dry season owing to the increased productivity of the system during the wet season. The success and dominance of both species in the St Lucia system can therefore be attributed to their dietary strategies. Under extreme environmental conditions, O. mossambicus has the added advantage of its wide tolerance of different environmental conditions, particularly salinity, thus allowing it to proliferate. / M.Sc. University of KwaZulu-Natal, Durban 2014.