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

Abstract

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.

Piscivory does not cause pansteatitis (yellow fat disease) in Oreochromis mossambicus from an African subtropical reservoir

“…Their diet was herbivorous/detritivorous, and the dominant food items identified indicated that they mostly fed in the pelagic zone. Based on the stomach contents and mixing model results, their diet in LR is very much what would be expected according to previous studies in their native range (Bowen, ; De Moor et al ., ; Zengeya et al ., ; Dyer et al ., ). Both analytical methods showed that C. hirundinella was the dominant food source.…”

“…While the consumption of rancid (rotting) fish following fish kills may explain the occurrence of pansteatitis in crocodiles, persistent symptoms of the disease observed in mature O. mossambicus in LR are not easily explained. In their native range, their diet usually consists of detritus, phytoplankton, periphyton, macroalgae, diatoms and zooplankton (Bowen, 1979;De Moor, Wilkinson & Herbst, 1985;Zengeya et al, 2011;Dyer, Perissinotto & Carrasco, 2013). However, O. mossambicus are known to exhibit trophic plasticity under different environmental conditions (Bowen & Allanson, 1982;Dyer et al, 2013).…”

“…In their native range, their diet usually consists of detritus, phytoplankton, periphyton, macroalgae, diatoms and zooplankton (Bowen, 1979;De Moor, Wilkinson & Herbst, 1985;Zengeya et al, 2011;Dyer, Perissinotto & Carrasco, 2013). However, O. mossambicus are known to exhibit trophic plasticity under different environmental conditions (Bowen & Allanson, 1982;Dyer et al, 2013). In Australia, they have been shown to prey on juvenile indigenous fish in both laboratory and field situations (Doup e et al, 2009), and to completely digest fish prey in <1 h leaving little evidence of this food source in stomach contents (Doup e & Knott, 2011).…”

Temporal and spatial dietary dynamics of the longspine glassy (<i>Ambassis ambassis</i>) in the St Lucia estuarine system, iSimangaliso Wetland Park

“…In order to determine the likely contribution of each potential food item to the diet of A. ambassis at the five study sites during the wet and dry season, SIAR (stable isotope analysis in R) version 4.0 was used to generate mixing models, based on the standard corrected carbon and nitrogen ratios (Parnell et al, 2010). A trophic enrichment factor (TEF) was incorporated in the model, using values of 3.4‰ for δ 15 N and 1‰ for δ 13 C (Smit, 2001;Carrasco et al, 2012;Dyer et al, 2013). A standard deviation of 1‰ was used for both carbon and nitrogen signatures, to remove any bias in the variability of trophic fractionation among the sources (Caut et al, 2009;Inger et al, 2010).…”

A stable isotopic study of the diet of <i>Potamonautes sidneyi</i> (Brachyura: Potamonautidae) in two coastal lakes of the iSimangaliso Wetland Park, South Africa

“…Despite the ecological importance of P. sidneyi (Purves, 1991;Dyer et al, 2013), no work has been done on the species in terms of population studies, diet, trophic position and tolerance to anthropogenic and natural disturbances or habitat modification. While Reavell and Cyrus (1989) classified the species as a 'benthic scavenger ', Dobson (2004), in a statement referring to all African freshwater crabs, suggested that in addition to this they had a preference for plant-based detritus.…”