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Piscivory does not cause pansteatitis (yellow fat disease)in Oreochromis mossambicus from an African sub-tropical reservoir
Citation:
Dabrowski, J., G. Hall, N. Lubcker, P. Oberholster, D. Phillips, AND S. Woodborne. Piscivory does not cause pansteatitis (yellow fat disease)in Oreochromis mossambicus from an African sub-tropical reservoir. FRESHWATER BIOLOGY. Blackwell Publishing, Malden, MA, 59(7):1484-1496, (2014).
Impact/Purpose:
A manuscript by South African government and university scientists and an EPA scientist examines the diet of an important river fish species in locations with and without widespread occurrence of pansteatitis, a disease that is also having a major effect on crocodile mortality in South Africa. The study’s aim was to determine if the diet of affected fish included significant amounts of other fish, as this disease is generally associated with consuming fish fats. This study used stable isotope analysis to determine the dietary importance of fish, zooplankton, algae, dinoflagellates, sediment, and detritus. Isotopic “mixing models” based on those developed by EPA scientists were used sort out the importance of food sources in the fish’s diet in locations with and without the disease, calculated from the carbon and nitrogen isotopic composition of the fish and their prey. Diseased fish were not found to have diets that included other fish, which opens the possibility that elevated pollutant levels in the diseased locations could be a cause, possibly indirectly through effects on food quality. Application of these modeling tools is leading to a better understanding of environmental causes of this disease in important South African ecosystems, including the iconic Kruger National Park.
Description:
1. Pansteatitis (yellow fat disease) is ubiquitous in the free-ranging population of Oreochromis mossambicus from Loskop Reservoir (LR), South Africa. The disease is nutritionally mediated and associated with a diet high in polyunsaturated or rancid fats, frequently of fish origin. Piscivory has never been reported in dietary studies of O. mossambicus in their native range, but they are known to be opportunistic omnivores, demonstrating a high degree of trophic plasticity under variable environmental conditions. As a result, opportunistic piscivory associated with fish kills, or as a regular part of their diet, cannot be ruled out of the aetiology of pansteatitis without a comprehensive dietary study. 2. The diet of O. mossambicus from LR (n=91) was compared to a population from Flag Boshielo Reservoir (FBR; n=81) located less than 100 km downstream, where no pansteatitis occurs. The stomach contents and stable isotope signatures (δ15N and δ13C) of fish and food sources were evaluated across four seasons. Isotope signatures were also compared over various time scales from historic samples and mortalities collected from LR. Five additional fish species were sampled at both reservoirs in order to compare isotopic niche metrics within and between fish communities. 3. There was no evidence of piscivorous feeding behaviour in fish from either location, or from historic LR samples. The results of the SIAR mixing model and stomach contents analysis showed that the dinoflagellate, Ceratium hirundinella, was the dominant food source followed by zooplankton, detritus and Microcystis aeruginosa in LR. The diet of fish from FBR differed substantially, and was dominated by sediment and detritus. The meso- to eutrophic status of LR compared to the oligotrophic status of FBR was reflected in the higher diversity of food items consumed by fish in LR. 4. The distinguishing feature of the dietary comparison between reservoirs was the abundance of planktonic food items dominated by C. hirundinella in the diet of fish from LR. Another feature unique to LR is the presence of numerous pollutants predominantly associated with wastewater and coal-mining in the catchment. The lack of evidence for piscivory among O. mossambicus from LR suggests that the classic aetiology of pansteatitis does not apply. This highlights the need to further explore direct (environmental exposure) and indirect (dietary exposure) links between pollutants and pansteatitis. Identification of the major dietary constituents in this study will facilitate future research into the nutritional and chemical composition of these items.