Abstract:
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. While piscivory has never been reported in dietary
studies of O. mossambicus in their native range, their opportunistic and omnivorous
feeding habits mean that piscivory cannot be ruled out as a cause of the disease.
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.
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 was less diverse than fish from LR, and was
dominated by sediment and detritus.
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. 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 to pollutants) and indirect
(dietary exposure) links to pansteatitis. This study identified the major dietary
constituents for O. mossambicus, which enables future research to focus on their
nutritional and chemical composition.
