Pansteatitis in tilapia (Oreochromis mossambicus) from Loskop Dam, South Africa : links between the environment, diet and thyroid status

Abstract

The Olifants River is recognised as one of South Africa s hardest working rivers . Deteriorating water quality has been attributed to increased discharge of effluents from mines, industry, and wastewater treatment works, along with escalating demand for power generation and agriculture. Between the years 2005 and 2009 mortalities of Nile crocodiles, Crocodylus niloticus (Laurenti), and Mozambique tilapia, Oreochromis mossambicus (Peters), were reported at Loskop Dam and were attributed to pansteatitis. Downstream in the Kruger National Park (KNP), the disease was also reported in crocodiles and sharptooth catfish, Clarias gariepinus (Burchell) during the same time period. Environmental conditions were investigated in Loskop Dam, as well as Flag Boshielo Dam, located < 100 km downstream, where pansteatitis has never been reported in any species. Frequent blooms of Microcystis aeruginosa and Ceratium hirundinella at Loskop Dam were the cause of high fluctuations in physico-chemical conditions, especially dissolved oxygen and pH. The trophic state of Loskop Dam was classified as meso- to eutrophic, and analysis of historic data showed increasing trends of dissolved inorganic nitrogen and phosphorus. In contrast, Flag Boshielo Dam was classified as oligotrophic with a decreasing trend in inorganic nitrogen, which reflected in very low chlorophyll-a Pansteatitis is a nutritionally mediated disease reportedly associated with a diet high in polyunsaturated fats, frequently of fish origin. In order to determine whether piscivory (opportunistic or otherwise) was a contributing factor, the diet of O. mossambicus from both reservoirs was studied using stable isotopes and stomach contents. There was no evidence of piscivory in fish from either reservoir, including samples collected at Loskop Dam during historic fish mortality events. Fish diets differed markedly between reservoirs. The dinoflagellate C. hirundinella was the dominant food source in Loskop Dam, followed by zooplankton, detritus and M. aeruginosa. In Flag Boshielo Dam, sediment and detritus dominated the fish diet. This suggests that the classic aetiology of pansteatitis does not apply. A distinct characteristic of the disease is an abundance of mesenteric fat. The thyroid status of O. mossambicus from both reservoirs was evaluated as a biomarker of exposure to xenobiotics, and to determine whether disruption of the thyroid cascade could be affecting metabolic processes leading to lipid accumulation. Fish from Loskop Dam had elevated T3 concentrations along with increased thyroid follicle size, vacuolisation and epithelial cell height. Plasma cholesterol and triglycerides, hepatocyte size, and liver fat content were all elevated in fish from Loskop Dam and were indicative of high levels of energy storage. These indicators of nutritional status were all positively correlated with elevated T3 concentrations. Positive correlations between several indicators of nutritional state and thyroid hormones showed that elevated thyroid activity in fish was partly due to high nutritional inputs, and no disruption of the thyroid axis was detected. Although no definitive aetiology of pansteatitis was determined in this study, the results provide valuable information for hypothesis building, thus facilitating future research.