Abstract:
The principal reason for the construction of Loskop Dam was to provide irrigation water to wheat farmers settling in the Olifants River valley in the 1920s. Agriculture has since developed in the area and today, the Loskop Irrigation Board supplies water to > 700 properties with an area of 25 600 ha farming cotton, wheat, citrus and grapes near the town of Groblersdal. Serious concerns were raised about deteriorating water quality when the crocodile population began to decline and the frequency of large fish kills increased from 2006. Crocodile (Crocodylus niloticus) and Mozambique tilapia (Oreochromis mossambicus) mortalities were linked to pansteatitis which is characterised by obesity and lipid peroxidation. Known impacts on water quality include eutrophication and acid mine drainage from coal mining with associated increases of soluble metals. The aims of this study were to: i) determine whether pansteatitis could be linked to any specific parameters in the water chemistry and limnology of Loskop Dam; ii) measure concentrations of aluminium, copper, iron, manganese, selenium and zinc in various tissues of O. mossambicus to determine whether bio-accumulation was occurring and could be related to pansteatitis; iii) assess the metazoan parasite communities of O. mossambicus to determine whether they are effective indicators of ecosystem health in Loskop Dam. Four established sampling sites were used at Loskop Dam and a reference site was located at neighbouring Kranspoort Dam. Surface water quality samples were collected monthly between July and December 2010 from each site and analysed for 27 constituents including nutrients, major ions, total metals, pH and dissolved oxygen using standard methods. Orthophosphate and total inorganic nitrogen results frequently categorised Loskop Dam as eutrophic and the transitional zone of the dam was characterised by very alkaline conditions resulting from algal blooms (median pH 9.67) which increase the solubility of metals like Al. A combination of active and passive biomonitoring techniques were used for fish collection. Fish gills, brain, muscle, liver and bone were analysed for Al, Fe, Mn, Zn, Cu and Se concentrations. The most striking result was an unanticipated significant deficiency in liver Cu concentrations of fish from Loskop Dam (mean 3.4 mg kg-1) compared to fish from Kranspoort Dam (mean 62 mg kg-1). Both endo- and ectoparasites were identified and enumerated on the fish and infection rates were calculated as mean intensity (I), mean abundance (A) and prevalence (P). Fish from Loskop Dam had extremely low infection rates and two fish had no parasites whatsoever. The ratio between monoxenous and heteroxenous parasites was calculated and was very high in fish from Loskop Dam compared to fish in Kranspoort Dam and Tompi Seleka, indicating a degraded aquatic ecosystem. While no single factor was outstanding as a possible cause of pansteatitis in this study, these findings provide a good foundation from which to formulate further research questions. Copyright
