Ph.D. (Zoology) / The contribution of genetics in animal husbandry and agriculture has long been recognised. The application of genetics in fish culture has not yet become established because both overseas and locally the focus was until recently still on culture techniques. In South Africa the only genetic data on the rainbow trout (Parasalmo mykiss) stock is limited to one electrophoretic survey (Op't Hoff et al. 1982). During 1982 to 1985 a survey was done of trout angling management in provincial trout angling waters by the Transvaal Division of Nature Conservation. The goal was to find and eliminate inhibiting factors affecting trout angling in order to provide a better service. Angling results were improved and management of trout waters brought into line with its own potential. The percentage returns of trout caught however, seemed to reach a level that could only be breached by genetically improving the trout strains used for stocking. The inherent inability of inbred domesticated P. mykiss strains to grow and survive in nature has been shown to affect its survival when stocked in streams and dams for angling purposes (Kincaid 1981; Ersbak & Haase 1983).

Thesis (MScAgric (Conservation Ecology and Entomology)--University of Stellenbosch, 2007. / Small farm dams (< 20 ha) in the Western Cape Province provide adequate water conditions for intensive cage production of rainbow trout (Oncorhynchus mykiss). A major environmental concern of cage aquaculture, however, is the high inputs of nutrients via commercial diets and the subsequent eutrophication of the water source. Eutrophication can result in the degradation of the general water quality (increasing pH levels, oxygen depletion, increased hydrogen sulphide and free ammonia) and shifts in the phytoplankton structure (increased biomass, single species dominance). Deterioration of water quality will affect the success of the fish farming enterprise as well as the performance of irrigation equipment by increasing the risk of clogging and corrosion. Water quality, phytoplankton and zooplankton compositions were monitored at four sites from June 2005 to November 2006 to determine the effects of cage culture on the farm dam environment, its associated biota as well as irrigation water quality. The distribution of nutrients, nitrogen and phosphorus, was mainly influenced by the stratification and mixing regime of the water bodies. Nutrient concentrations increased during the winter mixing period while in the summer months, they seem to settle to the lower part of the water column. Nutrient concentrations of production sites and reference sites were comparable except for the ammonia levels that were significantly higher at the production sites. Phytoplankton corresponded with nutrient availability resulting in high biomass during winter. In terms of biomass, phytoplankton was approximately two times more abundant in production sites compared to reference sites. Assemblage dominance by cyanophytes (Anabaena circinalis, Microcystis spp.) was found more often in production sites, while reference sites were dominated by dinophytes (Ceratium hirundinella, Peridinium spp.). Zooplankton biomass concurred with high phytoplankton biomass in winter. Zooplankton assemblages in production sites sustained much higher biomass. Effects of cage culture on irrigation water quality are evident from increased algal biomass and shifts in species composition. These results indicated that at its present production level, cage culture had impacts on the farm dam environment and irrigation water quality. The most significant evidence was given by increased plankton biomass and single species dominance in production sites. However, these findings can not solely be ascribed to the introduction of aquaculture as various other factors may also contribute to the water quality of these ecosystems.