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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/2206 |
| Date | 12 1900 |
| Creators | Du Plessis, D. |
| Contributors | Leslie, A., Salie, K., University of Stellenbosch. Faculty of Agrisciences. Dept. of Conservation Ecology and Entomology. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | University of Stellenbosch |
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