The effects of land use on freshwater fish in South Africa, with particular reference to Natal

Abstract

Freshwater fish live in an environment that is liable to modification by human activities. Instability of South African rivers makes them particularly vulnerable to changes brought about by settlement of the land. Indigenous species number 89, of which 47 belong to the Cyprinidae; eight exotic species are also established in South African natural fresh waters. These fish vary considerably in their tolerances, but the habitat of most species has undergone deterioration as a result of agriculture, pollution or water abstraction. Agricultural land-use effects are mainly (i) an increase in soil erosion leading to higher silt loads, which adversely affect the habitat, both for fish and for fish food and (ii) reduced low flows and higher flood-crests. Domestic and industrial pollution are liable to have disastrous effects, but fortunately serious pollution has so far been of localized occurrence and has affected mainly smaller streams. Dam building has had a major impact on fish populations. Below the dams some natural habitats have been impaired and migratory movements curtailed. The impoundments themselves on the other hand, have provided an enlarged and stable environment leading to population increases. Many of the larger impoundments carry indigenous fish, other dams have provided a habitat for exotic species. Progressive development of the land makes changes in natural waterways inevitable but conservation of mountain catchments and the building of dams are likely to prevent decimation of the more economically important species. Special steps may be needed to maintain the habitats of certain rare endemic species. Progressive development of the land makes changes in natural waterways inevitable but conservation of mountain catchments and the building of dams are likely to prevent decimation of the more economically important species. Special steps may be needed to maintain the habitats of certain rare endemic species.