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Some aspects of the ecology of the Groot Letaba River in the Northern Province, South Africa
Abstract
The aim of this pilot study was to evaluate the current ecological status of the Groot Letaba River and to compare this information with historical data. The objective was to determine the effects of various impacts on the fish populations of the river. This was done by analysing the water quality and by considering the effect of weirs and dams, as well as various illegal angling activities, on the fish community.
The Groot Letaba River is not highly polluted and the decline in its flow seems to be the greatest threat to the system. During a preliminary study to develop the river's resource potential, it was stated that the annual water allocation from Tzaneen Dam was 103.9 million m3/annum for irrigation, 8.4 million m3/annum for domestic and industrial use and 14.7 million m3/annum for environmental purposes. However, the yield from Tzaneen Dam was only 98 million m3/annum, suggesting that more water had been allocated than was available. As a result only 20% of the simulated natural flow is observed at Letaba Ranch Weir at the lower end of the river.
Over the past few years many weirs and dams, none of which have fishways, have been constructed in the Groot Letaba River, impacting on the flow regime and on the migration potential of many fish species. Tiger fish (Hydrocynus vittatus) and the largescale yellowfish (Barbus marequensis) are two of the more prominent species influenced negatively by these barriers. This problem is aggravated by the illegal netting of fish stranded below these barriers during their spawning migrations.
Keywords: ecological status; Groot Letaba River; South Africa; fish community; barriers; flow reduction; fishing; water quality
(Afr J Aqua Sci: 2000 25: 76-83)
The Groot Letaba River is not highly polluted and the decline in its flow seems to be the greatest threat to the system. During a preliminary study to develop the river's resource potential, it was stated that the annual water allocation from Tzaneen Dam was 103.9 million m3/annum for irrigation, 8.4 million m3/annum for domestic and industrial use and 14.7 million m3/annum for environmental purposes. However, the yield from Tzaneen Dam was only 98 million m3/annum, suggesting that more water had been allocated than was available. As a result only 20% of the simulated natural flow is observed at Letaba Ranch Weir at the lower end of the river.
Over the past few years many weirs and dams, none of which have fishways, have been constructed in the Groot Letaba River, impacting on the flow regime and on the migration potential of many fish species. Tiger fish (Hydrocynus vittatus) and the largescale yellowfish (Barbus marequensis) are two of the more prominent species influenced negatively by these barriers. This problem is aggravated by the illegal netting of fish stranded below these barriers during their spawning migrations.
Keywords: ecological status; Groot Letaba River; South Africa; fish community; barriers; flow reduction; fishing; water quality
(Afr J Aqua Sci: 2000 25: 76-83)
