Water tests by the Department of Water and Sanitation (DWS) indicate that the mass fish deaths in the Hartbeespoort Dam earlier this month were due to extremely low oxygen levels in the dam, typical of sewage contamination.
Tests by the Sefako Makgatho Health Science University also found insufficient oxygen in the dam, but the report states that oxygen levels are unlikely to be the main cause. DWS took samples at four different sites of the dam, including at the dam edge with overgrown hyacinth. It also took samples 60 metres away from the edge of the dam and at the Ifafi Aquatic Club where the fish deaths occurred, to determine the cause. “The results revealed that excessive algal growth caused by high nutrient (phosphorus and nitrogen contents) levels in the water led to oxygen depletion and fish mortality. This occurrence of low oxygen concentration in water is typical in sewage-contaminated systems with high organic matter, and could not be attributed to high water temperatures given the average temperature of 22 º C at the time of sampling,” said DWS spokesperson Wisane Mavasa. According to the report by the Sefako Makgatho Health Science University, the current condition of the dam, which is worsened by the uncontrollable growth of common water hyacinth, might have led to the fish deaths. “Preliminary tests were conducted on 13 April 2023 to establish the primary causes for this eventuality (fish deaths). The hypothesis for this preliminary investigation was that insufficient oxygen content may be the driver of this fish mortality,” the report stated. “Water samples were taken from sites in the dam where fish deaths occurred, as well as from sites not affected. The dissolved oxygen level showed no difference between the site where the mortality happened and the other site with no mortality, so it is unlikely that the dissolved oxygen level could be the sole driver of this mortality. Detailed chemical and pathological analyses are still to be conducted to profile other water quality parameters and fish health.” The report recommends a comprehensive water quality and fish health study on native fish species. This will help understand the magnitude of the threat to this waterbody. “Moreover, biochemicals such as hormones and enzymes that can provide early warning signs of exposure to water pollution and its effect should be investigated.” DWS said to address the hyacinth and algal growth at the dam, the DWS has recently appointed Magalies Water for a period of three years, to develop and implement a programme that will deal with the invasive plant and the algae that infest the dam. “The entity is expected to develop a resource management and remediation plan to address the poor water quality, minimise and control the plant and algal growth in the dam and the upstream catchment which lead to its pollution and compromise water quality and use of the dam. Part of the scope for Magalies Water is to develop a short-term intervention plan to remove the hyacinth, review the algal management strategy and develop a catchment management plan to address the receiving of water into the dam from the upper catchment,” Mavasa said. The plan will also involve the repurposing and readapting of the Metsi a Me programme to focus on the upper catchment which contributes to the eutrophication of the dam.” The programme is expected to start in mid-May and will incorporate the Biological Control Programme managed by the Department of Forestry, Fisheries and Environment (DFFE).