Lack of oxygen has killed about 56 tonnes of fish in the Rietvlei Wetland Reserve where nutrients have gradually built up over 30 years, the City of Cape Town said.
Acting nature conservation manager Dalton Gibbs said tests had confirmed the fish deaths were caused by a lack of oxygen in the water.
”This set of conditions will continue until water is further oxygenated by winds, which are not expected before the weekend.”
High nutrient levels combined with calm conditions and hot weather had led to high bacterial activity which depleted oxygen in the water, he said in a statement on Thursday.
Nutrients had gradually built up over 30 years in the water body from urban storm water and river catchment run-offs to the mostly land-locked water body.
The nutrients are broken down by bacteria which use oxygen.
Water tests had been done at Rietvlei and incoming canals for 15 years and tests done just over a week ago did not raise any concerns. The fish deaths posed no threat to drinking water in the surrounding Table View and Flamingo Vlei suburbs, he said.
A report on long-term remedial measures for the reserve is being prepared for the city council. The reserve remains closed but the other 22 reserves under the control of the city are open.
Clean-up operations have been stepped up with 40 additional contract staff working under four nature conservation officials. – Sapa