Residents near the Zandvlei estuary have been warned not to eat or collect fish washing out of the water as this is a health hazard and illegal.
The City of Cape Town has now conducted tests at the estuary following the die-off of thousands of fish in the vlei over the past few weeks.
Residents had complained of illegal fishing as well as a lack of response from the city and CapeNature.
Brett Herron, mayoral committee member for transport, roads and stormwater, said a lack of oxygen in the water was a symptom but not the cause of the problem.
Herron said test results from samples taken at the estuary on Monday showed that golden algae was responsible.
“The latest test results have confirmed that an extensive algal bloom comprising the species Prymnesium parvum, commonly referred to as ‘golden algae’, is responsible for the (fish dying).”
He said golden algae was also responsible for killing fish in Zandvlei in 1973.
Lionel Adendorf, spokesman for the department of agriculture, forestry and fisheries said: “The public is reminded that it is illegal to collect any of the species that wash out in the estuary without a permit and if found in possession of any fish, they can be fined up to R500 per fish.”
He said fish were dying because of the algal bloom that had been in the estuary for the last six weeks.
“The density of the algal bloom, caused by the high nutrient load that comes from the various sources in the catchment, ranging from stormwater run-off from both urban and industrial areas and sewerage overflows, increased dramatically over the last four weeks causing low oxygen levels in the estuary,” he said.
He said conditions in the estuary were expected to remain the same for at least another month and urged residents to avoid the area.
CapeNature also responded to residents’ queries about why the mouth of the estuary could not be opened to let the fish out.
Pierre de Villiers, co-ordinator of CapeNature’s estuaries programme said: “The aim of opening the mouth during spring tides is to ensure the inflow of marine water at high tides. If the mouth is kept open for lengthy periods the water level will drop drastically.”
De Villiers said the size of the “natural filter” within the estuary would therefore also decrease.
He said CapeNature, the city and partners were setting up an estuary management forum because they needed to work together to ensure the die-off did not occur again.