No end to big fish die-off in Zandvlei
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Muizenberg residents say they are still in the dark about what is killing the fish in the Zandvlei estuary, after yet another weekend removing thousands of fish from the vlei.
When the Cape Argus reported on the problem last month Belinda Walker, the mayoral committee member for economic, environmental and spatial planning, said oxygen depletion in the water was probably the cause.
But on Monday, estuary management staff, law enforcement officers and residents were still removing dead fish from the vlei while thousands more appeared to be dying.
Muizenberg resident Pierre Niehaus said they had removed thousands of dead fish, big and small, from the water.
He said he used to let his dogs swim in the water but has stopped because the dogs developed a skin irritation.
Bob Craske, a Marina da Gama resident for the past 10 years, said other residents who had lived in the area longer than he said they last had a problem like this in the 1970s and 1980s.
Craske said the problem would have been worse had it not been for the small Zandvlei estuary staff who had been working with residents to remove the dead fish and take survivors to the sea: “Without them this would have been 10 times worse. It would have been disgusting.”
Garnet Prince, a member of the Cape Piscatorial Society, said some members fished in the vlei but had stopped when the fish started dying.
Poachers had used pitchforks and dived into the water to catch the fish, some of which were endangered, which was not allowed.
Prince said the incident was unfortunate because the vlei was one of the healthiest. “Two months ago we saw steenbras in the vlei – they have not been seen here for the past 10 years.”
They had rescued a lot of fish over the weekend, some weighing 20kg.
Dr Stephen Lamberth of the inshore resources research branch of the Department of Agriculture, Forestry and Fisheries, said that based on more than 10 years of fish monitoring, the current group in the estuary was the healthiest yet recorded and that the vlei was not contaminated or toxic.
Lamberth was responding to an e-mail from resident Chris Fallows on the possible cause of the fish deaths.
“Much of this can be attributed to existing (estuary) mouth management that allows intrusion of saline (salty) water much further upstream than was the case throughout most of the 1990s,” said Lamberth.
The reappearance of fish such as pipefish and white steenbras, as well as expansion of sandprawn beds, showed well oxygenated bottom water.
“The recent event in which fish became concentrated in the lower reaches was most likely due to two separate events happening at the same time.”
First was day and night fluctuations in oxygen levels over the past weeks, which was a natural product of plant (such as algae and pondweed) photosynthesis and respiration.
Low levels of oxygen at night were made worse by algae blooms.
“Second, throughout the winter rainfall zones of SA, fish in rivers and estuaries take the first significant autumnal pressure drop as a cue to begin moving towards the mouth as this usually indicates the onset of rains and time to be flushed out to sea.
“So, fish in Zandvlei began moving towards the mouth but had to contend with low nighttime oxygen levels…”
This was “near identical” to the response of fish in other temporary open or closed estuaries in SA, while sampling surveys indicated long-term recovery of the estuaries.