Zandvlei fish kills: don’t ignore the algal wildcard

17 April 2012

Zandvlei near Cape Town, South Africa, is in the throes of an extended fish kill, to all intents quite a substantial event {see link}.  As with many, seemingly-inexplicable fish kill events, the cause is attributed to oxygen depletion – which can indeed cause fish kills.  However, in the shallow, windy environment of Zandvlei, dominated by pondweed as opposed to algae, nighttime oxygen levels should not drop so far as to cause a sustained fish kill.  

It is important to look beyond the easy choices, though.  Back in the late 70s, early 80s, Zandvlei suffered from a similar mass kill, caused by an icthyotoxic alga belonging to the Chrysochromulina/Prymnesium grouping.  As I recall the timing was somewhat similar: fairly substantial first autumn rains (the ‘traditional’ Easter weekend downpour), delivering a plug of nutrients into the vlei.  The fish kill followed and the algal bloom – comprised of algae that are not commonly noticed in routine samples) dissipated once the nutrients were exhausted.

I have an idea that a similar event in the nearby Rietvlei, in which fish species not commonly-affected by low oxygen levels, were included in the die-off.  This suggests a biological cause, not one brought about by oxygen.  The City of Cape Town should be aware of these possibilities and direct its scientific staffers to explore all possible avenues.  Also important to note that this is a natural extinction that cannot be managed for as the inter-annual cyclical period is usually long – so predicting when is impossible.

Bill Harding is a lake biologist and Certified Lake Manager who studied the Zandvlei ecosystem over a period of 10 years.

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