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This is why dead fish are washing up on a popular KZN beach

Dead Mozambique tilapia and redbreast tilapia have washed up on uMdloti beach in KZN. Picture: MER

Dead Mozambique tilapia and redbreast tilapia have washed up on uMdloti beach in KZN. Picture: MER

Published Sep 22, 2021


Durban – When dead fish began washing up on uMdloti beach, on the KwaZulu-Natal North Coast, residents and fishermen feared the worst.

And while blame was at first laid at the door of the UPL chemical factory which was burnt down during the July looting, a report by the Marine & Estuarine Research (MER) ecological consultancy has provided a simpler explanation.

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According to the MER – which specialises in providing expert monitoring, assessments and advice in coastal aquatic habitats – the mass fish kill was the result of “osmotic shock” when the nearby estuary was breached.

According to the report, written by Nicolette Forbes and Professor Anthony Forbes, on September 11 MER received multiple calls about dead fish washing up at uMdloti.

The fish were quickly identified as tilapia of possibly two species, namely the Mozambique tilapia and redbreast tilapia.

The MER team identified that the species came from a river and estuary, not a marine habitat.

An inspection was started in the north as later reports and observations indicated that there was a concentration of fish on the northern and uMdloti beaches and these were carried southward by sea conditions.

The report found that the mouth of the estuary was breached on the night of September 10 or early the next morning.

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Before the breach, the estuary had remained closed for just over three months since a suspected artificial breach on June 5.

When the estuary was breached, the tilapia were washed out to sea.

While the fish species can survive in saline water under certain conditions, the report found that the rapid surge in salinity in the water coupled with the sudden temperature change caused significant osmotic shock.

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“The relevance of this description is that the fish kill occurred following the breach of the uMdloti estuary, the next estuary to the north of uMhlanga, after a three-month closure.

“During this time the population of Mozambique tilapia would have increased and established in the lower estuary. The population is likely to have been washed out the mouth during the breach, being subject to the osmotic shock in combination with slightly colder temperatures adding to the physiological burden …There is no indication that this fish kill was in any way, influenced or driven by conditions event in the uMhlanga estuary catchment,” the report said.


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