Durban – When dead fish began to wash up on uMdloti beach on the north coast of KwaZulu-Natal, locals and fishermen feared the worst.
And while the blame was first laid at the door of the UPL chemical plant that was set on fire in the July looting, a report by ecological consultancy Marine & Estuarine Research (MER) provided a simpler explanation.
According to MER – which specializes in monitoring, assessments and expert advice in coastal aquatic habitats – the mass fish mortality was the result of an “osmotic shock” when the nearby estuary was ruptured.
According to the report, written by Professor Anthony TBEN and Nicolette TBEN, on September 11, MER received several calls about dead fish washed up in uMdloti.
The fish were quickly identified as tilapia of two possible species, namely Mozambique tilapia and red-breasted tilapia.
The MER team identified that the species originated from a river and estuary, and not from a marine habitat.
An inspection began in the north as subsequent reports and sightings indicated that there was a concentration of fish on the northern and uMdloti beaches and these were carried south by sea conditions .
The report found that the mouth of the estuary had been pierced on the night of September 10 or early the next morning.
Prior to the breach, the estuary had remained closed for just over three months since an alleged artificial breach on June 5.
When the estuary was ruptured, the tilapia were washed away by the sea.
While fish species can survive in salt water under certain conditions, the report found that the rapid rise in salinity in the water associated with the sudden change in temperature caused significant osmotic shock.
“The relevance of this description is that the fish were killed following the rupture of the estuary of uMdloti, the next estuary north of uMhlanga, after a three-month closure.
“Meanwhile, Mozambique’s tilapia population has reportedly increased and established itself in the Lower Estuary. The population is likely to have been taken by mouth during the breach, being subjected to osmotic shock in combination with slightly colder temperatures adding to the physiological load… There is no indication that this fish mortality was of however influenced or driven by conditions in the uMhlanga estuary watershed, ”the report says.