A Rhodes University lecturer has weighed in on the mysterious die-off of thousands of fish in the Nahoon River upstream of Abbotsford causeway over the weekend.
Dr Cliff Jones, a lecturer in the ichthyology & fisheries studies department, believes that while the sudden “fish kill” could result from a number of different causes, it was the death of barbel which was most concerning. He said this meant there was probably another cause which occurred alongside low oxygen levels.
On Saturday, Nahoon resident Que Hundermark captured a video footage of thousands of dead, floating fish including barbel, eel, bass and carp.
The Dispatch sent the footage and photographs of the river to Jones for his input on Tuesday.
“Most [causes] are usually a result of a sudden change in water chemistry.
“This can occur naturally and it can also result from man-made interventions,” he said.
“One of the more common causes is hypoxia [that is, the depletion of dissolved oxygen in the system].
“Since you noted that freshwater ‘barbel’ [Clarias gariepinus] were among those in the fish kill, it is unlikely the mortality here was a result of low oxygen levels alone, because this fish has an adaptation to its gill structure that allows it to absorb oxygen from air.”
Jones continued: “The introduction of a toxin from industrial or agricultural run-off, might also result in a fish kill.
“This could be tested but many toxic compounds have short half lives and might not be detectable after the event.
“It appears that the system is overgrown with water hyacinth.
“It is unlikely that the hyacinth alone would cause a sudden fish kill, but it might contribute to low oxygen levels in the water at night and it would also contribute to the build-up of debris in the anoxic layer in the sediment.
“This in turn [coupled with a other changes in the environment] could release compounds that result in mass mortality of fish.”
He said without a thorough investigation, it would be very hard to identify the cause of this fish kill.
Questions sent to BCM spokesperson Samkelo Ngwenya on Monday have not drawn an answer.
johnh@dispatch.co.za..