Viral Infection behind Fish Mortalities in Zeekoevlei

11 March 2016


The Department of Environmental Affairs (DEA) has noted the recent fish kills in the Zeekoevlei area around the City of Cape Town.

Over the past two weeks, an estimated 20 tons of fish have been washed up on the banks of Zeekoevlei. Other nearby freshwater and estuarine systems (Keysers River, Westlake and Zandvlei) have also experienced fish mortalities, but to a lesser degree.

Symptoms observed on the dead fish suggest that the mass mortalities were due to infection of the Koi Herpes Virus, also known as Cyprinid Herpes Virus 3, which is known to exist in Cape Town’s Carp populations, and has caused similar mortalities in previous years.

The Department has ruled out the possibility that the fish mortalities were caused by harmful algal bloom or water quality issues, as this would have affected more than one species of fish.

Furthermore, preliminary results from laboratory tests indicate that oxygen and algal toxin levels in Zeekoevlei are within the limits stipulated by water quality guidelines.

During a viral epidemic, 80 - 90% of the Carp population can die, especially if the outbreak coincides with Carp breeding when Carp densities are high in localised breeding areas. The virus also tends to spread when fish densities increase as water volumes decrease towards the end of the dry season.

Triggered by an environmental event, such as warm temperatures, outbreaks of the disease tend to recur.

Typically, Koi Herpes Viral epidemics form when the water temperature is between 22°C and 26°C, which is common in the late summer season.

The summer of 2015/16 has been particularly hot and dry, which might have contributed to the recent viral epidemic and mortalities in Zeekoevlei.

Because the fish mortalities occured in freshwater, the matter is being handled by the Department of Water and Sanitation (DWS) and other stakeholders.

Most of the dead fish have been removed and clean-up operations are continuing by the relevant authorities.

The Department would like to point that although the dead fish washed up are not poisonous per se, they should not be consumed by people under any circumstances. As the fish are not ‘fresh’, they may contain bacteria harmful to humans.

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Zolile Nqayi
Cell: 082 898 6483