Common Carp (Cyrprinus carpio) have been dying in large numbers in Zeekoevlei over the past few weeks. This is assumed to be as a result of the Koi Herpes Virus that is presently active in the vleis in the south of Cape Town. Common Carp are an alien species of fish originating in Europe and Asia. They have been extensively introduced around the world for food and recreational fishing where they are often detrimental to local fish fauna.

The disease was first noticed in the area when carp started dying in Zandvlei in December 2014 and was subsequently confirmed with DNA tests. Koi Herpes Virus, as its name suggests, affects only Carp/Koi and is species specific. The disease can be carried by Grass Carp and Goldfish, both of which are different species to Common Carp/Koi and immune to the disease.

The disease can lie dormant in a population and outbreaks are often triggered by environmental events such as the water temperature rising to 25 deg C. Such conditions have been experienced during the recent warm summers days and dead carp have been observed in large numbers in Zandvlei, Rondevlei, Zeekoevlei and Strandfontein. There is no cure for the disease, which can result in 80-100% mortality rates amongst carp populations. Such outbreaks have to eventually work their way out of the population.

The disease can be transmitted by infected fish, water, soil or any other living particle transported between water bodies and may be done so by water birds. Although no evidence exists for it, it is possible the disease was introduced into local water bodies by the release of domestic fish such as Goldfish or Koi. This underlies the importance of never releasing any pet or domestic animal into a natural system.

The dead and dying fish are readily eaten by predators such as otters, water mongoose and water birds and have no negative affect on them. Like any fish taken from water bodies in Cape Town, it is not recommended that they are eaten by people.