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Carp Fish dying

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.

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  1. Liz01-26-15

    Thanks Dalton for a really useful post.
    I am interested to note, though, that there are also a lot of small Tilapia dead or dying – at least on our side. May be something different – and linked to warm waters. Your thoughts?

  2. Trevor01-27-15

    Did notice it as well and forgot to call Rondevlei. We are seeing the baby fish looking as if their flesh has been eaten by parasites

  3. Dalton Gibbs01-28-15

    Hi Liz

    The Tilapia are not meant to be affected by the Koi Herpes virus – maybe low oxygen due to the warm days and algal bloom in the water? If it persists please let us know.

  4. James01-29-15

    Wikipedia- Koi herpes virus (KHV) is a viral disease that is very contagious to the common carp Cyrpinus carpio. It is most commonly found in ornamental koi, which are often used in outdoor ponds or as feeder stock. The first case of KHV was reported in 1998, but not confirmed until later in 1999.

    KHV is a DNA-based virus. After discovery, it was identified as a strain of herpesvirus. Like other strains, KHV stays with the infected fish for the duration of their lives, making the recovered and exposed fish potential carriers of the virus. Koi fish infected with KHV may die within the first 24-48 hours of exposure.

    KHV is listed as a nonexotic disease of the EU, so is watched closely by the European Community Reference Laboratory for Fish Diseases.

    Symptoms of KHV include:
    Gill mottling
    Red and white patches appearing on gills
    Bleeding gills
    Sunken eyes
    Pale patches
    Blisters

  5. Dagny01-29-15

    Interesting! Explains the increased seagull presence. Thanks Dalton. However I sure this will impact our bird life as a declined good source going forward. I have also noted a lot of smaller fish along the various waters edges. Perhaps banded tilapia fingerlings? They too are dying, but perhaps due to factors noted previously. Have seen kids catching the by hand at the end of Victoria road. The water is also very low presently. Rain would be welcome!

  6. Charlene01-29-15

    I have never seen the water so low in summer in the ten years I have lived here. the little fish at the water’s edge seem to have ‘Ick’ or something similar that I have seen in aquarium fish. Is the water level so low because of the wind we have had since October last year?

    • Dalton Gibbs01-30-15

      It is indeed a very low water level and can be attributed to the constant wind and warm weather. There is however also very little ground water on the Cape Flats as we did not have such a late winter last year. Rondevlei is drawn down at the moment in preparation for an ecological burn in March/April, but in other years when this has been done it has not affected the Zeekoevlei water level.

      When it comes to water levels on the Cape Flats it is useful to realise that at Strandfontein Sewage treatment Works they have an evaporation tank to measure evaporation. Over 2500mm of evaporation is measured each year with only about 615mm rain. The wetlands around here rely extensively on ground water and run off to remain full.

      The natural cycle is that our wetlands would dry up in summer (this is a winter rainfall area) and flood in winter. With urbanisation and human induced changes our wetlands remain artificially full all year round.

  7. Dalton Gibbs01-30-15

    Latest news: The Koi Herpes Virus has been positively confirmed based upon a lab based DNA test.

  8. alan hopper02-10-15

    we are experiencing similar fish deaths at wildevoelvlei
    thanks for input

  9. lionel presence05-12-16

    How long before the fishing can go on at zeekoevlei dam

    • Dalton Gibbs05-12-16

      Zeekoevlei is open to fishing; as per usual the eating of fish from the water body is not recommended.

  10. Debbie Kroukamp06-10-16

    Good day, not sure if this will help due to the size of the vlei…but see the link….http://www.nationalfishpharm.com/KHV.html
    Thanks, Debbie

    • MoonDance06-10-16

      I think the final score was like 20 tons of fish dead so nope, isolating them would take a bit to much effort 🙂 but thanks anyway

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