What does it look like?
Koi carp are an ornamental strain of common carp. Their colour is variable, often a blotchy pattern, which can include black, red, orange, gold and white. They have two pairs of barbels around the mouth.
Koi carp grow up to around 700mm long. They are bottom-feeding omnivores, planktivorous as juveniles. Adults mainly eat macroinvertebrates, but also some plant material and fish eggs/larvae. Koi carp live for approximately 8-12 years in New Zealand.
Still or slow moving freshwater bodies, especially shallow, warm water. The species also prefers well-vegetated areas for spawning. Although they may spend a lot of time in one area, some individuals undertake long distance movements (greater than 20km in some cases), particularly during spawning season. Tolerant of moderately low oxygen levels.
Why is it a problem?
When feeding, koi carp suck up and expel material from the bottom like a vaccuum, filtering out edible material, which in the process disturbs the substrate (bottom) of a lake or river. Because of their fast breeding and large size, this usually results in constant 'muddying' of the water, with no chance for this mud to settle. As a result, the light levels are reduced, which negatively impacts plant life and other native fish and invertebrates. This also makes for unpleasant swimming at recreational freshwater water sites.
Koi carp are also capable of hybridising with goldfish; there is the potential here then for gene flow to make them more invasive if the offspring were viable.