Koi carp (Cyprinus carpio Linnaeus, 1758) are a brightly coloured fish native to Asia and Europe. They are an ornamental variant of common carp (Cyprinus carpio carpio) and were developed by selective breeding from the Asian subspecies Cyprinus carpio haematopterus. They are one of the most popular ornamental pond fish in the world – some colour variants sell for thousands of dollars (especially in Japan). In the wild, koi carp keep their colour but not the forms that command high prices overseas. Feral koi carp have become a pest fish found on every continent in the world except in Antarctica.

An introduced species

The species is thought to have been introduced to New Zealand waters in the 1960s. They may have been released accidently from ponds during flooding. Some may have been released for fishing purposes. Feral breeding stocks were first noticed in the Waikato River in 1983. They are now very common throughout the lower Waikato River regions – spreading into streams, lakes and wetlands. They make up the largest fish biomass (up to 80%) in the lower Waikato River regions where slow, turbid water and shallow lakes and wetlands provide ideal habitat.

What do they look like?

Wild common carp tend to be a green colour, but our stocks are derived from the ornamental Japanese koi so they often exhibit patterns of black, red, orange, gold and white splotches. Koi carp in New Zealand can grow up to 12 kg in weight and 75 cm long. Koi have two pairs of whisker-like feelers called barbels at the corner of their mouths. About 1% of the Waikato koi are goldfish hybrids – identified by their large size relative to goldfish. These koi do not have barbels or they have a single pair of barbels.

Breeding

Waikato koi rarely exceed 9 years of age. Females average 5.2 years and males 4.6 years of age. An average fish weighs 3 kg. Females produce 100 000 eggs per kg of body weight. A typical female can produce 300 000 eggs annually (or more if they spawn more than once). Koi carp spawn throughout the summer. As they gather for spawning or feeding in the shallow margins of the river, koi biomass can reach 4000 kg/ha.

What damage do they do?

Koi carp feed like vacuum cleaners – moving along the bottoms of streams, lakes and rivers. Koi carp are opportunistic feeders eating a wide variety of benthic organisms – plants and animals. They suck up and expel material from the bottom, filtering out edible material as they go. They grind their food to a pulp with powerful teeth located in their throats, making a noise that can be heard underwater through a hydrophone.

This activity destroys native plant, fish, invertebrate and water bird habitat. It stirs up the substrate and can greatly increase the turbidity of the water. Waterways become muddy and unattractive. Aquatic plants are dislodged and are unlikely to re-establish. Oxygen is depleted.

Where are they found?

Koi carp prefer still waters, spreading from rivers into lakes, streams or backwaters in rivers. They are highly tolerant of poor water quality – surviving well in degraded water and contributing to the decline.

Although koi tend to stay within a restricted home range, they are powerful swimmers, and individuals may make extensive journeys to spawning habitats or for feeding. One tracked fish made a return trip of 104 km in 197 days while another made a return trip of 208 km in 196 days. Other fish have been recorded travelling upstream from Lake Whangape into the Waipā River, a distance of about 120 km. The Karāpiro dam prevents koi movement further upstream in the Waikato.

Koi carp are widespread in the Waikato and Auckland and are spreading further north. They have also been found in Wanganui, Hawke’s Bay and Wellington but have not yet been found in the South Island.

Containing and controlling koi carp

Initiatives have been set up in the Waikato to help contain and control koi carp. Koi carp recreational fishing (especially bow fishing) is encouraged in certain lakes in the Waikato. Competitions are set up annually near Huntly. In the spring of 2010, 8.6 tonnes of koi were caught from a lake in one weekend. Koi carp caught are made into berley for fishing.

The Waikato Regional Council is working to eradicate koi carp. Gates, traps and digesting machines are currently being set up to catch and destroy koi carp in lakes near Huntly. These fish are made into fertiliser for riparian planting along the river and around lakes.

Nature of science

Scientific priorities depend on the society and culture in which it is conducted. For example, scientists are concerned about the effects of introduced species on our environment. Koi carp have had major negative effects on the river and the surrounding catchment. This has led to on-going science work to try to negate these harmful effects.

    Published 19 March 2014