Brooke Hartigan of Northland Regional Council doing pest fish survey work

Archived content: This media release was accurate on the date of publication. 


A recent pest fish survey done by the DOC and Northland Regional Council has provided useful advice for planning the battle against two Northland pest fish, koi carp and rudd.

Date:  19 July 2019

“The survey covered 12 pest fish sites in Northland and the good news is that we confirmed that two historic koi populations have been successfully eradicated, and no new koi sites were located during the survey. The bad news is that the battle has just begun, with significant pest fish populations in waterways across Northland,” says Amy Macdonald, DOC Freshwater Technical Advisor.

“DOC and Northland Regional Council will use this survey information to develop a pest fish surveillance and eradication programme to help protect Northland waterways. This will enable us to prioritise sites for control and eradication, and to work with communities to prevent the pests spreading to new sites.

"Pest fish upset the balance in our freshwater systems, affecting ecological, cultural and recreational values. Koi carp and rudd are both bad news for water quality so they are also a problem for our agriculture and tourism industries - we’ve all got a reason to work together to stop them spreading and taking over.”

Koi are big and brightly coloured; they breed prolifically and can have devastating impacts on New Zealand waterways. Rudd are smaller but also prolific breeders; they eat the growing tips of native aquatic plants, and can turn lakes into barren algae-filled waterways that nobody wants to swim in. Pest fish expert Helen McCaughan from Wildland Consultants has flown in from Christchurch to support the operation. 

The survey was funded from Budget 2018 which allocated DOC $76 m over four years to invest in targeted biodiversity initiatives across land, freshwater and marine ecosystems to address New Zealand’s biodiversity crisis.

This included $4.5 m over four years to contain key aquatic pest species populations and  reduce the likelihood of them spreading to sites with high biodiversity values, where it’s more difficult and expensive to control them. The aim is to contain at least four serious freshwater pests (koi, gambusia, rudd, hornwort) which have the potential to expand to other parts of New Zealand.

The new fund will also be used to control invasive aquatic plants with a high risk or impact on freshwater biodiversity values, to reduce their impact on river, lake and wetland ecosystems and reduce the likelihood of dispersal to other indigenous habitats. The aim is to increase reduction of invasive aquatic plants to at least 10 sites per year.

New Zealand’s freshwater ecosystems face a number of threats - pollution, deforestation, loss of habitat, changing land-uses, sedimentation and nutrient enrichment, and the impact of invasive aquatic animals and plants such as koi carp and rudd.

Koi carp

Koi carp superficially resemble goldfish, except they grow much bigger and have two pairs of whisker-like feelers, also called barbels, at the corner of their mouth. They are highly variable in colour, often blotchy shades of black, red, gold, orange or pearly white.

Koi carp can often be observed during summer months in ponds and slow flowing water during the day when they are close to the water surface. When koi carp feed they stir up the bottom of ponds, lakes and rivers, muddying the water and destroying native plant and fish habitat.

They are opportunistic omnivores, which means they eat a wide range of food, including insects, fish eggs, juvenile fish of other species and a diverse range of plants and other organic matter. They feed like a vacuum cleaner, sucking up everything and blowing out what isn’t wanted. Aquatic plants are dislodged in the process and are unlikely to re-establish. Koi carp cause habitat loss for plants, native fish, invertebrates and waterfowl and degrade water quality.


Rudd are stocky fish with distinctive red fins and large, shiny scales that range from silver to pale or  burnished orange in colour. Rudd normally grow to about 25 cm, and about 500 g. As they are the only herbivorous freshwater fish they are the 'possums of the waterways', feeding voraciously on native aquatic plants.

They are found in ponds, lakes and slow-flowing streams where  they build up to high numbers quickly and destroy the food and habitat for native species.


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