Control and monitoring methods


Electric fishing boat

LERNZ has New Zealand's first electrofishing boat designed specifically to fish non-wadeable habitats. The ability to catch pest fish in fresh water that is too deep for wading (1-3 m) is the key to determining the abundance, impacts, and biological effects of pest fish. A major advantage of boat electrofishing is its time efficiency, with the ability to fish kilometres of habitat in a single day without the need for repeat visits. With our on-board GPS unit we estimate the distance and area fished, and can then calculate density and biomass per unit area. Since its launch in July 2003 we have was used in over 100 locations in the North Island of New Zealand to catch over 20,000 fish (total weight over 5 tonnes). We have developed multiple-pass boat electrofishing to give removal population estimates (Hicks et at., 2006). 

Contact: Brendan Hicks (

Pod traps

Pod traps are pyramid-shaped nets equipped with an automated wildlife feeder that frequently adds fresh bait to the trap to attract fish. Baits lose most of their attraction properties within an hour in the water, so by adding fresh bait trapping rates are greatly improved. Once inside the pod trap one-way doors keep fish within the trap until emptied. Pod traps are particularly effective at trapping koi carp, and have been shown to improve catch rates compared with other types of nets. Baited traps such as the pod trap lose their effectiveness after the bulk of the population has been removed because food becomes plentiful, making bait less attractive to fish, but they are nevertheless as useful method for monitoring koi carp populations.

Poison baits

Baits laced with toxins can be used to reduce carp numbers, but flavouring is often necessary to mask their unpleasant taste. LERNZ researchers have investigated the effectiveness of floating baits made mainly from brewer's yeast and grain laced with 'bold' flavours, such as vanilla or strawberry essence. Fish were given the choice of different bait flavours in tanks at the University of Waikato fish lab. All flavours were readily consumed indicating that any of the formulations could be used with equal success. One advantage of using floating baits is that unconsumed pellets can be removed from the water surface before they sink and toxins become available to native species feeding at night or on the bottom, such as eels. LERNZ researchers have also investigated the use of cube root powder and sodium nitrite as toxins in floating baits. The dose required to kill half the test animals (LD50) was 136 mg/kg for cube root powder and 122 mg/kg for sodium nitrite, indicating they were viable for inclusion in poison baits for koi carp.

Learn more...

Morgan, D. K. J., Kumar, H., Ling, N., & Hicks, B. J. (2014). Toxicity effects of oral gavage of aqueous solutions of cube-root powder and sodium nitrite in common carp. Wildlife Research, 40(8), 647-653.

Morgan, D. K., Verbeek, C. J. R., Rosentrater, K. A., & Hicks, B. J. (2013). The palatability of flavoured novel floating pellets made with brewer's spent grain to captive carp. New Zealand Journal of Zoology, 40(2), 170-174. DOI:10.1080/03014223.2012.719912


Exclusion barriers provide opportunities to prevent adult invasive fish from breeding in lakes or wetlands where they are not currently present or where they have been removed as part of an eradication programme. They typically comprise metal bars spaced to allow access to native species but not large pest fish. The bars swing one way to allow access downstream (i.e., out of the water body) but not upstream, and they are placed at constrictions on outlet streams. One such trap was installed by the Department of Conservation, with the assistance of LERNZ researchers, on a culvert at the Lake Ohinewai outlet following koi carp removal. It has proven effective at excluding breeding koi carp while requiring little ongoing maintenance.

Bottlenecks & Spawning Aggregations

This joint project with Waikato Regional Council is building on previous LERNZ research that documented the migratory behaviour of koi carp from river systems into lakes and wetlands to breed. We are determining the environmental cues initiating these migrations and determining key 'bottleneck' locations in the lower Waikato River floodplain where carp concentrate to support the deployment of mobile cages for carp trapping. In addition, we are investigating the use of unmanned aerial vehicles equipped with high resolution cameras and photo recognition software to survey lake margins for spawning aggregations.

Contact: Kevin Collier (, Bruno David (

Environmental DNA

Sensitive monitoring tools are needed to determine the success of invasive fish eradication and provide early warnings of invasion into new areas. LERNZ researchers are working with the Department of Conservation to investigate the feasibility of using fish DNA extracted from water and sediment samples to detect the distribution and abundance of pest fish such as koi carp, and the spread of invasive fish such as gambusia. This work will contribute to the management of pest fish control work and help with the assessment of changes to eradication techniques to ensure efficient use of resources.

Contact: Jonathan Banks (