Koi carp control tools
IntroductionInformation about koi carp control tools that are both currently available and being explored as future options.
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These tools are currently available options for controlling koi carp.
Recreational fishing of koi carp is permitted. All koi must be killed when caught. Recreational fishing likely has minimal impact on the overall koi population.
DOC sponsors the Koi Carp Classic (held in the Waikato) which has been running for 30 years. The event attracts bow hunters from across the region, and their efforts remove several tonnes of koi from Waikato waterways each year.
Fish can be manually removed from a water body using nets, draining the water body, electric fishing and traps. The logistics of managing these operations are labour-intensive and are more suited to enclosed water bodies.
Manual removal methods are not selective, affecting both native and pest fish, and make it challenging to return native species unharmed. It is useful to complement other control options, but on its own it is a temporary measure unless reinvasion can be prevented.
Koi traps can be effective, but:
- require a large capital investment
- take a long time to gain the necessary approvals or consents, and
- have ongoing maintenance costs.
A trial fish trap was installed in the mid-2000s at the outlet to Lake Waikare. It proved the concept and removed 35 tonnes of invasive fish.
To reduce the koi population over time and enable ecosystem recovery, koi traps need to be operated at a significantly larger scale and at multiple locations. They are not currently used as a viable option due to costs and site selection.
Rotenone (cube root powder) is the most common fish toxin (piscicide) used globally. It is a naturally occurring organic chemical used as a traditional fishing method across Asia and South America for generations. It’s also used for controlling pest fish in freshwater and is reasonably well known and used in New Zealand in smaller water bodies and enclosed systems.
Rotenone is not selective and is lethal to all fish species – that means it cannot be used in a widespread manner to control koi. Extensive investigation is required to determine how to manage effects on ecosystems before a control operation takes place.
Barriers can be used to selectively contain and exclude fish from areas that can be isolated from a wider catchment area. There are natural waterfalls and other barriers (eg culverts) in streams that have prevented koi carp reaching a number of waterways in the Waikato region.
An example of an artificial waterfall barrier was one constructed to specifically exclude koi and other pest fish from the Puketirini Stream in Huntly. This barrier is designed to protect a key native fish habitat as it prevents pest fish passage but allows climbing native fish to getpassed. .
Another trial at Lake Ohinewai excluded large koi through a selective one-way barrier and extensive fishing reduced koi in the lake. This design did not exclude juvenile koi, so after four years the numbers of fish in the lake was estimated as being similar as before construction.
Total barriers prevent up and downstream migration by New Zealand native fish and are not an option due to freshwater regulations.
These tools are being explored as further options to detect, estimate population size and control koi carp.
eDNA detection tool
All species which enter or live in a waterway shed DNA. DOC is investigating the use of eDNA (environmental DNA) as a detection tool. Water samples can contain traces of DNA from animals and plants present in those water bodies. Laboratory tests can determine species present from the eDNA collected in a water sample.
There is ongoing development of this tool, with a number of organisations collaborating on its use.
Drone surveillance tool
DOC continues to work on the use of drones to detect koi in waterbodies. Initial results have determined the optimal flying height, video/photograph technical specifications and proof of concept to detect koi in known infested sites.
Further work will explore the ability to find koi when in very low numbers and the ability to estimate koi abundance (biomass).
Koi herpes virus
Koi or Cyprinid herpes virus (CyHV-3) has been the subject of an extensive research and investigation programme in Australia's National Carp Control Programme.
Results show CyHV-3 has so far been found to be highly specific, only affecting carp species, and has not been found to affect other species of fish, including some common to New Zealand like short-fin eel and inanga. CyHV-3 is a potential biological control tool for managing koi populations in the North Island.
There is currently no planned release of CyHV-3 for New Zealand. A programme of work has been scoped for consideration, with the first steps to test whether CyHV-3 will affect New Zealand koi, while not harming native or other desirable species. Extensive tangata whenua (iwi, hapū, whānau), stakeholder and public engagement is also required before progressing through the many stages of assessment and trial prior to release.
It is important to note CyHV-3 is not considered an eradication tool. It will require other control measures to maximise the benefits and achieve reductions in koi populations to a point where freshwater ecosystems can recover.
Koi gather in the shallows and in large groups at spawning time in spring. Disruption of these spawning activities can be done in several ways, including:
- manually removing spawning adults
- physically blocking access to spawning areas
- liming of eggs, and
- isolation and/or spot poisoning of nurseries.
This work is still at the investigation stage.