Skip to main content
Published: 2012-11-02 00:00:00

A joint trial project to tackle the widespread pest fish koi carp in the Waikato River is about to get underway at Lake Waikare’s fish pass.

Koi carp are one of the causes of reduced water quality in the river – they stir up sediment and nutrients as they feed (although they also incorporate some of these nutrients into their flesh as they grow).

Lake Waikare in North Waikato is one of the major breeding sites for carp and large numbers of fish frequently enter the lake through a specially constructed fish pass.

Now, under the “Carp-N Neutral” project, a special fish trap and a fish “digester” for turning fish into fertiliser for growing native plants are due to be installed in and beside the fish pass and be operational by Christmas.

Trapped carp will be killed and then fed into the digester which will turn them into a nutrient-rich potting mix.

The co-location of an on-site fish trap and specially built digester is believed to be a world first. Co-location helps reduce haulage costs, resulting in a more energy efficient process, and eliminates the risk of the trapped pest fish being spread accidentally during transport to a disposal site.

The project involves funding from Waikato Regional Council, the Waikato River Authority, and Genesis Energy in partnership with Waikato-Tainui and the Waahi Whaanui Trust.

Latest developments include:

  • The planting of 150 native trees on site in September which were grown with nutrients from carp captured at the fish pass. Plants were propagated at Whaingaroa Harbourcare nursery as part of a native plant growth trial using the carp product.
  • Site works at Lake Waikare have begun.
  • The construction of pre-cast concrete channels for the trap is underway in Hamilton.
  • The Australian-built fish trap has arrived at the council’s Gordonton depot and will be assembled on site later this month.
  • A crane is due to be deployed on site this month to help install the trap and digester.
  • The digester is scheduled to be at the site by the end of November.

There will be a period of testing after that to ensure the system is operating as planned.

It’s hoped the trap and digester could be a model for other such projects in the region in the future.

“By removing large numbers of carp from strategic locations where they want to breed we can start to improve the local environment for native fish and reduce the negative influence of carp on water quality,” said regional council freshwater scientist Dr Bruno David.

“As an added bonus we can also recycle the nutrients stored in carp flesh for use in riparian planting, which further helps water quality through shade and filtering of surface runoff.”

The trial will initially focus on further proving the concept of the fish trap and refining the carp mix production process. Eventually, the project could be replicated at other sites.

“Carp are one of many issues that need to be controlled to improve river health in the lower Waikato basin,” said Dr David.

“Large numbers of carp can certainly hamper river rehabilitation efforts. However, if the new trap and digester site is successful, and enough of these units and other devices can be deployed in the right places, then the negative effects of this invasive species should be significantly lessened.

“We will work closely with all involved to study how well the new system works and how this technology can best be spread to other sites around the region where carp are a problem,” said Dr David