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Published: 2012-05-07 00:00:00

A project to tackle the widespread pest fish koi carp in the Waikato River has been given a major boost with an $85,000 funding injection to the regional council from the new Waikato River Authority (WRA).

The “Carp-N Neutral” project will involve capturing carp – officially described as an “unwanted organism” and a “noxious fish” – in a special trap at Lake Waikare. They’ll then be turned into plant food on site using a fish “digester”. Recent work by the council has shown that, through their feeding, carp incorporate both nitrogen and phosphorous into their flesh. When digested and dried, the carp mixture is ideal for growing native trees. In effect, processing the carp will serve to recycle these excess nutrients diffusing into the aquatic environment as a result of intensive farming practices.

The authority’s $85,000 – subject to a final deed of funding being put in place – will go towards construction of the digester. The council had previously been advised that the other 50 per cent funder of the $170,000 digester would be Genesis Energy.

“The WRA’s decision is fantastic news as it will help us combat the koi carp plague in a way that protects rivers and aquatic wildlife but also help to grow plants in nurseries which benefit the environment,” said council freshwater scientist Dr Bruno David.

Since the early 1980s, koi carp have “exploded” in the lower reaches of the Waikato River, competing with native species for food and stirring up sediments and nutrients which can contribute to the growth of toxic algae.

In 2009, a multi-agency trial showed how large numbers of fish could be captured at Lake Waikare, with minimal harm to other species, using a specially designed trap.

The council subsequently agreed to fund the building of its own trap and this is due to be installed this year.

The digester being funded by the WRA and Genesis will be installed alongside the trap allowing a fully functioning “demonstration site” to operate at Lake Waikare.

“The long-term objective of installing the digester is to have an operation that can supply community nurseries with plant food. The idea is to remove some of the excess nutrients diffusing into the river from farming activities and incorporate those nutrients into long term plant growth which will help to protect the river and the environment generally,” said Dr David.

It’s hoped the trap and digester could be a model for other such projects in the region in the future, Dr David added. “This trap and digester operation will eventually help improve water quality and aquatic diversity, and create new environmental opportunities. In time, we’d like to see more of these types of trap and digester sites at other strategic locations within the lower Waikato River basin to really put a dint in the carp population.”