Aaron Leaman looks at a trial project tackling the invasive pest fish koi carp.
Beneath the turbid surface of Lake Waikare, a brilliant plague has taken hold.
In the lake's dark, waveless waters, a beautiful invader lurks, feeding and breeding in relentless numbers.
In little over three decades, the ornamental koi carp has morphed from colourful curiosity to freshwater curse.
Resembling a goldfish on steroids, the carp has spread throughout the lower Waikato River basin, stirring up river and lake beds and creating havoc with native ecosystems.
North Waikato farmer and regional council chairman Peter Buckley first saw a koi carp in the early 1980s.
"At the time I thought, what a beautiful fish, because it was this bright orange colour. What nobody realised then was their potential to breed and the harm they would cause."
Buckley, who has lived near Te Kauwhata for more than 60 years, has seen firsthand the damage inflicted on the river and lakes as carp numbers swelled.
His speech quickens and the lines on his forehead deepen as he talks about the urgent need to control the invasive pest.
"Prior to the koi carp, the water in the Whangamarino catchment would come clean. But because the fish stir up so much fine sediment when they feed, the water stays dirty. The fish also damage our stopbanks and drainage systems by burrowing into the banks to feed. The financial cost is potentially huge."
Faced with this vast orange tide, a trial project has begun at Lake Waikare, near Te Kauwhata, that aims to tackle the koi carp and, in the process, give the region's native plant life a potent boost.
Lake Waikare is one of the major breeding sites for carp and large numbers enter the lake through a specially constructed fish pass.
It's here Waikato Regional Council staff have been trialling the Carp-N Neutral project.
The trial project features a special fish trap and "digester" for turning carp into fertiliser. It's midday Wednesday and the smells of life burden the air as council freshwater scientist Dr Bruno David and biodiversity officer Dave Byers lift a haul of carp from the water.
The seething orange mass heaves and squirms in the lifted fish trap before being emptied onto a metal chute.
David, careful not to inundate the system, asks Byers to bring up the bounty in two lifts.
In 30 minutes, about 800kg of carp is lifted from the water.
David and Byers work seamlessly, guiding the fish down a chute where turning blades kill the fish instantly.
Once killed, the fish are fed into the digester.
Some fish twist and turn, thrusting their bodies into the air in defiance.
Most disappear quietly down the chute.
Importantly, the fish trap has been designed to screen carp and other pest fish, such as catfish, while allowing smaller native species, such as eel and smelt, to pass through unharmed.
Inside the digester, geothermal bacteria process the fish, generating heat of between 60 to 75 degrees Celsius.
The end product is a dry, powdery fertiliser David jokingly calls carp-puccino.
From start to finish, the digestion cycle takes 48 hours.
In a season, it is estimated the trap will snare about 30 tonnes of pest fish.
One tonne of fish produces 300kg of dried powder.
Previous studies show carp, through their feeding, incorporate nitrogen and phosphorous into their flesh.
When digested and dried, the carp mix has proved an ideal potting mix for growing native plants, such as flax, manuka and cabbage.
David says the project is based on the idea of "turning something bad into something good".
"In effect, the carp are helping us recycle the excess nutrients diffused into the environment. The energy that was stored in their flesh is now transferred and stored in the plant which is the idea of the game."
David uses the example of Canadian salmon to explain the cycle of energy. Salmon live most of their lives in the sea and absorb nitrogen in their flesh. They then swim up the streams to spawn and die.
Bears feed on salmon and defecate in the forest, fertilising the trees.
Core samples taken from the trees show the nitrogen there is derived from a marine environment.
"The salmon situation is a natural movement and flow of energy, whereas what we've got in the Waikato is an unnatural movement of energy and an unnatural amount of energy in the environment. The carp is being very kind in recycling that for us which we then store in trees. So the carps' mortality is not in vain. It's not a case of putting them in a hole in the ground. The trees they've fertilised will grow and the benefits will accrue for the lives of those trees."
Plants grown in the carp mix can be used in riparian planting, which helps water quality through shade and filtering of surface runoff.
A long-term objective is to have the Carp-N Neutral project supply community nurseries with plant food.
The regional council paid $120,000 for the fish trap while Genesis Energy and the Waikato River Authority contributed $75,000 and $85,000 respectively towards the digester and its operation.
River Authority co-chairman Tukoroirangi Morgan says innovative projects, such as the fish trap and digester, will go a long way to restoring the health of the Waikato River and its catchments.
The authority administers the Waikato River Clean-Up Trust, with the Government committing $210 million over 30 years.
Morgan grew up by the banks of the Waikato River at Huntly and first saw koi carp in a fish pond at Auckland Zoo.
"When they appeared in the Waikato River, it came as a major surprise to all of us. Seeing their numbers now is incredible."
Earlier this year, the authority revised its funding strategy with a greater emphasis on co-funding river cleanup projects.
Morgan says the authority understands it cannot clean up the river alone.
"This project is symbolic of our desire to stretch the dollar. It's also sensible to work with others who have the same aspirations."
Byers and David have worked on the koi carp project since its inception eight years ago. Byers has an intricate knowledge of the project and can explain in detail how the geothermal bacteria inside the digester produce their own heat when breaking down the fish protein.
It is a remarkable process and Byers is excited to have the fish trap and digester reach trial stage.
"Unlike a pest control operation, where you don't actually get to see what's happening, it's great to have something tangible on the ground.
"You can see the pest problem going in and something good coming out."
Although the project is still in its trial stage, the pair are already thinking of ways to improve its running. In time, the pair would like to see the trap and digester operation used at other locations where carp breed.
"By doing this here, we won't solve the carp issue but it's the first step in a strategic framework for putting units at other locations within the lower Waikato River basin," David says. "There are at least five or six other places where this set-up would work very well. If enough of these traps and digester sites are put in the right spots, then the negative effects of carp can be significantly reduced."
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