Koi carp look like goldfish when they are small, but the carp have paired barbels or feelers around their mouth which goldfish lack. The carp use their barbels and big lips to fossick for food in lake sediments. Photo: RNZ / Alison Ballance
The Carp-N Neutral project in the Waikato is trying to make good from a bad pest fish, and turn back the orange tide that has taken over the streams, lakes and wetlands of the Waikato in the last few decades.
Freshwater ecologist Bruno David is spearheading the Waikato Regional Council initiative, which aims to harness the nutrients contained in nuisance koi carp and repurpose them for beneficial environmental outcomes.
From bad fish to good fertiliser - going down the Carp-N Neutral biodigestor path
Each spring, thousands of koi carp leave the streams and wetlands of the Waikato and head to lakes such as Waikare to spawn.
The project literally heads the migrating koi carp off at the pass, sending them up a specially-designed fish pass at the Lake Waikare outlet where they get caught in a fish trap. The fish trap is a cage with a mesh that is large enough to allow smaller native fish to swim through, but which traps the much larger koi carp and other pest fish such as bullhead catfish.
“One female koi carp can potentially produce 400,000 eggs, so [when we catch] one fish we’ve prevented nearly half a million baby fish,” says Bruno.
The fish trap at Lake Waikare. The biodigestor is housed in the white container. The muddy water of Lake Waikare, visible in the background, is caused by thousands of pest fish stirring up the sediment. Photo: RNZ / Alison Ballance
Many Waikato lakes, such as Lake Waikare, suffer from excess nitrogen and phosphorous runoff from surrounding farmland. Although the koi carp stir up sediment in the lake as they feed, their advantage is that they also remove nutrients.
“[Koi carp] are aquatic vacuum cleaners basically. They use their feelers and big ‘juju’ lips to fossick around in the sediment after small organisms.”
Rather than waste the nutrients that end up incorporated into the fish flesh, Bruno has been developing a Carp-N Neutral project that uses the fish as a source of environmentally friendly fertiliser, rich in potassium, phosphorous and nitrogen, that can replace the expensive fertiliser tablets currently used in revegetation programmes, which use synthesised nitrogen and mined phosphorous..
“Effectively what we’re doing is recycling a whole lot of nutrients,” says Bruno. “We’re trying to replace that fertiliser tablet with an invasive organism, and using a recycled nutrient rather than one that was mined or synthesised.”
Small koi carp look like goldfish (although they have barbels around their mouth that goldfish lack), and it is thought they were introduced to New Zealand in a consignment of goldfish during the 1950s. It wasn’t until the 1970s that they reached large enough numbers to have a noticeable effect in lake and rivers.
In the four years that it has been running, the trial Carp-N Neutral project has removed 35 tonnes of invasive fish from Lake Waikare.
The Carp-N Neutral process uses thermophilic digestion to turn koi carp into fertiliser pellets. After the fish are euthanased their bodies are put into a large digestor along with a ‘starter’ that contains the bacteria Bacillus subtilis.
Bruno says the aerobic bacteria “like to eat protein and generate heat … and all we need to do is provide them air.”
The digestion process is rapid: it takes 48-72 hours to break down 3-4 tonnes of fish into a dry granular fishmeal that Bruno has nicknamed ‘carpuccino.’ This is then mixed with locally sourced biochar – they are currently using waste corn - and sent to a factory to be turned into pellets.
The biochar provides carbon and structure, binds ammonia, reduces aroma and helps create a slow-release fertiliser.
The fertiliser pellets are being trialled at four dune restoration sites. The idea is that there used to be large numbers of seabirds that would have once naturally supplied lots of nutrients in the form of guano.
Another by-product of the digestion process is an oily liquid that can be diluted and used as fertiliser.
Bruno says they have also been trialling the fishy-smelling liquid as a bait for stoat and rat traps, with promising results.
“It’s the concept of using one invasive species to control another invasive,” says Bruno. “We’re trying to make a few wins like that.”
In future, the Waikato Regional Council would like to consider creating biochar from invasive trees such as willows that they remove in large numbers from the Waikato’s wetlands.
Bruno David won a national environmental Green Ribbon Award in 2015 in the "caring for our water” category for the prototype Carp-N Neutral project, which he hopes to expand to several other sites in the Waikato.
Bruno David won the 2014 New Zealand National Fielddays Society Innovation Award for his work with native freshwater fish. He was on Our Changing World last year talking about ‘kokopu condos’ and ‘tuna townhouses’, which are innovative homes for freshwater fish in urban streams.
