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CarpN Neutral Project

Turning something bad into something good

The CarpN Neutral project aims to remove invasive pest fish and convert them into products to address other environmental issues, with potential for cost recovery.

Invasive fish cause major problems in New Zealand and globally, with costs and resources to keep them under control significant and ongoing.

Koi carp were introduced as an ornamental fish, but pose a significant threat to our freshwater ecosystems by uprooting water plants, lowering water quality and eating insects normally consumed by native fish. Koi carp are considered to be one of the most invasive freshwater fish in the world.

Many shallow lakes infested by invasive fish in the lower Waikato River basin are nutrient enriched from agricultural land use, and high densities of invasive fish exacerbate the poor water quality, hampering rehabilitation efforts.

Removing large numbers of these fish reduces the re-suspension of lake nutrients caused by them. Also, because these fish feed on nutrient-rich lake bottom sediments, excess nutrients assimilate into their flesh.

So, removing the fish removes excess nutrients (embodied in the fish) from the aquatic ecosystem and reduces the likelihood of problematic algal blooms.

The CarpN Neutral project

To lessen the impact of koi carp on our environment, we have been undertaking a range of trials to generate a net environmental positive from a negative.

In 2012, Waikato Regional Council had a koi carp trap and digester built to efficiently remove invasive pest fish from Lake Waikare.

Over four years, our demonstration site has removed over 35 tonnes of invasive fish from entering the lake. Of those, only three fish have been native species. These three fish were returned alive upstream of the structure. Trapping upstream of the structure shows large numbers of native fish are passing through our invasive fish cage unscathed.

The long term aim is a self-funded, cost neutral invasive fish removal programme with outputs supporting other environmental initiatives. The same model could be applied by other agencies where invasive fish are problematic.

How it works

  • Fish trap screens koi carp and other pest fish, while allowing native species to pass through unharmed.
  • Thermophillic bacteria rapidly digests harvested invasive fish into a granular, nutrient-rich fertiliser/fishmeal we call carpacinno.
  • Fertiliser pressed into pellets.

Making something good

At the trial site, thermophillic bacteria rapidly digest harvested invasive fish into a granular, nutrient-rich fertiliser, which we’ve used to create other products.

  1. Use of the material as an adjunct to potting mix to propagate native plants. Testing of leaf tissue (using stable isotope analysis) in flax and manuka seedlings shows that the greater the concentration of fish meal in the Dalton’s native potting mix, the greater the nutrient uptake by the plant. This shows that the excess nitrogen from the carp flesh is being taken up and stored in the plant rather than in the soil, removing the likelihood of it entering waterways.
  2. ‘Recycled’ invasive fish meal fertiliser pellets are being trialled in dune restoration planting and replace commercial fertiliser tablets typically made from synthesised nitrogen and unsustainably-mined phosphorous components. Planting trials suggest many dune ecosystems need this additional fertiliser for improved plant survival and growth because historical marine-derived nutrients provided by large colonies of nesting sea birds has been significantly reduced.
  3. In this project we have also compressed invasive fish meal into baits for use in rat and stoat traps and chew cards. It’s a simple concept – using one invasive animal to control another. Preliminary trials indicate the baits and chew cards are attractive to rats and also hedgehogs.

    Dune planting trials: A golden opportunity

    Currently hundreds of thousands of natives are planted using artificial fertilisers to improve establishment and early growth.

    However, there are excellent opportunities to use CarpN Neutral pellets in restoration planting programmes. 

    Planting trials undertaken at Port Waikato, Nukuhakari and Whitianga by care groups in 2015 have shown CarpN Neutral fertiliser pellets perform at least as well as commercial tablets, and 3-4 times better than control plants going into the ground without fertiliser.

    Further testing is being undertaken to fully assess the effect of the pellets.

    CarpN Neutral: An award-winning project