Koi Carp - Turning a Pest Fish into an Environmental Gain
Koi carp (Cyprinus carpio) populations have exploded in fresh water systems of the lower Waikato since introduction, contributing to poor water quality. The species is now officially classified under various central and local regulations and Acts as a ‘noxious fish’, an ‘unwanted organism’, and a ‘containment animal pest’. They are opportunistic omnivores eating a wide range of food including insects, fish eggs, juvenile fish of other species, and a diverse range of plants and other organic matter.
Photos: Koi carp have exploded in number in the lower Waikato River basin. They grow to 75cm in length and weigh up to 10kg (top). Directing koi carp down a chute where they are sorted prior to digestion (bottom).
This Waikato Regional Council Carp-N Neutral project managed by freshwater scientist Dr Bruno David aims to trap large numbers of koi carp and “digest” them into a range of products. The fish trap screens carp and other pest fish such as catfish while allowing smaller native species such as eel and smelt to pass through unharmed.
The Waikato Regional Council and Coastal Restoration Trust have teamed up to evaluate the use of digested koi carp in a range of products for positive environmental gains in community-based restoration initiatives. These can be applied to a range of ecosystems - coastal dunes, riparian zones, wetlands, forest ecosystems. Potential uses of processed koi carp include:
- slow release fertiliser – replacing artificial fertiliser for planting on sand dunes
- animal repellent – applied to foliage of palatable planted natives to deter browsing
- rodent bait – as lures for control operations targeting rodents and mustelids
- nursery propagation – additive to potting mix and liquid fertiliser
- fungal and insect control – foliar protection of planted natives
- carbon/organic matter – mixed with bio-char from willows to boost carbon storage
This project promotes the concept of sustainability and traceability of “energy’’ flow within and between ecosystems – for instance, harvesting nutrients in pest fish and using this as an organic fertiliser replacing artificial petro-based fertilisers to boost planted natives on sand dunes.