Erina Watene-Rawiri from Waikato-Tainui College for Research and Development is working with others to reduce numbers of koi carp in the lower part of the Waikato River and its connecting waterways. Koi carp is a pest fish species1 that is causing a lot of damage. These fish occupy 80% of the biomass2 in the lower regions of the river catchment3, contribute to poor water quality and damage or destroy the habitat4 of native5 fish and plant species.
Lake Waahi – one-way gate trap
Erina’s research involves installing a one-way koi carp gate in the only water connection between the Waikato River and Lake Waahi. The carp will be able to push their way through the one-way gate to get out of the lake and into the river – but the gate won’t allow them to come from the river into the lake. Local people will fish the carp from the lake using specially made koi carp traps. Erina hopes to be able to build a more sophisticated trap at Lake Waahi based on a pilot model at Lake Waikare.
Lake Waikare – pilot model
The trap at Lake Waikare is based on a trap designed by researchers from the South Australian Research and Development Institute. It was installed in a fish pass between a river tributary6 and the lake and became fully operational in November 2010. An estimated 1565 kg of koi carp were removed from the trap in the first 2½ days, with minimal bycatch7 of native fish.
Waikato Regional Council scientist Dr Bruno David said, “The success of the experimental trap is very encouraging and will likely lead to the development of a proposal to install a permanent automated koi trap in the Lake Waikare fish pass.”
How the trap works
The trap at Lake Waikare sits in the tributary. It was designed to exploit8 the natural behaviour of koi carp. Koi carp like to push through things. At the bottom of the trap are finger-like doors that can be pushed open – one way. The carp push against them and enter the trap. They remain there until someone brings the cage up out of the water. This works like a lift9. At the push of a button, the cage comes up – usually full of carp. The cage is tilted and the carp fall down a shoot into a digester.
The digester kills the fish and then grinds them up. The ground carp enter a large drum that contains thermophilic10 bacteriaThermophilic bacteria11 thrive at relatively high temperatures. They generate their own heat12 as they grow (getting up to about 60–70°C) and can decompose the fish at a fast rate. The drum turns slowly for 2–3 days, by which time the carp has dried out and become a brown, granular substance rather like coarse ground coffee beans. Bruno jokes that it makes ‘carppuchino’. The koi carp have actually been turned into an effective fertiliser13
The fertiliser is currently used for riparian planting14 along the riverbanks and around the lakes. Care is taken that not too much fertiliser is used so that the nutrients15 in it don’t end up back in the water, causing unwanted algal16 growth.
Excluding native fish
The trap is effective, even to the point of excluding native fish. Bruno tested this by putting out some nets behind the trap so that anything getting through the system would be caught in the nets. Hundreds of eels were caught overnight – showing that the eels are getting through the trap. The trap is designed to catch only large carp. Smaller fish and even small carp are able to escape through small holes. Once carp reach a certain size, they are unable to escape.
Erina and Bruno don’t think they will be able to completely eliminate all koi carp but hope that they can drastically reduce numbers.
Nature of science
Scientists often respond to environmental problems. The problem with koi carp has led to the careful observation of carp with a view to exploiting their behaviour to catch them. The carp behaviour of pushing against things has been incorporated into effective traps.
Acknowledgements
Lake Waahi Restoration Project (Genesis, Waahi Whaanui Trust, Waikato Raupatu River Trust)
Dr Bruno David
Dr Adam Daniels
- species: (Abbreviation sp. or spp.) A division used in the Linnean system of classification or taxonomy. A group of living organisms that can interbreed to produce viable offspring.
- biomass: 1. Organic matter, such as trees, plants, reject fruit, straw, algae, dairy effluent or tallow (waste fat), which can be turned into biofuel. 2. The mass of living biological organisms in a given area or ecosystem at a given time.
- catchment: An area that collects all the water that drains to a particular lake, river or reservoir. Also known as a watershed or a drainage basin.
- habitat: The natural environment in which an organism lives.
- native: A species that lives naturally in a country, as opposed to species that have been introduced by the activity of humans.
- tributary: A stream that flows into a larger stream or another body of water.
- bycatch: Also called incidental catch. The harvest of fish or shellfish other than the species for which the fishing gear was set.
- exploit: Making use of something, often for a profit
- lift: In aerodynamics, upward force produced by a difference in pressure due to airflow.
- thermophilic: Requiring very high temperatures for normal growth and development (thermo=heat; philic=fond).
- bacteria: (Singular: bacterium) Single-celled microorganisms that have no nucleus.
- heat energy (heat): Heat energy: the transfer of energy in materials from the random movement of the particles in that material. The greater the random movement of particles the more heat energy the material has. Temperature is a measure of the heat energy of a material.
Heat: the flow of energy from a warm object to a cooler object. - fertiliser: Compounds that are given to plants to promote growth.
- riparian planting: Planting in the strip alongside a stream or river.
- nutrient: A substance that provides nourishment for growth or metabolism.
- algae: A large, diverse group of photosynthetic eukaryotic organisms. Algae have no stems or leaves and grow in water or on damp surfaces.
