Trapping koi carp

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 species that is causing a lot of damage. These fish occupy 80% of the biomass in the lower regions of the river catchment, contribute to poor water quality and damage or destroy the habitat of native 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 tributary 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 bycatch 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 exploit 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 lift. 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 thermophilic bacteriaThermophilic bacteria thrive at relatively high temperatures. They generate their own heat 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 fertiliser

The fertiliser is currently used for riparian planting along the riverbanks and around the lakes. Care is taken that not too much fertiliser is used so that the nutrients in it don’t end up back in the water, causing unwanted algal 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.