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  • Rights: University of Waikato. All rights reserved.
    Published 19 March 2014 Referencing Hub media

    Waikato Regional Council scientist Dr Bruno David describes how the digester installed at Lake Waikare works. The digester helps to reduce koi carp and other pest fish – by turning them into fertiliser with the help of thermophilic bacteria.

    Point of interest
    The trap also captures other invasive species such as the brown bullhead catfish from North America, goldfish and goldfish/koi carp mixes.


    The trap’s designed by the South Australian Research Development Institute to try and control invasive carp in wetland systems. We’ve had a pretty good working relationship with the guys in Australia, and we’ve got a similar problem here, so we used some of the existing technology that they’d developed, got them over here and put one of the cages here on the shores of Lake Waikare.

    The trap exploits various aspects of carp behaviour. So the bottom of the trap has a series of fingers on it, and when the fish come up, they push through the fingers, and carp like to push through things. So once they’ve pushed through the finger and go in, the finger closes behind them with the flow, and they get trapped inside the cage. It’s a one-way system, but with the cage in a New Zealand context, we’ve designed mesh sizes on the cage here so that all of the native fish can pass through but we can screen out the smallest possible invasive fish. This is also designed to catch other invasive fish in the Waikato basin

    The idea of this system here is to take the carp which have been free ranging through the Waikato system and assimilating a whole lot of energy into their body, and then they are euthanased very quickly and humanely, and then they’re fed into a bacterial digester that’s beside the trap there. The bacterial digester uses a thermophilic bacteria, and these bacteria generate their own heat. And they like eating proteins, so they consume the fish very quickly, and it gets up to about 60 or 70 degrees in there.

    We’ve been trying to propagate native trees in various mixes of the fertiliser, so the idea being that we take the energy that’s in excess within the environment, and we try to store that into native tree growth, and then as those trees grow, they provide a whole range of other functions like shade, root-holding capacity for banks and habitat for birds as well. So the idea is to take a problem, which is carp, and then try to result in a general positive benefit from having done that.

    Dr Bruno David, Waikato Regional Council

    The Waikato Tainui College for Research and Development acknowledges the financial support given by the Waikato River Cleanup Trust Fund which is administered by the Waikato River Authority.

    The Waikato River Cleanup Trust does not necessarily endorse or support the content of the publication in any way.

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