Controlling pest fish

Pest species of fish and plants will need to be controlled or eradicated if they are threatening the success of your restoration project.

Adult trout are predators of many native fish, particularly migrating juvenile galaxiids. Perch can also reduce native fish and crayfish.

What to do:

Remove pest fish by either repeat electric fishing or rotenone treatment. Prevent recolonisation by installing a barrier that hinders the movement of trout but not of native fish. Such barriers have been successfully applied in the past, in Australia and Otago, and are planned for streams in Rotorua where trout threaten the survival of koaro.

You need permission from the regional council to install barriers. To remove acclimatised fish such as trout, salmon, tench and perch, a Fish & Game Council permit is required.

In shallow streams (<20cm deep) with a low gradient and many pools, gambusia can displace threaten inanga in summer. Gambusia can only be eradicated by rotenone treatment, and only DOC staff are permitted to kill fish with this pesticide. To prevent gambusia recolonising, eradicate these fish from all upstream sources (e.g. farm ponds). Also, install barriers downstream to restrict the fish moving upstream from estuaries.

Only eradicate pest fish if it is technically feasible (i.e. if rotenone can be used successfully), and if downstream barriers can be installed to limit upstream recolonisation.

Restoration to reduce pest fish can include habitat alteration which favours native fish and discourages alien species. A good example is stream channel shading using riparian  cover – habitats with cooler water will discourage pest fish such as koi carp, tench, and gambusia because they tend to  favour warm water. As another example, discourage gambusia by increasing stream flow – gambusia are poor swimmers and cannot tolerate even small flows.

Non-fish pest species include exotic macrophytes and dense growths of emergent weeds. Reduce the growth of these plants with riparian shading to reduce sunlight. Consider mechanical, chemical or biological methods.

Grass carp as a biological agent for weed control

Rotenone application to remove fish. Credit: NIWA