Keeping freshwater ‘possum’ out of Ruatuna 

A grill barrier has been installed to keep one of the country’s worst pests out of an Ōhaupō lake.

The Lake Ruatuna barrier is designed to prevent the spread of koi carp.

Ruatuna represents one of Waikato’s most important freshwater ecological restoration projects and the barrier is part of the Living Water project, the conservation partnership between Fonterra and the Department of Conservation.

Don Patterson and Doc Freshwater Technical Adviser Katie Collins at Lake Ruatuna. Photos: Department of Conservation.

Koi carp is a noxious pest fish resembling giant goldfish and are widespread across the Waikato – but have not been found in Ruatuna. They operate late vacuum cleaners and present a serious ecological threat to the health of freshwater ecosystems and are variously described as freshwater possums and rats of the river.

The fish are harvested in huge numbers each year by bow hunters but are not good eating.

The Lake Ruatuna grill barrier is designed to prevent access of adult koi carp upstream and serves as a final barrier between the lake and the pest fish during flooding events. The grill bar has spacings of 30mm allowing movement of native fish like tuna while also preventing adult carp passage.

Dion Patterson, DOC’s Waikato Peat Lakes Site Lead, says there are limited options available to manage koi carp, and barriers act as a last line of defence.

“Koi are like the possums of freshwater. They’re very hard to eradicate and are highly destructive. They feed like vacuum cleaners, sucking up anything in their wake and blowing out anything they don’t eat – which isn’t much.”

The carp stopping grille

Koi feed on insects, fish eggs, small fish, plants and almost any other organic matter. Their manic feeding stirs up the bottom of waterways, further degrading water quality. And unfortunately, koi thrive in degraded water.

Catfish, rudd and goldfish are present in the lake and the ongoing control through net fishing is intended to reduce their numbers. The work of staff involved in pest fishing at Lake Ruatuna acts as a surveillance should koi somehow find their way into the lake.

A long-term goal at Lake Ruatuna is the reintroduction of macrophytes – tiny aquatic plants.

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