Possums of freshwater: Invasive koi carp stopped from accessing local lake

DoC Waikato's Peat Lakes site lead Dion Patterson from and DoC freshwater technical adviser and science lead Katie Collins at Lake Ruatuna. Photo / Department of Conservation

Lake Ruatuna, near Ohaupo now has a special barrier to prevent the spread of the invasive pest fish koi carp.

Koi carp are an introduced pest fish resembling giant goldfish and are widespread across the Waikato. The species is unwanted and noxious and therefore a serious ecological threat to the health of freshwater ecosystems. Especially to Lake Ruatuna as it is one of Waikato's most important freshwater ecological restoration projects.

The new grill barrier, a project by the Department of Conservation and Fonterra, is designed to prevent access of adult koi carp upstream. The grill bar has spacings of 30mm allowing movement of native fish like tuna while also preventing adult carp passage.

The newly installed koi carp barrier at Lake Ruatuna, near Ohaupo. Photo / Department of Conservation

DoC's Waikato Peat Lakes site lead Dion Patterson says there are only limited options to manage koi carp and barriers act as a last line of defence to help control the spread of this pest fish.

"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."

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.

The invasive pest koi carp are highly destructive and feed on almost any organic matter. Photo / Supplied

"The design and functionality of this trap looks great, and we are thankful to the expertise consultant John Gumbley has in this field. The trap will require ongoing checks for blockages and potentially weed clearance. Maintenance will be scheduled as part of regular predator trapping lines DoC rangers and volunteers do around the lake."

Catfish, rudd and goldfish are present pests in the lake and the ongoing control through net fishing is supposed to reduce their numbers.

"Koi carp have not been detected in Lake Ruatuna so maintaining this 'koi-free' status is a priority for this site," Patterson says.

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 tiny aquatic plants called macrophytes but water quality is vital for that to occur and it will be more difficult if koi carp get into the lake.

If koi carp are caught, they must be killed immediately and check, clean and dry all gear that comes into contact with water when moving between waterways to prevent the spread.