Pests threaten ecosystem

A giant catfish. Photo / Thinkstock

New Zealand streams, rivers, lakes and wetlands are under threat from aquatic pests, including catfish and red-eared turtles.

Increasing presence of koi carp, catfish, gambusia and rudd worry environmentalists and anglers, says Rotorua-based Department of Conservation partnership ranger Caraline Abbott.

"Once established, these pest fish stir up sediment making water murky, they prey on native fish and their eggs and compete with other fish for food. Evidence also suggests they increase nutrient levels and algal concentrations," Ms Abbott says.

Koi carp, introduced here as an ornamental fish, are now established in lakes across New Zealand. They can grow up to 10kg and feed like a vacuum cleaner, sucking up everything and blowing out what isn't wanted.

They stir up sediment, destroying native plants and fish habitat, often undetected until populations reach large numbers. Little can be done to eliminate these aquatic pests.

Waikato is just one region shown to have become home to koi, catfish, rudd and exotic turtles.

Although New Zealand temperatures are usually too low for turtles to breed, there is concern geothermal venting in the central North Island could provide habitats suitable for reproduction.

People may be unaware of the long-lived nature of terrapins and release them into waterways when no longer wanted.

Once in the water, the amphibians which live up to 40 years, have the potential to cause great harm to plant and animal life including eating invertebrates which are usually a food source for native fish and trout.

The Biosecurity Act also prohibits the release of aquarium contents into waterways. People should never release aquarium weeds, exotic fish, goldfish or amphibious turtles into lakes, rivers and streams.