Grass carp will be used as lake cleaner

Graham Surrey releases grass carp into Lake Wainamu near Bethels Beach. Photo / Richard Robinson

Releasing carp into a lake you've just spent five years cleaning up may seem like an odd thing for a council to do.

But the Auckland Regional Council says the grass carp released at a popular West Coast swimming lake are not like the blotchy pests that infest other waterways.

About 270 silvery fish were released at Lake Wainamu - a 15ha freshwater lake 2km northeast of Bethells Beach - to eat an invading water weed.

Unlike their relatives koi carp - an introduced pest conservationists are struggling to purge from freshwater lakes - grass carp are considered unable to breed in the wild in New Zealand because of their strict spawning and rearing requirements.

The grass carp, which have been tagged so ARC staff can monitor them, will spend the next three or four years nibbling at oxygen weed.

When their work is done, they will be removed from the lake so that native aquatic plants can grow again.

The ARC has spent the past five years removing more than 10,000 exotic pest fish from Lake Wainamu, which is an easy 20-minute walk from Bethells following a stream bed.

But the cleaner water that resulted helped large mats of oxygen weed grow, getting in the way of lake users and smothering native plants.

"We investigated using aquatic herbicides and mechanical removal of the weed but using grass carp is more cost-effective and environmentally sustainable," said Graham Surrey, leader of the ARC Freshwater Science Project.

Grass carp have been successfully used to control invasive weeds at other North Island lakes since being introduced in the 1960s.

A barrier net has been installed across the outflow of Lake Wainamu to stop the fish escaping.