DOC seeks to put toxin in waterways to kill trout

Wellington waterways may be poisoned in an attempt to eradicate introduced brown trout.

The Conservation Department has applied for resource consent to introduce the natural toxin rotenone to the upper Karori reservoir and the streams flowing into it this summer. Rotenone is found in the roots of tropical plants and has been used in Central and South America to kill fish that are then collected and eaten.

It posed no danger to humans and would be contained in Zealandia's upper lake and only released once water was tested and shown to be safe, the wildlife sanctuary said.

"This operation will provide the first opportunity for us to trial the use of rotenone in flowing water with the major advantage of being able to close the valve on the top dam, ensuring there is no effect on the downstream system," DOC spokesman David West said.

"Rotenone has been used in New Zealand to great effect for a number of years, but only in lakes and ponds. If this trial is successful, rotenone could be a major breakthrough in protecting and restoring native freshwater ecosystems, where there are threatened species of native fish."

Dr West said when used correctly it posed "little if any" risk to public health.

Zealandia's Raewyn Empson said the restoration of the stream would help bring it back to its pre-human state. "If trout can be removed from this area, we can provide a safe haven for some of our most endangered fish," she said.

To protect native banded kokopu and freshwater crayfish already present, as many as possible would be caught and held in cages until the operation was over.

With trout gone, native species were expected to recover quickly.

Brown trout, introduced to New Zealand in 1867 as a sports fish, were a predator and competitor, eating native fish and freshwater invertebrates.

- © Fairfax NZ News