Weed has trout rivers in its grip

Two popular trout rivers have been effectively closed and restrictions will be put on a third in the next few days as an invasive foreign weed threatens some of New Zealand's most beautiful waterways.

As the trout fishing season kicks off this weekend, around 200km of Southland's lower Waiau and Mararoa Rivers have been declared biosecurity zones, effectively barring anglers and whitebaiters, while restrictions will be put on the Buller River soon.

The invasive aquatic plant didymo (pronounced did-ee-mo) has been found in all three rivers. A microscopic organism that blooms into thick, unsightly clumps, didymo suffocates riverbeds, threatens trout and other fish and makes wading dangerous.

It is suspected of fouling drinking water in municipal supplies in the United States.

Biosecurity New Zealand response manager Kerry Bodmin said scientists were working to come up with a solution but eradication had never been attempted anywhere in the world.

In the meantime, river users are being asked to go elsewhere.

"We would rather visitors stayed away but if you live in that area and use only that river, don't use any other river," she said.

"It's very serious. Didymo likes clean, clear flowing rivers so our best rivers are at risk."

Didymo can be spread in microscopic form in a single drop of water.

It is not a human health risk but swimmers can get itchy eyes or irritated skin.

"It makes rivers look like they are polluted; it looks like toilet paper. It's not good at all," said Fish and Game Southland spokesman Maurice Rodway.

If the weed found its way into a popular trout-fishing river such as the Waikato or its tributaries, the result would be "disastrous", Mr Rodway said.

The weed grows up to 100mm thick and looked like a blanket on the riverbed or like big clumps of "sheep's fleece", he said.

It could have been in the Mararoa-Waiau catchments, near Te Anau, for up to five years.

Trout fishers, whitebaiters, kayakers or anyone using rivers risk a fine of up to $100,000 or five years' jail if they knowingly spread it.

Any equipment potentially infected should be scrubbed and soaked for at least a minute in 60C hot water using a 2 per cent bleach solution or with a 5 per cent solution of nappy cleaner, salt, antiseptic hand cleaner or washing detergent and left to dry for 48 hours.