'Too late' to stop river algae

It is almost certainly too late to eradicate the invasive algae that are threatening the South Island's world-renowned trout fisheries, says the project's leading scientist.

Cathy Kiljoy, a freshwater biologist with the National Institute of Water and Atmospheric Research, has been studying didymo since it was discovered in the Mararoa and Waiau Rivers in Southland late last year.

But the discovery of the weed, commonly known as rock snot, in the Buller River near Westport and Otago's Hawea River meant it was now widespread.

"While it was contained in one catchment there was a chance we could have done something," she said. "Once it spread into two or more, the chances of being able to eradicate it become much less."

Didymo is an algal species that forms dense clumps that look like mats of wet tissue paper.

It suffocates fish life, clinging to rocks and the riverbed, fouls fishing gear and can be spread in a single drop of water.

As the trout-fishing season opens today, anglers and other river users are facing strict controls on all four rivers. Anyone using them is asked to clean gear thoroughly and not to use other rivers.

Biosecurity NZ yesterday rejected claims it had not done enough to control didymo (pronounced did-ee-mo).

Senior adviser Amelia Pascoe said the agency believed didymo had actually been blooming in the Lower Waiau and Mararoa Rivers, south of Te Anau, for about four years.

"Because we are starting to find it in other rivers doesn't mean it has only spread in the last few months."

Massey University environmental science lecturer Mike Joy said the response so far from Biosecurity New Zealand and the Government was "pathetic". He knew of about 20 scientists who shared his views.

Freshwater Sciences Society president Neil Deans said: "Disaster is not putting it too strongly. The question now is, can we keep it out of the North Island?"

How to help
To stop the spread of didymo, equipment should be cleaned in a solution containing bleach (2 per cent), salt (5 per cent) or detergent (5 per cent) and left to dry for 48 hours.