North Island rivers being checked for weed growth

Checks have begun on North Island trout fisheries for signs of the invasive weed didymo.

Since didymo was discovered in October last year, Biosecurity New Zealand has done one survey, at 62 South Island sites around the original find, on the Waiau and Mararoa Rivers. None was found.

Since then, didymo, also known as rock snot, has been discovered at Buller, 500km north, and in the Clutha and Oreti and Hawea Rivers.

The "delimiting" survey now launched by Biosecurity will be done at 700 sites including 160 in the North Island in world-renowned angling rivers such as the Tongariro near Turangi.

Biosecurity didymo response manager Kerry Bodmin said the survey was not done earlier because the algae was thought to bloom at this time of year and would be more noticeable.

Otago University senior ecology lecturer Dr Gerry Closs said there was a strong possibility didymo would eventually reach North Island rivers.

The Waikato River near Lake Taupo and the lake itself were areas where it could bloom to nuisance levels.

"The best places for it always seem to coincide with the best fisheries," he said.

Didymo forms dense mats that look like wet paper or wet sheep's fleece on rocks and the beds of rivers. It is not a health risk but affects invertebrates that fish feed on and makes wading dangerous.

Meanwhile, Biosecurity faces pressure to can an international jet boat race.

Hundreds of enthusiasts are arriving in the country for this weekend's World Jet Boat Marathon but Environment Southland chairman Stuart Collie is demanding the race be stopped.

He accused Biosecurity of being unable to make an unpopular decision.

Two rivers infected with didymo, the Clutha and the Oreti, are part of the event. The race takes in eight rivers in Southland and Otago.

"To even still be contemplating hosting an event that introduces boats and equipment to almost every river in Southland and Otago ... seems to me to defy any sort of logic," Mr Collie said.

Otago Regional Council chairman Stephen Cairns said his council had had to step into a leadership "void" left by Biosecurity in response to didymo.

The council had organised its own staff to clean boats during the race.

Biosecurity spokesman Peter Thomson defended his agency's response, saying a final decision on on the race had not been made.