Rocksnot spreads in Tasman region

The invasive algae known as "rocksnot" has been found in three more rivers in the Tasman region at the top of the South Island.

But though the number of infested rivers in the region has now jumped to 21, a Department of Conservation spokeswoman Trish Grant said there were still only two rivers known to be infested in the Marlborough and Kaikoura regions, and there had been no new infestations recorded there in the past year.

She said a survey of 40 waterways found didymo in the Riwaka and Pearse rivers and it was confirmed to be in the Maruia River from a sample collected by a member of the public.

A Biosecurity New Zealand spokesman said this brought the number of infested rivers in the South Island to at least 80 and possibly as many as 90 -- the uncertainty was because the database was being tidied up to remove duplications where the same river system was known by more than one name.

This was estimated to be about a third of the significant waterways in the South Island, though the incursion had reached some rivers without the algae "blooming".

The northern hemisphere algae was first found in the Lower Waiau River in 2004.

The reason for variations in its growth and spread were not known but research was under way into the roles played by temperature changes and the chemical composition of the water, including whether it contained phosphates.

The algae, scientifically known as Didymosphenia geminata, has tended to most heavily affect clear, fast-running rivers which get a lot of light and have a stable bed, but apparently it does not flourish in spring-fed waterways.

Lindsay Vaughan, who leads the Top of the South regional didymo management group, said most river users followed the "check, clean, dry" procedure to stop their gear spreading aquatic pests, but obviously some had failed to do that as they moved between waterways.

Nelson/Marlborough DOC manager Martin Heine said the department particularly wants trampers, anglers and other visitors to avoid spreading didymo into habitat for the endangered native whio/blue duck.

"Didymo is in several waterways that are important for whio, including the Fyfe and Baton rivers, Sandstone Creek, and now the Pearse River," he said. It was important to keep didymo out of the parts of these rivers, and other Kahurangi National Park waterways, on which whio live, including the Wangapeka River.

The long term effect of didymo on the insects -- caddis fly, mayfly and stone fly -- on which whio fed was not yet known, but the algae could form massive blooms on the bottom of the streams, and potentially reduce the food supply.