Biosecurity New Zealand (BNZ) is investigating a confirmed find of the invasive alga Didymosphenia geminata (Didymo - or 'Rock snot') in the West Coast's Buller River and is asking all river users to avoid the Buller if they intend using other rivers.

It is likely that a Controlled Area will be declared on the Buller in the next few days, prohibiting use of the Buller River if users are intending to use other rivers. BNZ is working with local councils, DOC and Fish and Game to identify and contact river users. BNZ is aware that the fishing season starts on Saturday and that the whitebaiting season has already started.

Didymo was first identified in Southland's Mararoa and lower Waiau Rivers in October last year - the first time it had been found in the Southern Hemisphere - and extensive public awareness was carried out to try and contain the alga. A Controlled Area was declared on the Southland rivers on 19 August 2005. Didymo forms a 'wool-like' blanket and snuffs out life in rivers. Bad news for trout fishermen and whitebaiters.

BNZ Didymo response manager Kerry Bodmin says the latest Didymo find confirms the importance of all river users cleaning their equipment after use regardless of what rivers they are using - a message promoted for some time by a variety of agencies to limit the spread of several aquatic weeds

"When Didymo was first discovered scientists expected that it would be found elsewhere. This is the first find outside of Southland, but BNZ acknowledges the possibility Didymo might be in other South Island rivers and will be working to gather further information. We are grateful that several councils have already offered support," says Kerry Bodmin.

Didymo is microscopic and can be spread in a single drop of water. In bloom, it attaches itself to the streambed by stalks to form streamers that can turn white at the ends. Larger blooms form a thick white or brown layer that smothers rocks, submerged plants or other materials.

The algae looks slimy, but feels like wet cotton wool. When washed up on stream banks it looks similar to clumps of wet tissue paper.

Didymo has been declared an unwanted organism under the Biosecurity Act 1993. That means that people knowingly spreading it face a fine of up to $100,000 or up to five years' in prison. Among other things, Didymo can affect stream habitat and sources of food for fish and make recreational activities unpleasant. It is not a human health risk, though swimmers might notice itchy eyes or irritated skin from the silica in the algal cells.

Further information on Didymo and cleaning methods is available online at www.biosecurity.govt.nz/didymo.