Spread of invasive algae threatens southern trout, salmon rivers
Written by Stuff - New Zealand   
Monday, 20 December 2004 19:25
An invasive aquatic weed known in North America as "rock snot" has probably been in two southern river systems for up to two years, scientists say.

The Didymosphenia geminata (didymo) algae in the Lower Waiau and Mararoa rivers south of Te Anau has caused increasing concern overseas for its ability to spread rapidly, choking waterways and killing invertebrate life essential for fish survival.

"This algae looks like it would be slime but actually feels like wet cotton wool and is known in the United States and Canada as rock snot," Biosecurity New Zealand senior adviser Amelia Pascoe said.

"It can form flowing rat tails and when dry it can turn white and look like tissue paper."

Just one or two cells, invisible to the naked eye, can be enough to spread the algae to a new river system. It is not known how the algae reached New Zealand but could have come on anglers' gear, boats or even earthmoving equipment brought from North America.

In New Zealand, anglers or other river users who knowingly spread didymo will be liable, under the Biosecurity Act 1993, for a fine of up to $100,000 dollars and/or imprisonment of 5 years.

Ms Pascoe said the first concern was to try to contain the algae while scientists worked on issues such as the best ways to clean equipment - such as rods, reels, lines, kayaks, waders, and boats - which had been in the rivers. They were also seeking other ways the incursion could be tackled, but little research had been done overseas, and there was no suggestion of a suitable biological control.

She said there were legal issues in terms of proving that someone deliberately spread the algae. But if people were told that they could not be sure of not spreading it unless their equipment was thoroughly cleaned, then it could be argued that any failure to clean equipment left them liable.

The two rivers would be signposted, "information officers" would patrol the banks and ask people to avoid going in the water if they could, and sports outlets would be given fact sheets asking people to stay out of their water, and to clean up thoroughly afterwards if it could not be avoided. Some of the information officers would be designated as "authorised persons" able to enforce the Biosecurity Act.

But the restrictions would do nothing to stop the algae being spread by waterbirds to nearby river systems.

A technical advisory group (TAG ) convened by Biosecurity New Zealand, which met in Te Anau recently, said the algae had not so far been eradicated anywhere else in the world where it had invaded river systems.

"We still know very little about didymo, so our strategy is focused on attempting containment and researching how far it has spread and the treatment options river users can use to kill the algae to ensure they don't spread it to other waterways," Ms Pascoe said.

"This algae has not been eradicated anywhere else in the world so control and containment are likely to be our only options."

Biosecurity NZ and the Southland regional council had launched a campaign to inform the public about the risk of spreading didymo and how they could reduce the potential for its spread to other river systems.

"We have put up signage on Southland and Otago waterways to inform people they should avoid using the Lower Waiau and the Mararoa and providing them with information of how to clean any gear that does come into contact with these rivers," Ms Pascoe said.

The "information officers" would be advising people on how to clean their gear and ensuring people are aware of the problems the algae can cause.

Overseas, the didymo was found in cooler northern hemisphere river systems and had degraded some fishing rivers.

There was some anecdotal evidence that it hurt the ecology of the rivers by limiting the range of food for some species and it also clogged up water intakes.

Biosecurity NZ has declared Didymosphenia geminata to be an unwanted organism under the provisions of the Biosecurity Act 1993.

But the Ministry of Health only considered it to be a nuisance species and not a significant human health risk.

"Swimmers may experience scratchy, red, watery eyes after swimming," Ms Pascoe said.

Water taste and odour might be affected but overall water quality did not appear to be degraded.

"If people do use the river they should ensure that any boats, fishing equipment and other possessions that come into contact with the river and the Didymosphenia geminata algae should be thoroughly cleaned," she said. A bleach solution or nappy cleaning products could be used to kill the algae.

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