KEY POINTS:
It's been called rock snot and worse, but Steve Smith's description of didymo is particularly colourful.
"It's like this scungy-looking dead sheep, [or] dirty toilet paper going down the river," he says.
With the trout-fishing season opening on Monday, Mr Smith, Fish & Game's Rotorua-based regional manager, is warning anglers to be on heightened alert for the invasive algae. One of Fish & Game's main messages this year is that lakes as well as rivers are at risk from the invasive algae.
It was previously thought that didymo could not survive in lakes, but that view has been disproved. It is now established in several South Island lakes, including Queenstown's Wakatipu, and Mr Smith said the risk of it reaching North Island waterways was ever-present.
"There's an amazing amount of movement backwards and forwards between the two islands, and it's not just for fishing. There's kayakers, multi-sporters, people just on holiday, and they all pose a risk."
So far, no didymo has been detected in the North Island, and Mr Smith said to stop its transfer, those fishing for trout needed to be vigilant about cleaning their gear.
"We're really asking anglers to be more careful than ever, and particularly those who use lakes who might previously have thought they did not need to worry too much."
Fish & Game plans to hand out spray bottles for cleaning at popular trout-fishing spots on the Rotorua lakes on Monday. The bottles have already been available at local fishing stores to encourage freshwater fishers to make cleaning a habit, in the same way as those who fish or dive in the sea clean their gear to protect it from salt.
Mr Smith said a solution of 5 per cent dishwashing liquid mixed with water killed didymo, but one item that remained a problem was felt-sole waders.
"Even with the best will in the world, they're a high-risk item," he said.
The waders, coveted by anglers for their non-slip qualities, posed a problem because they were porous and could stay wet for weeks, providing a perfect breeding ground for the algae.
Freezing waders overnight until they were solid was a method of killing didymo, and drying other items worked, but only if they were completely dried.
Anglers were encouraged to use alternatives to felt soles such as new rubber boots, which had improved adhesive properties.
Didymo first invaded the South Island three years ago, and Mr Smith says it shows no sign of disappearing.
"There's no positive news from the point-of-view it was a bloom-bust cycle."
Little was known about the algae when it arrived, but new forms of genetic analysis were making it easier to detect. "If there's a single cell organism, the DNA analysis will give us the opportunity to detect it when it's not visible in any other form," Mr Smith said. "That increase in knowledge has been phenomenal so every month didymo is not in the North Island provides increased capability to deal with it. With greater public support, there's got to be a real chance of at least delaying its North Island incursion for quite some time."
Menace from US
* Didymo was first detected in New Zealand three years ago and is believed to have come to this country from its native North America.
* Declared an unwanted organism by Biosecurity NZ, it is unsightly and can adversely affect freshwater fish, plants and invertebrate species.
* At its worst, didymo forms a mat up to 200mm thick on river beds and around lake edges, hardening like papier mache or egg cartons when it dries on contact with air.
* When rivers flood, it gets caught in trees, and unlike native algae, it is coarse, gritty, and difficult to pull apart.
