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Didymo is steadily creeping through South Island rivers with its deadly spread being helped by unwitting anglers - the very people who hate it most.
The alga, also known as rocksnot because of its slimy appearance, was discovered in October 2004 in Southland rivers.
Biosecurity New Zealand spokesman Phil Barclay said 26 South Island rivers were now known to be infected.
"We've spent a couple of million on the public awareness campaign and we're getting into millions on the research side of things."
Biosecurity NZ has no idea how didymo was introduced to New Zealand, but it was probably introduced on fishing gear or kayaks coming into New Zealand from overseas.
"We're dealing with a fairly significant curveball thrown to us by Mother Nature," Mr Barclay said. "This is a reasonably fragile alga that somehow survived a trip halfway around the world and then somehow got in the river. We're never going to know how it got here."
Didymo is an ugly, slimy alga that covers rocks and tree stumps in river systems. It smothers invertebrates, which are the main food source of fish and in some cases threatens their population. Anglers hate the stuff.
Central South Island Fish and Game officer Graeme Hughes said he could not think of a worse thing to affect the rivers.
"I've seen grown men almost in tears on the Waitaki. It's such a disgusting mess ... Anglers who have visited just cannot believe the change - it's not even nice to look at any more. Half the fun of fishing is catching fish, of course. The other part is being in nice surroundings."
Mr Hughes said it was not just the sight of the didymo that disgusted anglers, but the hassle of hooking it when they cast out.
"You pull these things up like sea-monsters. Anglers do believe that it is the end of the Waitaki for them."
He said fish in the river were still in excellent condition because the food was still there - but the supply was going downhill.
Otago Fish and Game officer Niall Watson said anglers might well turn to lake fishing as didymo in lakes only developed in small areas near the shore.
He said fast-flowing or flooded rivers cleaned out didymo for a period and anglers could fish then with relative ease.
"I'm hopeful that not all rivers with didymo are similarly affected at one time, but I'd be surprised if it didn't have an impact on anglers' choice of where to go."
South Island fisherman Peter Carty said he could no longer fish in the Buller, his local haunt.
"You hook a fish and you end up with a kilo of didymo on the line. It's just not very pleasant."
He put the blame on fishers for the part they played in the spread of the alga.
"It's getting spread because people aren't getting the message, are they?"
Mr Carty said anglers had been asked to clean their gear after fishing in any waterway in the South Island. "Some of them say they know about it, but they don't know what to do - it's quite frustrating."
Fish and Game officer Jay Graybill agreed the message to clean fishing gear was not getting through to some anglers.
"For the opening of the high country season here, some of our rangers were out and only about two-thirds of the anglers that were spoken to were taking active steps to clean their gear.
"We're still trying to get the message through."
Mr Graybill said if fishers were using felt-soled waders they needed to be soaked in a detergent or mild bleach solution because the didymo cells could stay alive in the felt for days.
Biosecurity NZ's Mr Barclay said staff would visit the rivers to push the cleaning message.
"There will always be people who think it's not an issue for them: this is an issue that affects all New Zealand rivers."
He said once didymo was introduced to a river system, it was almost impossible to get rid of: "That would require removing every microscopic cell of the stuff."
He stopped short of saying it was inevitable that didymo would reach North Island rivers and said there was lots of information about cleaning measures at ferry terminals.
But he warned: "It's really getting that behaviour change and getting people out of the habit of just casually using the environment."
Alga detected in two more rivers in South Island
Didymo has been found in two more rivers in the north of the South Island, Biosecurity New Zealand announced this week.
Positive finds of the invasive alga, commonly known as rocksnot, were made in the Pelorus River at a site about halfway between the Pelorus Bridge and Canvastown, with more in the Speargrass River further inland.
The Pelorus find was made during a survey being undertaken by the Department of Conservation as a result of the discovery of didymo in the Takaka River in December and was microscopic.
The Speargrass find was confirmed after a report by a member of the public and was in the early stages of bloom.
Biosecurity New Zealand and DoC are encouraging all river users to check, clean and dry all items that come in contact with the water between different waterways to help prevent the spread of the alga.
A nationwide survey of high-use recreational rivers at about 140 sites will be carried out in late January. The Northern Hemisphere pest has been detected in 30 South Island rivers since its discovery in Southland in 2004, but has not been found in the North Island.
- NZPA
