Oct. 28 (Bloomberg) -- New Zealand officials say two of the nation's biggest industries, tourism and fishing, are under attack from exotic algae and sea creatures.

Didymosphenia geminata, or didymo, has been found in at least six rivers in the past 12 months, prompting the closure of waterways and a campaign to halt the spread of the dense, slimy algae. A permit is now required to access some of the rivers in Fiordland National Park, the country's largest nature reserve.

``It's terrible stuff,'' said Mike Molineux, owner of Manapouri, South Island-based tour company Fish Fiordland and a fishing guide for 25 years. ``It makes the rivers un-fishable. With every cast it was on your line, or on your fly,'' he said in a telephone interview on Oct. 26.

Didymo is one of two exotic species found in New Zealand waters in the past year. Biosecurity New Zealand, a government agency in Wellington, is also trying to curtail the spread of a small oceanic creature called a sea squirt that may threaten New Zealand's NZ$200 million ($141 million) mussel farming industry. Scientists say it may not be possible to eradicate the pest that smothers shellfish.

The sea squirt, called clubbed tunicate, or Styela clava, has been found in harbors in New Zealand's North and South islands. It spawns as often as every 24 hours and is a prolific breeder, according to the provincial government of Canada's Prince Edward Island, which hasn't managed to arrest its spread in the seven years since it was found there.

Economic Effect

New Zealand's fish and shellfish exports were worth NZ$1.1 billion in the 12 months ended Aug. 31, making it the nation's sixth-largest export, according to government figures last month.

In Canada's Prince Edward Island, the clubbed tunicate adds 11 cents to 15 cents a pound to the cost of harvesting mussels, about 23 percent of the catch value, said Neil MacNair, an aquaculture biologist at the province's Agriculture, Fisheries and Aquaculture department. The island's mussel industry is valued at about C$60 million ($51 million).

``It continues to be a serious problem,'' MacNair said. ``It grows on everything. We haven't found the magic answer.''

Didymo, which comes from North America and northern Europe, can form ``a thick brown layer that smothers rocks, submerged plants and other materials,'' according to the World Conservation Union's invasive species Web site.

Biosecurity New Zealand is still assessing the economic impact of didymo, which is found in 24 countries, said Phil Barclay, a spokesman.

Didymo Spreads

The microscopic organism ``can be spread incredibly easily,'' Barclay said. ``No country has eradicated it.'' Didymo doesn't affect human health. Its major impact ``seems to be aesthetic.''

Its discovery in New Zealand marks the organism's first recorded appearance in the southern hemisphere. A single drop of water is enough to spread the algae, the agency said.

New Zealand is separated from its nearest neighbor Australia by more than 1,000 miles of sea. Its isolation has delayed the spread of pests endemic elsewhere, helping New Zealand establish a reputation as a pristine tourist destination.

``Clearly we trade on our clean, green image,'' said Darren Gibbs, senior economist at Deutsche Bank AG in Auckland. ``Any news of this kind is unhelpful.''

The threat to tourism comes as a rising New Zealand dollar is making the country a more expensive place to visit, he said. The currency has averaged 70.71 U.S. cents this year, compared with an average of 58.07 cents since it began trading freely in 1985.

Gibbs said New Zealand's NZ$7.5 billion international tourism industry faces bigger threats, such as bird flu.

Sea Squirt

The clubbed tunicate is a leathery filter feeder that grows as long as 6.3 inches, according to Biosecurity New Zealand. It anchors itself to wharves, docks, boat hulls, mooring lines, buoys and aquaculture structures and can form dense clumps. It originated in north Asian waters and probably hitched a ride on a ship.

In South Korea, it is considered a delicacy, and is used to flavor a broth of steamed angler fish. In Japan, it is eaten raw as sashimi.

In New Zealand, it hasn't yet reached mussel or oyster farms. Should it take hold, it may affect exports to the U.S. and Europe, said Lorna Holton, executive officer of the New Zealand Mussel Industry Council, whose 200 farmers produce 85,000 metric tons of the greenshell mussel annually.

``To have a new species here that we know is invasive, that has us concerned,'' Holton said in Wellington. ``It's likely that its natural predators aren't here.''

For Mike Molineux, the didymo invasion means spending one and a half hours cleaning his boat, then leaving it to dry for 48 hours before taking it to another river to help tourists fish for brown and rainbow trout.

``It's getting pretty difficult to find somewhere to take them'' to fish, he said. ``This is a big concern for me.''

To contact the reporter on this story: Jonathan Underhill in Wellington at junderhill@bloomberg.net.

Last Updated: October 28, 2005 00:13 EDT