An invasive algae considered a serious threat to Vermont's trout fishery has been spotted in the state's landmark fly-fishing trout river, the Batten Kill.

State officials said Monday that a small amount of the invasive algae Didymo, nicknamed rock snot, has been discovered in the upper Batten Kill, which winds south from Dorset through Manchester and Arlington and then heads west into New York. It also recently has been found in the upper Connecticut and White rivers.

Biologists with the Vermont Agency of Natural Resources discovered the Didymo algae late last week, according to agency officials.

This is the first time that the algae, which clogs the river bottom and smothers aquatic life, has been found in the Batten Kill, long considered one of Vermont's prime fisheries for native brown trout, as well as a popular river for paddlers and tubers.

Brad Wright, the information specialist for the agency, said Monday state officials are very concerned about the discovery, especially since it comes one year after their counterparts in New York found a two-mile-long bloom of the algae on the New York side of the river.

"We have reasons to fear the unknown right now," said Wright, who added that biologists from the agency will look for further evidence of rock snot in the Batten Kill in the coming weeks. "We need to do more research because we don't know too much about it."

The particular sample discovered by agency biologists was in West Arlington, near where the river turns toward New York, officials said.

The 50-mile-long Batten Kill is known across the country as a popular location for fly-fishing, drawing anglers from around New England, though the trout fishery has faced challenges in recent years. Rep. Cynthia Browning, D-Arlington, who is also the executive director of the Batten Kill Watershed Alliance, said Monday the discovery is "very bad news" for people who use and appreciate the river.

The Watershed Alliance is having its annual meeting Tuesday night, she added, and rock snot is sure to be a vital last-minute addition to the agenda.

"I think we now have to figure out what this means for us," she said. "I know there are different strains of this, so it's not clear that it would spread as it has in other places. But it poses a lot of questions for us."

The Batten Kill discovery last week follows similar discoveries of the algae in the Connecticut and White rivers earlier this year. State officials have posted signs in the area where the algae was discovered last month, including in the towns of Royalton, Bethel and Stockbridge.

Rich Langdon, a biologist with the Natural Resources Agency, said the algae was first seen in Europe and northern Canada, but that it has been discovered further south in recent years.

While it does not pose a risk to humans, the algae does form large mats across the bottom of streams and rivers, which affects the aquatic habitats and food sources for the animals in the water, he said.

There is also an aesthetic concern as the so-called rock snot Ñ which resembles a green and brown sludge, but feels like thick cotton to the touch Ñ makes bodies of water unpleasing in appearance for swimming and other activities, he said.

"It may take awhile to fully assess the situation in Batten Kill," Langdon said.

Until then, Langdon encourages people using the waterways to carefully check their equipment for signs of the algae. The bottoms of boats should be scrubbed and dried, and fishing gear should be properly disinfected in hot and soapy water for 30 minutes.

Wright said he hopes to soon gather environmental officials from New Hampshire and New York, states that are tackling similar problems with that algae, to develop a regional plan.

He added that Vermont would post signs along the Batten Kill on Tuesday warning people of the discovery.

For more information on the algae, including pictures of what it looks like, visit www.vtwaterquality.org.

Contact Daniel Barlow at Daniel.Barlow@rutlandherald.com.