Zebra mussel veligers -- the tiny, nymphal stage of the highly invasive, non-native mussel -- have been found in the Housatonic River near the Rocky River hydroelectric power plant in New Milford.

However, they have yet to be found in Candlewood Lake -- the lake that the Rocky River plant regulates by pumping in water from the Housatonic.

FirstLight Power Resources, which owns the lake, announced July 26 the veligers turned up as part of the zebra mussel monitoring program set up along the river.

"It proves our monitoring system is working,'' FirstLight spokesman Chuck Burnham said.

Mr. Burnham said the Rocky River plant has not pumped water into Candlewood Lake since the spring. It also stopped pumping into the lake in the spring and summer of 2011 to prevent the spread of the mussels.

"They deserve credit for being good public citizens,'' Howard Berger, chairman of the Candlewood Lake Authority, said of FirstLight.

Mr. Berger said the finding is strong evidence zebra mussel veligers are coming down the river from Laurel Lake in Massachusetts. The lake, which is badly infested with zebra mussels, has an outlet brook that feeds into the Housatonic.

"It's time for the Department of Energy and Environmental Protection and the attorney general's office to get involved,'' Mr. Berger said.

The small, striped-shelled mussels first turned up in the United States in the Great Lakes in 1988. They have now colonized waterways and lakes throughout the central and eastern United States.

They've traveled down the Mississippi River to the Gulf of Mexico and across the Erie Canal into the Hudson River.

Once established, the mussels can multiply by the millions. They can completely block the intake pipe of power plants, and cover docks and the bottoms of boats.

They also alter freshwater ecology, driving out native species and filtering out the phytoplankton that other species depend on. Where zebra mussels set up shop, the water gets unnaturally clear.

In 2010, researcher Ethan Nadeau found zebra mussels in Lake Lillinonah and Lake Zoar. Veligers have continued to be found in the two lakes, although there has yet to be an explosion of adult mussels in either lake.

Mr. Nadeau, working for FirstLight, has been conducting the utility's monitoring program. He found the veligers July 24 near the Rocky River plant, then confirmed they were there through enhanced analysis the next day.

Mr. Berger said the discovery should only enhance the diligence of people who use Candlewood Lake to carefully clean their boats after they use them in other bodies of water, to make sure they're not spreading veligers, as well.

He said the finding at the Rocky River plants -- where there are no boat launches -- shows the veligers are coming down the river from Laurel Lake.

Therefore, he said, DEEP Commissioner Dan Esty and state Attorney General George Jepsen, as well as U.S. Rep Chris Murphy, D-4, need to begin working with their counterparts in Massachusetts to remedy the situation.

"Otherwise, it's just the Candlewood Lake Authority and FirstLight doing the work,'' Mr. Berger concluded.

bmiller@newstimes.com;203-731-3345