BRATTLEBORO -- A dead lamprey eel that washed up on the banks of the West River caused some consternation last weekend. But a biologist said lampreys are not all that unusual in the Connecticut River and its tributaries.

"They are engaged in migrating up the Connecticut River and into tributary rivers and streams to spawn now," said Kenneth Cox, a fisheries biologist with Vermont Fish and Wildlife. "That people may see and report lamprey, alive and dead, in the West and Rock rivers at this time of year is not unusual."

Cox said sea lamprey are anadromous as are American shad and Atlantic salmon.

"The adults migrate from the marine environment into freshwater to spawn. Spawning typically occurs in stony, riffle sections of streams where the adult excavates a depression in the streambed into which the eggs are deposited."

Unlike shad and salmon after spawning, said Cox, all adult lamprey die, which may explain the weekend sighting of a dead lamprey.

Once the eggs hatch the juvenile fish -- ammocoetes -- move into areas of the stream where the substrate is mud or silt. They feed on plankton and live in freshwater four to six years before migrating out to sea to mature to adults.

After one to two years at sea where they feed on other fish, they return to freshwater to spawn and die," said Cox. "While in freshwater, anadromous sea lamprey do not feed. This is unlike the landlocked population that occurs in Lake Champlain which complete their entire life cycle in freshwater.

He said anadromous sea lamprey provide an important ecological service.

"First they recycle nutrients between marine and freshwater environments. When lamprey die in freshwater, nutrients obtained out of sea are returned to inland freshwater ecosystems nourishing aquatic insects, which in turn pass the nutrients up the food chain to fishes important to fisheries, such as trout, bass and other panfishes."

Cox also noted that lamprey are completely harmless to people.

"As for how far up the Connecticut River lamprey migrate, they are observed passing through Vernon and Bellows Falls fish ladders and juveniles have been observed as far up into the watershed as the White River," he said. "Amazingly, lamprey are quite adept at swimming above many low-head dams and falls."

A low-head dam is a man-made structure designed to recreate a pool or lake of water.

Lampreys were considered by the Vermont Agency of Natural Resources as a potential representative important species for the discharge permit for Vermont Yankee nuclear power plant, said Cox.

"But because they are quite tolerant of warm water and there were other warmwater fishes being represented in the suit, we decide there was no need to include lamprey," he said.

Bob Audette can be reached at raudette@reformer.com, or at 802-254-2311, ext. 160. Follow Bob on Twitter @audette.reformer.