Often referred to as vampires of the deep, sea lampreys, a parasitic eel-like fish, can be found in some of New Hampshire’s rivers. Like Atlantic salmon they are anadromous, living a portion of their lives in freshwater, migrating to salt water as adults and returning to freshwater to spawn.

As adults, sea lampreys prey on other ocean fish, using mouths filled with concentric rings of teeth and a razor-sharp rasping tongue to latch onto other fish and feed on their blood. They secrete an enzyme that prevents blood from clotting, similar to how leeches feed.

Often reviled, sea lampreys actually play an important ecological role in freshwater, according to the New Hampshire Wildlife Action Plan. Their nests improve spawning habitats for other fish and the larval stage, known as ammocoetes, are filter feeders that may improve water quality. Adult sea lampreys, which swim upstream into rivers to spawn, die after spawning and their carcasses release marine-derived nutrients into freshwater rivers.

When it’s time to spawn, the adult sea lampreys are drawn into coastal rivers, following pheromones from the larval ammocoetes. They construct nests in the gravel substrate riffle sections of freshwater. The ammocoetes live for up to five years as filter feeders before developing teeth and becoming adults, at which time they swim downstream to the ocean in search of host fish.

According to a species profile from the New Hampshire Fish and Game Department, the sea lamprey, native to the Atlantic Ocean, inhabits coastal rivers throughout eastern North America and western Europe as far south as the western Mediterranean Sea and the gulf coast of Florida. In New Hampshire, sea lampreys migrate into the Connecticut River, Merrimack River and other coastal rivers as far inland as the first impassable barrier.

Sea lamprey populations in the Granite State are generally declining or stable at low levels, primarily due to dams blocking access to spawning habitat, as well as the overfishing of marine host species.

Naturalist Kim Noyes, environmental education coordinator at the Northfield Recreation and Environmental Center and supervisor of the Turners Falls Fish Way in Massachusetts, said spawning is determined by water temperature, which is variable. It generally occurs in the last half of June. While the Connecticut River is their primary access to inland fresh water, most of the spawning takes place in tributaries like the Ashuelot River.

“The adults stop feeding for the spawn,” Noyes said. “They die after spawning and become an important food source for scavengers. They also provide nutrients to plants along the rivers. The ammocoetes become a food source for fish, crustaceans like crayfish and small mammals like raccoons.

Much of the sea lamprey’s bad “vampire” reputation comes from the Great Lakes, where it became an invasive species and devastated a thriving fishery in the mid-20th century. Originally confined to Lake Ontario, an early 20th-century shipping canal linking Lakes Ontario and Erie bypassed the natural barrier of Niagara Falls and allowed lampreys to invade all the Great Lakes.

Unlike marine fish, which co-evolved with the lamprey, their predation on freshwater fish is often fatal. The Great Lakes Fishery Commission estimates each lamprey is capable of killing up to 40 pounds of fish during its 12- to 18-month feeding cycle.

One of the main functions of the Great Lakes Fishery Commission is sea lamprey control in an ongoing effort to restore the fishery. Large fish caught in the Great Lakes often bear the scars of lamprey feeding.

People looking for an opportunity to learn more about sea lampreys have an opportunity to do so on Wednesday, June 21, from 6:30 to 8:30 p.m., at the Northfield Recreation and Environmental Center at 99 Miller Falls Road (Rt. 63) in Northfield, Mass., when Noyes will discuss the life history of this creature and its important contributions to river ecology and lead a field trip to a nearby tributary in search of sea lampreys and spawning activity. Call 800-859-2960 to register. Participants should wear appropriate footwear and dress for walking along and in a small river. The event is free.