If you are wading or paddling the Connecticut River or one of its tributaries this month and spot a long, eel-like fish building a nest of stones, or come across one dead on the riverbed, the official advice is simple. Leave it be. That fish is a native sea lamprey, it is doing exactly what it is supposed to do, and biologists consider its presence a sign that the river is getting healthier.
State officials issued the reminder as spring spawning season peaks along the river that forms the border between New Hampshire and Vermont. As the Vermont Fish and Wildlife Department announced, anglers and the public are being asked to avoid disturbing spawning sea lamprey now present in the Connecticut River and several of its feeder streams. The message matters because sea lamprey carry a reputation problem, and the agency wants Granite Staters and Vermonters alike to understand the difference between a pest and a protected native species.
A Conservation Success, Not a Menace
Sea lamprey are easy to misjudge. They are jawless, snake-shaped, and as adults at sea they latch onto other fish and feed as parasites. That biology has earned the species a fearsome image, and in some waters it deserves it. But in the Connecticut River basin, the story is the opposite.
“Sea lamprey are native to the Connecticut River basin and play a vital role in the ecosystem,” said Lael Will, a fisheries biologist with Vermont Fish and Wildlife. The species is listed as a Species of Greatest Conservation Need in both Vermont and New Hampshire, a designation that places it among the wildlife the two states are actively working to protect.
Will also addressed the confusion head on. Vermont is home to a separate population of sea lamprey in Lake Champlain, where the fish are treated as an invasive nuisance and actively controlled. The same animal, two completely different management goals. In Lake Champlain the lamprey is a problem to be suppressed. In the Connecticut River it is a native to be safeguarded. The agency went out of its way to highlight that contrast so the public does not assume every lamprey is a target.
“If you happen to see a spawning sea lamprey or a lamprey carcass, don’t be alarmed,” Will said. “The fish provide a number of important ecological benefits.”
The Remarkable Life Cycle Behind the Warning
Understanding why officials protect these fish requires understanding how they live, because almost every stage of the sea lamprey’s life delivers something back to the river.
Each spring, adult sea lamprey return from the ocean to spawn in fresh water. They travel up the main stem of the Connecticut River as far as Wilder Dam and fan out into tributaries including the West, Williams, Black, and White rivers. Spawning runs strongest when water temperatures climb into roughly the 50 to 59 degree Fahrenheit range, which is why May and June are the peak window.
Here is the detail that changes everything about how to view them. Once they return to fresh water to spawn, adult sea lamprey stop feeding entirely. They are no longer parasitic. They spawn, and then they die shortly afterward. Those carcasses are not litter. They are fertilizer. The decaying adults release marine nutrients they carried inland from the ocean, cycling nitrogen and other nutrients into freshwater ecosystems that need them. A dead lamprey on the riverbed is feeding the next generation of river life.
The young continue the service. Larval lamprey spend their first years burrowed in sandy river bottoms, staying put and filtering detritus from the water for food, which helps clean the system. At around five years old they transform into juveniles and head out to the ocean, where they live as parasites attached to larger fish. Even then they remain part of the food web, serving as prey for fish, marine mammals, and birds in estuarine and marine environments.
This is an ancient arrangement. Sea lamprey have existed for more than 350 million years, and Atlantic populations have co-evolved with their oceanic hosts over that span. Where they are native, biologists consider their populations to be in balance, a natural part of the system rather than a threat to it.
Why the Numbers Are Climbing
The request to leave the fish alone comes at a moment of real progress, and the data shows it. In 2025, more than 17,000 sea lamprey passed the Holyoke Dam in Massachusetts, and more than 4,000 passed the Vernon Dam further upstream. Those counts reflect years of deliberate work to reopen the river to migrating fish.
The species is managed under the Connecticut River Migratory Fish Restoration Cooperative, a partnership that brings together four state agencies, two federal agencies, and members of the public. Among its efforts, Vermont Fish and Wildlife has been improving fish passage facilities throughout the Connecticut River drainage so native sea lamprey can complete their journeys to spawning habitat.
“Our goal is to continue to improve fish passage and flows on the river to not only help lamprey spawning, but to also support all migrating fish species,” Will said. That last point is the strategic core of the work. Infrastructure that helps lamprey move upstream helps shad, salmon, and other migratory fish too. A river engineered to let one native species complete its life cycle tends to serve the whole community of fish that depend on the same corridor.
What It Means for New Hampshire Residents
For people who live, fish, and recreate along the Connecticut River, the practical guidance is straightforward. Spawning lamprey and their carcasses are a healthy sign. Do not remove them, do not disturb the gravel nests they build, and do not assume the fish is invasive. If anything, a strong spawning run is a marker that decades of river restoration are paying off.
The reminder also fits a broader pattern of conservation attention across the Granite State this season. New Hampshire wildlife managers have been wrestling with how to protect iconic species under pressure, from the moose population battling a winter tick crisis to the volunteers fanning out to shield nesting turtles during their vulnerable season. Healthy rivers and forests are connected systems, and the same long-term thinking behind forest and watershed management drives the patient, multi-agency work on the Connecticut River.
A creature that looks alien and carries a bad reputation turns out to be one of the river’s oldest and most useful residents. The ask this June is small. Watch if you like, learn what you are seeing, and let the lamprey finish the work they have been doing for 350 million years.
Frequently Asked Questions
For related coverage, see our reporting on New Hampshire Fire Deaths Drop To 18 In 2025.
Are sea lamprey dangerous to people?
No. While adult sea lamprey feed as parasites on other fish in the ocean, they pose no threat to humans. During their spawning run in the Connecticut River they stop feeding entirely, so there is no risk to swimmers, anglers, or paddlers.
Why are sea lamprey protected in the Connecticut River but controlled in Lake Champlain?
Sea lamprey are native to the Connecticut River basin, where they play a beneficial ecological role and are a Species of Greatest Conservation Need. In Lake Champlain they behave as an invasive nuisance and are actively controlled. The same species can be a conservation priority in one watershed and a problem in another.
What should I do if I see a dead lamprey in the river?
Leave it alone. After spawning, adult sea lamprey die naturally, and their carcasses release ocean nutrients that nourish the freshwater ecosystem. A lamprey carcass is part of the river’s nutrient cycle, not pollution.
When do sea lamprey spawn in New Hampshire and Vermont?
Spawning occurs in spring, peaking in May and June when stream temperatures reach roughly 50 to 59 degrees Fahrenheit. The fish spawn in the main stem of the Connecticut River up to Wilder Dam and in tributaries including the West, Williams, Black, and White rivers.
Are sea lamprey numbers improving?
Yes. In 2025 more than 17,000 sea lamprey passed the Holyoke Dam in Massachusetts and more than 4,000 passed the Vernon Dam, reflecting years of work to improve fish passage along the Connecticut River through a multi-agency restoration cooperative.
