Sea lamprey have been called the least-understood fish of the Connecticut River.
Their very appearance can be startling: long, skinny, and eel-like, with a round mouth and no obvious gills. Their reputation from elsewhere gives them a bad rap in the Connecticut. In the upper Great Lakes, they’re a non-native invasive species that lives off native trout and other fish.
But in the Connecticut River and its tributaries, they’ve been coming home to spawn each spring since the glaciers melted some 14,000 years ago. And fish biologists have nothing but praise for the many good things this native species does for the ecosystem.
The difference between the Great Lakes and the Connecticut River populations of sea lamprey is that in the Great Lakes, they have become landlocked, helped upstream by nineteenth-century canals. Now they live their adult lives in the lakes as parasites on prized game fish such as lake trout.
Sea lamprey in the Connecticut River are anadromous, which means they live as adults in the ocean and return to fresh water to spawn, like Atlantic salmon. When in the river, lamprey aren’t parasitic, so anglers needn’t worry about the trout.
An ocean-dwelling adult uses its round, rasping mouth – filled with concentric circles of teeth – to scrape a hole in the side of a host fish like a mackerel or shad. They feed on blood and body fluids before letting go. In true parasite fashion, they weaken, but don’t kill, their hosts.
The great wave of returning sea lamprey that rolls up the Connecticut River every May and June benefits the river in two important ways. First, sea lamprey are one-time spawners that die after mating and egg-laying. Their 3-pound carcasses are a boon to scavengers as small as stonefly larvae and as large as otters and snapping turtles. Their dying bodies provide a large annual influx of nutrients from the sea to the river.
The second great benefit is their amazing ability to manipulate the stream bed. Sea lamprey adults, which can be up to 30 inches long, always spawn in swift water. Adults use their sucking mouths to move rocks as large as 9 inches in diameter, creating huge mounds that arch across the river bottom, from which they sweep away fine sediments by fanning their tails. The fertilized eggs are deposited in the hollows and carried by the current into the rock mounds, where they are safely incubated in the cool, oxygen-rich waters.
When adult sea lamprey remodel to create their nurseries, they loosen the riverbed, making great habitat for other things. Insects like mayflies and caddisflies use the spaces between rocks to hide, and fall-spawning salmon and brook trout find prime habitat in the clean-swept, rocky beds.
Sea lamprey young spend 3 or 4 years as worm-like creatures burrowed into the soft muds of the Connecticut, with just their mouths sticking up. They feed by filtering organic debris out of flowing waters. They have no natural defense other than being secretive, and they’re bite-sized for just about every predatory fish in the river. As they outgrow their burrows, they wiggle out and float downstream to start a new burrow. To avoid a legion of predators, they always make this short migration at night and often during a flood.
How fast a larval sea lamprey grows depends on the temperature of the water and the availability of food. When they reach 5 to 6 inches long, the young adults follow a biological urge to find the sea and rarely feed until they reach salt water.
Dams constructed in the eighteenth century to power the great mills of New England drastically depleted the stocks of all anadromous fish by blocking access to their spawning grounds. In the twentieth century, many dams were retrofitted with fish-passage structures. Sea lamprey are the biggest success story of these restoration efforts because of their unique ability to move upstream over barriers if the water doesn’t flow too swiftly.
Sea lamprey are great sprinters, swimming vigorously for short distances, but they need to rest between bursts. The same sucking mouth that can pierce a host fish or rearrange the river bed can also latch on to the face of a waterfall or dam. Their ability to suck their way up barriers has enabled sea lamprey to regain much of their historic spawning territory in the Connecticut River, even as high as Bellows Falls, Vermont. Nowadays, 80,000 to 100,000 returning adults annually pass over the dam at Holyoke, Massachusetts, constituting the largest population of sea lamprey on the Atlantic coast.
Petromyzon marinus, the Latin name for sea lamprey, means “seagoing stone sucker.” Let’s hope the success of tenacious sea lamprey in the twentieth century is a precursor for their migrating relatives like salmon and shad in the twenty-first century.