DULUTH, Minn. — Invasive, parasitic sea lampreys continue to hunt and kill Lake Superior fish above the levels biologists would like to see, especially after the COVID-19 pandemic caused parts of two summer seasons to be lost in the never-ending battle against the eel-like creatures.
Lake Superior now has an estimated 108,000 lampreys, a three-year rolling average, far above the goal of 48,000 lampreys set by the Great Lakes Fishery Commission.
Sea lampreys feed on the blood and body fluids of fish by attaching to them with a tooth-filled, suction-cup mouth. They file a hole through the fish's scales and skin with a razor-sharp tongue. They eventually fall off, but the trauma is sometimes too much for the fish to survive.
Many fish survive the attacks, however, and Lake Superior fish caught by anglers often have circular marks where lampreys were attached.
Sea lampreys are by far the most destructive of all aquatic invasive species to hit the Great Lakes. Native to the Atlantic Ocean and the St. Lawrence River, they spread into the Great Lakes via human-made canals that bypassed natural waterfall barriers.
Lampreys were first confirmed in Lake Erie in 1921, Lake Michigan in 1936, Lake Huron in 1937 and Lake Superior in 1938. They rapidly devastated fish populations, and within a few years, both commercial and sport fishing on Lake Superior came to a near standstill.
But scientists developed a poison that kills lamprey larvae and is otherwise mostly harmless to other creatures. The lampricide is spread in tributary streams along the Great Lakes where lamprey spawn, on a rotating schedule, and that effort has restored some Great Lakes fish populations to near historic levels. Lampreys are also caught in traps as they spawn in some areas, such as Wisconsin's Bois Brule River.
Control efforts chipped away at the total lamprey population, from 780,000 in Lake Superior before treatments began in 1958 to about 217,000 in 2017 and about 108,000 now.
But the battle never ends, and when 70% of the Great Lakes lampricide applications were canceled in 2020 due to the pandemic shutdown of field work for many natural resource agencies, and another 25% were canceled in 2021, those missed treatments allowed lamprey numbers to bounce back.
The full effects of the pandemic may not be seen for another year or two as the lampreys hatched in those years grow to adults, swim out of rivers and begin their attacks on Great Lakes fish.
"Before COVID, we were very, very aggressive with control, and (lamprey) numbers were down significantly across the board," said Marc Gaden, spokesman for the Great Lakes Fishery Commission, which is charged with waging the war on lampreys. "We were, so to speak, as well-positioned to weather the storm going into COVID as we could have been, though we didn't know it at the time."
Lampricide treatments have returned to pre-pandemic levels in 2022 and 2023, Gaden noted, but the lamprey population is expected to be elevated for at least a few years.
"Sea lampreys are slimy, noxious, crafty little buggers that take advantage of any relaxation in control," Gaden said. "We are hitting them back hard. But it's not clear how quickly we can recover. Hopefully, this uptick will be a blip."
While both lamprey estimates and the number of lamprey marks found on fish are above goals, the five-year average lake trout population estimate in Lake Superior remains high, a sign that the trout population has been able to withstand the lamprey onslaught.
From May into October this year, dozens of Lake Superior tributaries are scheduled to be treated with the lampricide in Minnesota, Wisconsin, Michigan and Ontario. Those include the Middle, Amnicon, Iron and Sand rivers along the South Shore and the Knife, Arrowhead Brule and Gooseberry rivers on the North Shore.
