Vampires of the Great Lakes': Parasitic sea lampreys exceed targets in all Great Lakes

First the bad news: sea lampreys exceeded abundance targets in 2024 in all five Great Lakes.

Now the silver linings: the findings weren't a surprise, the reason is well understood and the coming years will likely see levels of the destructive, invasive fish at or below goals.

A couple of assumptions are built into that optimistic forecast, including adequate staff and funding provided to the U.S. Fish and Wildlife Service to continue its sea lamprey control work and another pandemic doesn't crop up.

The status of sea lampreys in the region was the focus of a recent report by the Great Lakes Fishery Commission.

It included this year's sea lamprey assessments in each Great Lake and an explanation of the undesirably high levels.

Sea lampreys have been in the region for more than 100 years. But from perspectives ranging from sport angling to commercial fishing to native species, they still deserve our attention.

The recent upward trend in the sea lamprey abundance is only the latest reminder: this ancient, non-native species is a survivor that requires constant management.

Sometimes called the "vampires of the Great Lakes," the sea lamprey is a parasitic fish that ranks among the most destructive aquatic invasive species to enter the waters of the Upper Midwest.

The sea lamprey has a disc-shaped mouth it uses to suction to the side of a fish. Its scientific name is Petromyzon marinus, Latin for "sucker of stone from the sea."

Once attached, its rows of teeth dig into the host's flesh and a sharp tongue rasps through scales and skin.

The parasite then feeds on the fish’s body fluids; to keep the juices flowing, the sea lamprey secretes an enzyme that prevents its host's blood from clotting.

A single lamprey will eat about 40 pounds of fish during its lifetime and grow to about 2 feet long, according to the Great Lakes Fishery Commission.

The sea lamprey is native to the Atlantic Ocean but swam into inland North American waters after canals were built to ease shipping. The upper Great Lakes were especially vulnerable to invasive species after the Welland Canal was built to bypass Niagara Falls.

Sea lampreys were first observed in Lake Erie in 1921, in Lake Michigan in 1936, in Lake Huron in 1937 and in Lake Superior in 1946, according to the fishery commission.

Through the mid-1900s, the sea lamprey was a driving force in the "collapse of the Great Lakes ecosystem and the economy it supported; tens of thousands of jobs were lost, property values were diminished, and a way of life was forever changed for millions of people," the fishery commission said in a history of the region.

Coupled with overfishing, the sea lamprey had a devastating impact on lake trout, a top native predator species in the Great Lakes, said Chuck Bronte, research fishery biologist and project manager with the U.S. Fish and Wildlife Service.

Within a decade after the sea lamprey arrived in Lake Michigan, commercial lake trout harvests began to plummet. In 1943 Lake Michigan commercial fishermen landed 6.9 million pounds of lake trout, according to fishery commission data. The lake trout catch then went into a free fall to 4 million in 1946, 1.2 million in 1948 and 54,000 in 1950.

In 1953 commercial fishing was closed for lake trout in Lake Michigan. Burbot, the other native top predator fish in the lake, also suffered drastic reductions in its population.

The dreadful conditions sparked positive change.

Officials from the U.S. and Canada met in 1954 and charted a path to rehabilitate the lakes. It included the establishment in 1955 of the Great Lakes Fishery Commission, a bi-national body charged with conducting sea lamprey control, formulating a coordinated research program and coordinating fisheries management in the basin.

Sea lamprey control is conducted by Fisheries and Oceans Canada and the U.S. Fish and Wildlife Service, with research support from the U.S. Geological Survey and other agencies and institutions.

The primary means of sea lamprey control is a chemical called 3-trifluoromethyl-4-nitrophenol (TFM for short) added to tributaries where sea lampreys spawn. The chemical blocks the respiration process of the sea lampreys. Some rivers also have sea lamprey traps or barriers.

Decades of TFM use in Great Lakes tributaries succeeded in knocking down the populations of sea lampreys. Lake trout and burbot populations have increased, including a 2024 pronouncement that lake trout in Lake Superior have fully recovered.

However, the parasitic fish is still present in each of the Great Lakes.

And testing this year showed it increased above target levels.

The reason? Reductions in TFM treatments due to work restrictions during the coronavirus pandemic in 2020 and 2021.

In its recent report, the fishery commission said sea lampreys "spiked in numbers when field crews were constrained in their ability to conduct sea lamprey control."

Sea lampreys typically spend a couple of years developing in tributaries before heading into the lakes as adults and feeding on lake trout and other fish.

Because of this life cycle, scientists are now seeing the ramifications of the reduced control work in 2020 and 2021.

Sea lamprey populations are monitored by generating an adult sea lamprey abundance index for each lake. The index is calculated by assessment crews who capture migrating adult sea lamprey in index streams with traps during spring and early summer. A mark-recapture study is conducted on each index stream to generate a population estimate. Individual index stream population estimates are used to create the lake-wide adult sea lamprey abundance index.

The fishery commission sets sea lamprey abundance targets to correspond with acceptable wounding rates on fish.

On Lake Michigan, federal officials have established an abundance target of 20,526 adult sea lampreys.

In 2024 the Lake Michigan index showed 25,000 adult sea lampreys, or 22% higher than the target, according to the Fish and Wildlife Service.

Stream-specific estimates showed Michigan's Manistique and Big Manistee rivers contributed most to the lake-wide adult abundance estimate with 45% and 22%, respectively.

In Wisconsin, the Fish and Wildlife Service noted a concern this year when they documented sea lampreys upstream of the barrier on the Kewaunee River. The physical structure is designed to prevent the fish from reaching areas of the river more conducive to spawning.

The 2024 sea lamprey abundance indexes in the other Great Lakes also exceeded the targets. They were: Lake Superior, abundance index of 56,000, target of 10,000; Lake Huron, abundance index of 40,000 and target of 31,000; Lake Ontario, abundance index of 22,000 and target of 14,000.

Lake Erie has the lowest sea lamprey population and much more variable data. Its abundance index from 2022-24 was 3,800, and its target is 3,300.

Great Lakes Fishery Commission officials were candid about the last few years and the challenges associated with sea lamprey control. Due to restricted TFM treatments, the commission said in a statement "millions of larval sea lampreys that would have otherwise been removed, survive(d) and parasitize(d) millions of pounds of valuable fish."

“We continually strive to reach and maintain a level of sea lamprey suppression, which allows a fishery that supports the millions of people that live, work, and recreate in the Great Lakes,” said commission Chairman Ethan Baker. “The COVID-19 pandemic provided an unintentional, but valuable, lesson."

To fully understand what happened, the commission announced plans to fund a multiagency study, led by the U.S. Geological Survey, to analyze the effect of reduced control efforts during 2020 and 2021 on Great Lakes sea lamprey populations.

They are confident, however, that treatments in 2022, 2023 and 2024 and beyond will reduce the number of adult sea lampreys seen in the Great Lakes in the coming years.

"The sustained increase in sea lamprey abundances following a lapse in annual control effort highlights the continued need for ongoing sea lamprey control and continued research into new and innovative control methods in the Great Lakes," said Jim McKane, the commission’s vice-chair, in a statement. "After more than six decades of successful sea lamprey control, the reduced effort during the COVID-19 pandemic shows that if controls are ceased or relaxed for even a short period of time, sea lamprey populations will rebound, and the fishery will suffer.”