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Sea lampreys harm Great Lakes fish by attaching to them and feeding on their blood and body fluids. Their spread through the upper lakes was followed by a dramatic collapse in lake trout harvest, and they remain a management problem today. A binational control program has substantially reduced their numbers, but it has not eliminated them; in the 2024 report, abundance indices exceeded lake-specific targets in four of five lakes.
How sea lampreys harm Great Lakes fish
Sea lampreys are parasitic fish. They attach to a host with a suction-cup mouth and feed on its blood and body fluids. Some attacked fish die; others survive with wounds. During the period of highest abundance, up to 85% of fish not killed by lampreys bore attack wounds, according to the Great Lakes Fishery Commission (year not stated on its page). The Commission identifies lake trout, lake whitefish and ciscoes among the fish heavily affected. Great Lakes Fishery Commission: Sea Lamprey
How the invasion affected the fishery
The Commission says sea lampreys entered Lake Ontario in the mid-1800s and began reaching the upper Great Lakes in 1921. Before the invasion, Canada and the United States harvested about 15 million pounds of lake trout each year in the upper Great Lakes. By the early 1960s, that catch had fallen to approximately 300,000 pounds, about 2% of the earlier average. These are historical figures, not current harvest totals. The documented damage and timing show the scale of the crisis, but the figures alone do not establish lampreys as the only cause of every fishery or ecosystem change. Great Lakes Fishery Commission: Sea Lamprey
Are sea lampreys still a problem?
Yes. The Great Lakes Fishery Commission says control has reduced sea lamprey populations by 90% in most areas of the lakes, but the fish remain present and the program continues. The Commission’s latest annual report used here covers 2024 operations and was published in 2025. Its adult abundance indices exceeded lake-specific targets in four lakes; Lake Erie was below its target. An index is a monitoring measure, not a census of every adult lamprey.
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| Lake | 2024 adult abundance index | Lake-specific target | Relation to target |
|---|---|---|---|
| Superior | 55,551 | 10,421 | Above |
| Michigan | 24,799 | 20,526 | Above |
| Huron | 39,974 | 31,274 | Above |
| Erie | 870 | 3,263 | Below |
| Ontario | 22,374 | 14,065 | Above |
Source: Sea Lamprey Control in the Great Lakes 2024. Indices and targets are reported by the Commission and partner agencies; they should not be read as literal population counts.
How the Great Lakes control program works
Control targets vulnerable stages and locations rather than relying on a single method. Crews assess tributaries for larval lampreys, then use those findings to guide treatment and other actions. In 2024, agencies surveyed 421 tributaries and 69 lentic areas, and treated 84 tributaries and 22 lentic areas. The report also records 73 operated barriers: 48 purpose-built and 25 modified structures. 2024 annual report
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Lampricides target larvae
The primary control method is treating larval nursery areas with lampricides. TFM targets larvae before they develop the mouth used to feed on fish. The Commission says that at concentrations used, most other organisms are unaffected; this is a qualified description, not a claim that treatments have no ecological effects. Treatment planning considers larval assessments and conditions such as stream discharge, temperature and pH. Great Lakes Fishery Commission: Lampricide Treatments
Barriers and traps intercept adults
Barriers block adult lampreys from reaching spawning habitat, while traps capture adults, often near barriers. Trapping alone generally does not catch enough adults to prevent reproduction or remove the need for treatment. The Commission reports 2024 barrier operations in its annual report. Great Lakes Fishery Commission: Barriers and Trapping
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Supplemental methods address local challenges
Where standard methods are less effective or constrained, managers may use portable traps, seasonal electric barriers or sterile-male releases. The Commission describes local results: sterile-male releases begun in 2017 in a landlocked upper Cheboygan River population were followed by no new larvae detected in the mainstream of the Maple and Sturgeon rivers; in the Pigeon River, larval abundance and distribution were roughly 90% below historical averages. In the Black Mallard River, traps plus a seasonal portable electric barrier have removed or blocked 99% of adult lampreys from upper reaches since 2017. These are site-specific outcomes, not basin-wide effectiveness figures. Great Lakes Fishery Commission: Supplemental Controls
Why control has to continue
Reducing abundance is not the same as eradicating sea lampreys. Their numbers can rebound, and the Commission warns that elevated abundance can take years to remedy, setting back fishery and ecosystem recovery. Tributary assessments, treatment and other control operations therefore remain necessary. The Commission describes sea lampreys as “resilient beasts.” Great Lakes Fishery Commission: Sea Lamprey Control in the Great Lakes
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Control tradeoffs and effects on other fish
Barriers can protect spawning grounds and reduce the need to treat lamprey-infested tributaries, but they may also impede migrations and dispersal by desirable fish. The Commission says control design and research consider selective fish passage. Dams that also serve as lamprey barriers may be removed or deteriorate, creating additional control challenges. Great Lakes Fishery Commission: Barriers
Lampricide treatment is planned around larval surveys and environmental conditions. The management goal is to target lamprey larvae while limiting effects on other organisms, not to assume treatments are impact-free. The Commission says TFM at the concentrations used leaves most other organisms unaffected. Great Lakes Fishery Commission: Lampricide Treatments
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Why the stakes extend beyond one fish species
The historic decline was especially stark for lake trout, but lamprey attacks also affected lake whitefish and ciscoes. The Great Lakes Fishery Commission says the fishery is worth more than $5.1 billion annually and supports 75,000 jobs; its page does not state the year for those figures, so they should not be mistaken for a newly measured 2026 estimate. Great Lakes Fishery Commission: Sea Lamprey Control in the Great Lakes
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