Why sea lampreys are going to be a bigger problem for the Great Lakes

A cross-border program keeps sea lamprey populations at bay. But a two-year pandemic disruption was never part of the plan.

Top: Experts predict there will be at least a temporary surge to lamprey populations in the lakes following a dip in control efforts during the early stages of the Covid-19 pandemic. T. Lawrence, Great Lakes Fishery Commission via NOAA

This article was originally featured on Undark.

It’s a gray morning on Lake Ontario when Will Sampson, a sports fishing guide and recreational angler, sees one of the fishing reels on his father’s boat jerk. He reels it in. On the other end of the line is a Chinook salmon that he estimates weighs 22 pounds—a great catch, save for one wriggling detail: Latched into the salmon’s side dangles a two-foot long sea lamprey, suction-cup mouth clinging on, one eye peeking around its host’s fin. In its lifetime, that sea lamprey could kill up to 40 pounds of fish; the hooked Chinook was its latest target.

Here, just off the Toronto harbor front and 27 miles to Niagara Falls, the Sampsons have gotten to know the Great Lakes’ infamous eel-like invasive fish well. Out of every 10 big fish they catch, they say usually at least four will have clear signs of a lamprey bite, some with multiple wounds at different stages of healing. For the father-son duo, the lamprey on the morning’s Chinook is just another sign that this summer, the blood-sucking fish’s population has ballooned.

It’s a scenario that scientists are concerned about, too. During the 1940s and 1950s, when the region’s sea lamprey populations reached their peak, they decimated fisheries, wiping out livelihoods and wreaking havoc on the lakes’ ecosystem. Since then, the species has been the subject of a robust cross-border control program. But that program was disrupted significantly during 2020 and 2021 amid pandemic restrictions.

Now, as control measures return to normal, biologists, conservationists, and fishery scientists worry about how the consequences of this disruption will play out across the Great Lakes, from Lake Ontario to Lake Superior. Though many are cautiously optimistic that the region may only experience a short-term increase to lamprey population levels, the question also lays bare the challenge of predicting how invasive species in general will respond to years of pandemic-fueled gaps in control efforts.

“If you ease up control, even for a short time—like two years in Covid—they’ll take advantage of that,” explained Marc Gaden, communications director of the Great Lakes Fishery Commission, or GLFC. The commission, established by a 1954 treaty between Canada and the United States, oversees the sea lamprey control program across the five Great Lakes from hubs in Michigan and Ontario. In order to protect the Great Lakes’ $7 billion fishery, the two countries spend about $25 million annually on sea lamprey control, according to Gaden, with the U.S. contributing nearly 70 percent of the funding. Though there are over 180 invasive species in the Great Lakes, none are more destructive than the sea lamprey, Gaden said.

The control effort isn’t just expensive, but also labor intensive. To do the job right, aligning treatment times with the lamprey’s growth cycle is essential—a precise schedule that the pandemic threw two years off-target.

“The good news is you can control them, but they’re not going away,” Gaden said. “You’re never going to get that last mating pair.”


At the beginning of its lifecycle, a lamprey can seem harmless. At less than an inch long when hatched, with no eyes or teeth, the larval lamprey feeds on plankton and other detritus in streams. But after four or six years, on average, it goes through a dramatic metamorphosis. The tiny worm-like fish grows eyes and a suction cup mouth with barbed teeth along its tongue. Once the metamorphosis is complete, it begins to travel downstream to hunt. Within one year, these parasitic sea lampreys can grow over two feet long by latching onto fish and feeding on their blood.

The trick to controlling sea lampreys is preventing them from metamorphosing and moving beyond the streams, Gaden said. Each year, the GLFC maps out a selection of the approximately 500 streams in the region that produce most of the larvae and selects some for treatment. Then, from spring until fall, crews from U.S. Fish and Wildlife Service and Fisheries and Oceans Canada treat the streams with pesticides selective to lampreys, called lampricides, ultimately killing about 98 percent of them. (The chemical is non-lethal to other species, which are able to detoxify it, though it can sometimes have other harmful effects on the surrounding ecosystem.)

Using this strategy, the program has reduced the sea lamprey population in the Great Lakes by about 95 percent from the historic abundance in the 1940s and 1950s. “It’s a boots-on-the-ground, boats-in-the-water field program,” Gaden said. “By the time you come back to the stream, they’ll have built up again and you can get them.”

But in 2020, the pandemic hit just before spring arrived in the Great Lakes region. That year, crews only treated around 25 percent of the planned streams as pandemic restrictions and concerns prevented control teams from traveling. The following year, in 2021, crews accomplished 75 percent of targeted treatment.

“It would be Pollyannaish to think there’s not going to be any effect,” said Gaden. But what the effect will be on sea lamprey control—how severe, how long lasting, and how the region’s fisheries and other species will fare—is still playing out.


On March 11, 2020—the day the World Health Organization declared the coronavirus a pandemic—Margaret Docker, professor of biological sciences at the University of Manitoba, was visiting Ann Arbor, Michigan, for a sea lamprey control meeting. By the time Docker returned home two days later, norms had changed across the world. In Winnipeg, Manitoba, she began working from home. At the same time, out in the hundreds of streams around the Great Lakes, larval lampreys prepared to metamorphosize.

The lamprey spawning this spring and summer are the survivors of that 2020 season. Now, scientists are tracking those spawning numbers, working to understand the impact of the pause to control measures. “If you have streams not being treated two years in a row, there’s just no way that you’re going to get them all,” Docker said.

Docker predicts there will be at least a temporary surge to lamprey populations in the lakes. But she is also hopeful that the disruption might serve as a kind of unplanned experiment, helping researchers understand how lampreys react to shifts in their population abundance. She’s curious about how more lampreys in the lakes may impact the species’ birth rate, death rate, and sex breakdown, and how that information could inform control efforts.

“When you get increases in larval density, it often decreases the growth rate,” she said. “Because the number of sea lamprey has been reasonably consistent over the last 50, 60 years, we can’t really study that because we don’t have that variation.”

But predicting what will happen to lamprey populations in the coming years and decades—and what kind of intervention to invest in to keep the worst effects at bay—is a tricky science, said Michael Jones, a quantitative fisheries scientist and professor emeritus at Michigan State University. Jones models lamprey populations in the lakes to understand how variances in temperature, weather, control measures, and more affect lamprey population levels. In this work, decade-to-decade changes are much more significant than year-to-year ones, he explained.

“The actions that we take in any one year affect the situation two years later, they don’t really affect the situation 10 years later,” said Jones. Other factors, like warming waters from climate change, he predicted, could make the Great Lakes a much more favorable habitat for lampreys over the long term, with a potentially far greater benefit to the lamprey survival rate than a year or two of lapsed lampricide treatments.

“If you have streams not being treated two years in a row, there’s just no way that you’re going to get them all,” Docker said.

Jones’s confidence that lamprey levels will remain under control also comes, in part, from the GLFC’s ability to respond with full force in the coming years. Supported by significant investment from both sides of the border, the commission aims to play aggressive catch up with its lampricide treatments.

For other species control programs, though, the situation could be more dire. The invasive carp, for example, also threatens to enter the Great Lakes ecosystem, with no pesticide available to eliminate it. After the pandemic interrupted the construction of carp barriers, the invasive fish entered tributaries even closer to the lakes, said Marc Smith, Great Lakes policy director at the National Wildlife Federation. If carp make it into the lakes, no chemical treatment will be able to control them.

Biologists have expressed similar concerns about pandemic-fueled boons to other invasive species populations, including rats in New Zealandmice in Hawaii, and some plants in California.

When it comes to invasive species management, “You never want any delays, you never want a lack of focus or lack of pushing things out time-wise,” Smith said. “That’s kind of what Covid did.”