Image of a sexual mature male sea lamprey. The adults' skin colours change like salmon with the male lamprey developing a "rope-ridge" across their back.

Sea Lamprey: A Parasitic Pest in the French River and Beyond

Sea lampreys are vampiric invasive species that have caused significant damage to the native fish populations in the Great Lakes. The French River in Northeastern Ontario is one of the many waterways they have invaded. While sea lampreys are monitored and population controlled by the Ministry of Natural Resources and Forestry (MNRF) and Fisheries and Oceans Canada, it is also important for us – as anglers – to be able to identify and report our findings.

In this article, we will discuss their appearance, the historical impact on Ontario’s waterways, concerns, and measures taken to control the sea lamprey population.


The lampreys have a primitive appearance and remained largely unchanged over 340 million years, since the Paleozoic Era. During this, they survived four major extinction events according the to 2021 Annual Survey report by Great Lakes Fishery Commission.

Image of an Adult Sea Lamprey with its mouth sucking on the side of a fish tank.
Vampire of the Great Lakes. Photo Credit: Great Lakes Fishery Commission (GLFC)

They have eel-shaped bodies lacking true jaws with paired fins and soft cartilaginous skeletons. There are many types of lampreys with various colour patterns and shapes, but the sea lampreys are the most infamous among them.

Sea lampreys have a dorsal fin separated into two parts by a deep notch, the second part meeting with the caudal fin. Its mouth is merely a disc-shaped suction cup lined with horny cusps, with a similar jagged rasping tongue. The closely spaced teeth are arranged in curved, radiating rows.

Adult sea lamprey skin has a dark tan to olive or red-brown tone, with chocolate blotches; while spawning adults are purple or blue-black. The young are tan on their back with lighter bellies and with occasional darker mottling. They can grow to 120 cm (47 in) and nearly 2.3 kg (5.1 lbs.) according to FishBase in the Great Lakes regions.

Sea Lamprey’s Invasion of the Great Lakes

Originally native to the Atlantic Ocean, sea lamprey co-evolved with their host fish and did not normally kill them. They naturally inhabited the Atlantic from Greenland to Florida and from Northern Norway to the Mediterranean Sea. Sea lampreys would later ascend to freshwater to breed, some of which would occur in land-locked freshwater lakes.

Sea Lamprey map showing the native and non-native locations of Sea Lamprey by the USGS
Invasive species map generated by the United States Geological Survey. Interactive Map here.

Yet, they became an invasive species in the Great Lakes and caused significant damage to the native fish populations. Sea lampreys were first reported in Lake Ontario in 1835. But there’s a dispute if they are native to Lake Ontario, as some evidence suggests they were present there before European colonization.

Niagara Falls served as a natural barrier preventing sea lampreys from entering the other four Great Lakes until the late 1800s and early 1900s. Improvements to the Welland Canal bypassed Niagara Falls which provided a direct link between Lakes Ontario and Erie. With this connection, it allowed sea lampreys to invade the other Great Lakes.

Rapidly spreading throughout the Great Lakes system, they reached Lake Erie by 1921. Later, they populated the remaining Lakes Michigan (1936), Huron (1937), and Superior (1938). These lakes were abundant with host fish as they thrived with excellent spawning and larval habitat for them.

Impact of Sea Lampreys on Ontario’s Ecosystem?

Their devastating impact on the Great Lakes fishery cannot be understated as they inflicted considerable damage to the trout, salmon, whitefish, and other native fish species. Before the sea lamprey invasion, Canada and the United States harvested about 15 million pounds of lake trout in the upper Great Lakes each year. By the late 1940s, lake trout harvest had dropped to almost zero with the lamprey invasion. Sea lampreys also reduced the diversity and abundance of other fish species, like lake sturgeon, lake whitefish, ciscoes, burbot, walleye, catfish, and Pacific salmonids.

This forced a coordinated effort by Canada and the United States to restore the Great Lakes fishery before being irreversibly damaged. By 1955, the Great Lakes Fishery Commission was established to coordinate fishery management in the Great Lakes.

Lamprey’s Effects on Human Activities?

According to Fisheries and Oceans Canada, over 40 million people depend on the Great Lakes. Our fishery generates up to $7 billion annually which offers food, drinking water, tourism, and angling opportunities for five million people. The Great Lakes fishery provides 75,000 jobs in our region.

Sea lampreys affect human activities in the French River in various ways, depending on the perspective and the value of the species. Below are some of their impacts:

  • For some people, lampreys are a source of food and income. They are a traditional delicacy in Europe, especially in Spain, Portugal, and France, where they are still commercially fished. Some people in North America also enjoy eating sea lampreys, especially those of Native American heritage from the Pacific Northwest.
  • For many in North America, sea lampreys are a nuisance and a threat to the native fish populations. In particular, to the trout, salmon, and whitefish industries. They also affect the biodiversity and ecosystem’s health of the French River and other tributaries of the Great Lakes.
  • For others, sea lampreys are a fascinating and ancient species that deserve respect and conservation. As mentioned earlier, the species has lasted for 360 million years adapting to various habitats and conditions. Evidence suggests they play a role in the nutrient cycling and food web of the rivers they inhabit that are directly connected to the ocean.

Fortunately, lampreys are not interested in us humans since we are warm-blooded creatures.

Lifecycle of Sea Lamprey

The Lifecycle of sea lamprey typically lasts between 5 to 9 years and goes through a series of stages. Yet, some lampreys have been recorded to live for 13 years according to McClane’s New Standard Fishing Encyclopedia (1998, page 860).

Migration and Spawning

Sea lampreys are anadromous, meaning they migrate from their lake or sea habitats to spawn in rivers with moderately strong currents. According to the New York Department of Fish and Wildlife, a pair of sea lamprey build a nest called a “redd”. A type of spawning ground similar to Salmonids (e.g. salmon, trout, and char) selected by the female and built in the gravel stream bed.

Image of a sexual mature male sea lamprey. The adults' skin colours change like salmon with the male lamprey developing a "rope-ridge" across their back.
Image of a spawning male lamprey. Notice the “rope-ridge” on its back that develops during their spawn. Image credit GLFC.

The redds, or large depressions, are made by the adult lampreys moving large stones with their mouth. Females deposit tens of thousands of eggs in these nests with the male(s) fertilizing the eggs. While the sea lamprey matting system is not full understood yet, a 2021 survey by indicates they are polygynandrous – where both males and females may share multiple partners.

From their research they observed 114 tagged sea lamprey (56 male and 58 female) where, “[some] individuals would only visit one nest, while [others] could be seen on more than 5 nests.”  After spawning, the adult lampreys perish.

The lamprey eggs are laid in the small spaces between the gravel and get oxygen from the flowing water. After a few weeks, they hatch and repeat their complex life cycle.

Hatching and Laval Stage

With the new generation, young sea lampreys start as non-parasitic larvae. They filter feed on plankton and detritus for 3 to 10 plus years depending on environmental conditions and growth.

Image of a person holding a Sea Lamprey Ammocete (larva)
Image of a person holding a Sea Lamprey Ammocete (larva). Image credit: GLFC

The larval sea lamprey undergoes a dramatic metamorphosis and develops their eyes while their oral disks and tongues become covered with pointy teeth During this metamorphosis, like some species of worms, a sex becomes assigned to the individual sea lamprey. The mechanism of sex determination in sea lampreys is not fully understood yet. However, a 6-year field report published by the Royal Society in 2017 indicates that environmental conditions and nutrient levels may influence it.

Sea Lamprey Transformer
Example of a juvenile Sea Lamprey “Transformer”. Image credit: GLFC

Once the lamprey’s metamorphosis finishes, they begin migrating downstream to the lakes.

Parasitic Stage

Once larval sea lampreys have metamorphosed and migrated to the lakes, they become parasitic juveniles – feeding on host fish for 12 to 20 months.

Lamprey transformer undergoing metamorphosis from its larval stage.
Young lamprey undergoing further development from its earlier larval states. Photo Credit: GLFC

Sea lampreys attach to fish with their lips, which act like suction cups. They use their sharp teeth and hard tongue to cut through the fish’s skin and scales while feeding on its blood and fluids. After puncturing the fishing’s skin, the lamprey’s saliva secretes an anticoagulant, like leeches and certain species of mosquitos, making it easier for them to feed.

Effects on Fish Population and Food Chain?

As mentioned earlier, Sea Lampreys have reduced the number of sportfish in the Great Lakes. Depending on different reports, only one in seven fish survive an attack and destroy up to 18 kgs. (40 lbs.) of fish within the lamprey’s lifetime. Lampreys will feed on almost any fish in the Great Lakes.

The lamprey predation has caused significant population collapses of native fish in the Great Lakes such as lake trout, whitefish and chub which declined to extirpation in the 1940s and 1950s. Sea lampreys have harmed the Great Lakes ecosystem in Ontario in many ways and affected other aquatic species that relied on these fish as food.

How We Control and Stop the Spread of Sea Lamprey?

Controlling the sea lamprey population has not been easy as many measures have been taken. These include chemical, physical, and biological control methods. They have been successful in reducing sea lamprey populations by 90% according to the Department of Fisheries and Oceans Canada.

Below are the biological control methods that government agencies and scientists are using to manage sea lamprey populations:

  1. Lampricides: These are selective pesticides that target sea lamprey larvae in streams and rivers. The most commonly used lampricide is TFM (3-trifluoromethyl-4-nitrophenol) first discovered in 1958. A relatively harmless chemical to other organisms as it is used in about 250 Great Lakes tributaries every 4 years according to the Great Lakes Fishery Commission.
  2. Barriers: Barriers are physical structures that prevent sea lampreys from reaching their spawning grounds. Natural barriers like waterfalls such as the Sturgeon Chutes in the French River. There are 5 man-made types of barriers:
    • Low-head barriers: These are simple structures that create a vertical drop of two to four feet that sea lampreys cannot jump over. They also have a lip that prevents sea lampreys from climbing over them with their suction-cup mouth. These barriers are designed to allow most other fish to pass safely.
    • Adjustable-crest barriers: An improved version of low-head barriers that allow easier passage for native fish to migrate while restricting sea lamprey’s spawning migration. They have adjustable air bladders controlled by computers to raise and lower the height of the crest.
    • Traps: These are devices that capture sea lampreys before they can spawn and remove them from the stream. They also enable the assessment of sea lamprey populations. Some barriers have built-in traps or trap-and-sort fishways that separate sea lampreys from desirable fish.
    • Velocity barriers: These are structures that exploit the poor swimming ability of sea lampreys and create a high-water velocity that they cannot overcome. They are usually adjustable and can be lowered or raised depending on the water flow and the fish migration season.
    • Electrical barriers: These are devices that use electric current to deter sea lampreys from passing through. They are usually installed in streams where physical barriers are not feasible or desirable. They do not block the flow of the stream and can be effective in stopping sea lamprey migrations.
  3. Sterile male release: This method involves releasing sterile male sea lampreys into the wild to mate with female sea lampreys. Since the eggs laid by female sea lampreys that mate with sterile males do not hatch, this method can help reduce the number of sea lampreys in the wild.
  4. Pheromones: Pheromones are chemicals that are released by sea lampreys to attract mates. Scientists are studying ways to use pheromones to disrupt the mating behaviour of sea lampreys and reduce their populations.

Sea lamprey control has reduced their population and their damage to the Great Lakes fishery. According to the Great Lakes Fishery Commission, the program has saved about 100 million pounds of fish per year or about $500 million annually. Yet, sea lampreys are a persistent threat to our ecosystem. The Commission and their partners continue to improve sea lamprey control methods, assess their impacts, and continue their efforts to restore native fish populations.

French River Lamprey Treatments

Ryan Booth, Senior Biologist for Fisheries and Oceans Canada, and Shawn Robertson, Aquatic Science Technician IV for Fisheries and Oceans Canada provided the following information about the French River treatment program:

The French River is only treated in small sections due to its size, and has not been treated at all since 2012 (and prior to that in 2006, 1992 and 1976),” according to Ryan Booth and continued, “We have never released sterile males, or used any attractant pheromones in that system. It is such a massive system […], that even assessment can be quite challenging.

Shaw Robertson reaffirmed Ryan’s statement for the 2012 treatment adding:

There are only two sections of the French River that receive treatments with TFM or a combination of TFM and Bayluscide.  They are the Wanapitei and the Old Voyageur Channel.  The 2012 treatment was on the OVC and the last time the Wanapitei River was treated was in June of 2011.

A 50 inch French River Muskie with what looks like old Lamprey bite marks on its sides.
A French River Muskie (September, 2023) that could have old lamprey bites.

How Do We Prevent the Spread of Sea Lampreys?

After many years of study by scientists and conservationists, as anglers and boaters, we can help prevent the spread of sea lampreys by following these three simple guidelines:

  1. Clean your boat and equipment: Remove all plants, animals, and mud from your boat, trailer, and equipment before leaving the water. Rinse your boat and equipment with hot water or let them dry in the sun for at least five days before using them in another body of water.
  2. Don’t transport live fish: Don’t move live fish from one body of water to another. If you catch a fish with a sea lamprey attached, do not return the sea lamprey to the water. Kill it and put it in the garbage.
  3. Don’t help sea lampreys pass over dams and culverts: Sea lampreys are unable to pass over dams and culverts that block their spawning migration. Don’t move them over these barriers.

By following these recommended guidelines, anglers and boaters can help prevent the spread of sea lampreys and other invasive species in the French River and other tributaries in the Great Lakes region.

Article by Joe Barefoot, M.B., Outdoor Writer of Canada and Nationally Published Author & Photographer