Sea lampreys were the Great Lakes’ first notorious invasive species. Able to survive in both salt and fresh water, the Atlantic Ocean natives were documented in Lake Ontario in the 1830s and, in just over a century, they established themselves throughout the Great Lakes. These primitive fish may look like eels, but they’re closer to vampires as they feed on the blood of host fishes.
From harmless beginnings, a thirsty parasite
The sea lamprey, Petromyzon marinus, has a multistage life cycle that lasts several years. The most innocuous stage is as small larvae. During this time, lampreys live in freshwater tributaries and rivers, buried in the mud and feeding on algae and other microscopic nutrients that float by. As they grow into adults, lampreys migrate toward the open water of the Great Lakes, where they reach their better-known stage of life.
An adult sea lamprey is long—about 2 feet—and lean, with mottled black or brown coloration and seven pairs of gill openings. But the signature feature of lampreys is the gaping suction-cup of a mouth, filled with rings of sharp, hooked teeth that radiate out around a rasping tongue. This nightmarish maw, an exquisite adaptation for feeding on the blood of host fishes, earned lampreys the reputation as the vampires of the Great Lakes.
In open water, a lamprey attaches to the side of a fish and, with its small teeth, pierces and hooks onto the side of its host. Then it uses it tongue to rasp through the scales and skin to consume the blood.
Much like mosquitos that annoy humans during the summer months, lampreys produce an anticoagulant that maintains blood flow as it feeds. Unlike a small mosquito that just takes a small amount of blood from us, a sea lamprey will remain attached to a fish for hours, days, or weeks. While lampreys do not always kill their hosts outright, the afflicted fishes can be so weakened that almost all die due to blood loss or infection. Fewer than one in seven fishes survive lampreys feeding on them.
A sea lamprey continues feeding on fishes for about a year, then travels back upstream into a river or tributary to spawn. Soon after spawning it dies, and the life cycle begins again with the next generation.
Invasion and impact
Lampreys’ invasion from the Atlantic Ocean was made possible by manmade waterways built to connect the Great Lakes for ship commerce. The Erie Canal gave them entry to Lake Ontario, where they were first reported in 1835. They then spread through the Welland Canal, the water diversion built for cargo ships to bypass Niagara Falls, in 1880 and were found throughout the lakes by 1939.
Horrific as the image of a sea lamprey feeding on a fish is, the effect of sea lampreys on the Great Lakes is much worse. Invasive species are organisms outside their native range that do economic or environmental harm, and sea lampreys are among the most destructive species to have entered the Great Lakes. They have succeeded in this new environment for several reasons: They lack natural predators, females spawn up to 70,000 eggs in one season, and they are resilient to a wide range of water conditions.
Lake trout, historically one of the main predatory fish species in the Great Lakes, were especially hard hit by the exploding sea lamprey population. Combined with overfishing by humans, the lake trout population declined, and the natural balance of the Great Lakes food web was disrupted. At peak population numbers in the 1950s, there were an estimated 2 million adult sea lampreys in the Great Lakes, each killing up to 40 pounds of fish a year—that is, up to 80 million pounds of fish dying each year due to lampreys alone.
As lake trout fisheries collapsed throughout the Great Lakes, several control techniques were developed for sea lampreys, the most successful involving targeting the fish at their most vulnerable stage—as larvae in rivers and tributaries.
To kill the larvae, a lampricide chemical is applied in concentrated doses to streams and tributaries. The chemical disrupts the larval lampreys’ development but does not cause significant harm to native fishes and invertebrates.
This technique is expensive and time-consuming, requiring teams of technicians over multiple days to apply the correct dosage of lampricide to each targeted river and stream. Current research at Michigan State University is using the pheromones of male lampreys, a natural attractant for females ready to spawn, to lure females into unsuitable spawning habitats or traps before they can release their eggs.
It costs around $20 million a year to keep lamprey numbers under control, but these efforts have been effective and help maintain a $7 billion a year sport fishery. Sea lamprey numbers peaked in the 1950s, but they have been brought to near-historical lows since the fish spread throughout the Great Lakes, with the ultimate target of reducing the population to 10 percent of the size it was at its peak.
We may be controlling the sea lamprey population in the Great Lakes, but these vampires are here to stay. The lesson from sea lampreys is that the best and most economical protection against invasive species is prevention: Don’t let them get into the lakes in the first place.
Our native lampreys
Not all lampreys in the Great Lakes are invasive. Of the four native species, two, American and northern brook lampreys, aren’t even parasitic to fishes. The other two, chestnut and silver lampreys, do attach to fishes and are parasitic, but they rarely kill the native fishes they feed on.
Learn more about sea lampreys and see them for yourself in the At Home on the Great Lakes gallery at Shedd Aquarium.
—Sam Bugg; Scott Colborne, Ph.D., Daniel P. Haerther Center for Conservation and Research
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