A fish suffers with parasitic lampreys latched onto its sides (left). On the right, the blood-sucking mouth of the lamprey [Credit: Marc Gaden (fish with lampreys) and Ted Lawrence (lamprey mouth), Great Lakes Fishery Commission].
Jawless suction-cup mouths, hooked teeth and rasping tongues enable them to drain the life juices from hapless trout, salmon and whitefish. But sea lampreys, also known as “vampire fish,” have another attribute – a mighty sense of smell – that researchers hope to turn against this snake-like bane of the Great Lakes’ native fish.
In a long-awaited experiment designed to foil the lampreys’ mating process, a team of researchers from the U.S. and Canada will bait traps in ten Great Lakes streams this spring with a chemical lure: a man-made version of the pheromone secreted by male sea lampreys to attract spawning females. Researchers estimate that only 60 grams (a little over 2 ounces) of the seductive scent will be needed to complete the seven-week baiting program. They, along with a multi-billion dollar Great Lakes fishing industry that has battled invasive lampreys for more than half a century, hope the substitute will prove powerful enough to persuade females to follow their noses right into the traps.
“If you know what causes them to behave a certain way, you can then introduce a little trickery to manipulate their behavior,” says Marc Gaden, spokesman for the Great Lakes Fishery Commission, the agency responsible for coordinating sea lamprey control.
The synthetic lamprey pheromone has already proved itself in laboratory tanks and natural waterways. During recent tests in Michigan’s Ocqueoc River, researchers used it to snare lampreys from nearly half a mile away, although they believe the fish can detect the chemical over several miles. The pheromone has such a strong effect on spawning female lampreys that only 270 grams – not quite enough to fill a soda can – would need to be applied annually in the Great Lakes most severely impacted by lampreys: Michigan, Superior and Huron. And because the artificial pheromone is species-specific, it would not interfere with the matchmaking of other kinds of aquatic life.
Pheromones have been used effectively in managing insect populations since the 1960s, but until now no one has ever tried to use them to control a vertebrate species.
Insect researchers familiar with the strategy are optimistic it will work on lampreys, too. “These compounds are the main method of communication between male and female insects, and lampreys seem to be using them in a similar way,” says entomologist Jocelyn Millar, who studies pheromones at the University of California, Riverside and is not involved in the lamprey project in the Great Lakes. “You put the chemical out and the females come.”
Male sea lampreys release sex pheromones during a three-week spring mating season. When a spawning female catches a whiff drifting downstream, she instinctively swims upstream. If the lady lamprey loses the scent, she will turn back downstream, only to be lured upstream again once her nose recovers the odor. This process continues until she finds the source – whether a mate or a trap baited with a synthetic sex smell.
A lamprey, says Marc Gaden, is “like a swimming nose.” The description isn’t far off: a sea lamprey’s nose is bigger than its brain and it relies heavily on smell for sensory perception, especially during spawning.
This keen sense of smell and primitive reproductive strategy has been a target of scientific exploration and exploitation since the 1980s. The first major breakthrough came in 2002, when Weiming Li, a fisheries professor at Michigan State University, published the structure of a male sea lamprey sex pheromone called 3kPZS. Li and his colleagues have now demonstrated the utility of an artificial version of this single chemical compound.
The pheromone “love potion” may be even more effective underwater with sea lampreys than similar tactics used to trap flying insects. Lampreys “know where the odor is coming from because the current is always coming from the same direction, whereas for an insect, the wind could shift,” says Nicholas Johnson, a research associate in Li’s lab at Michigan State University. Johnson is the lead author of a paper describing the latest lamprey pheromone experiments that was published in the January 27 issue of the Proceedings of the National Academy of Sciences.
Sea lampreys survive as parasites in their native Atlantic Ocean, feeding on much larger fish, including sharks. But in freshwater lakes, where they are not native, they act as unchecked predators and can grow to nearly three feet long while consuming about 40 pounds of defenseless fish in a lifetime. For prey not killed from the loss of bodily fluids, the resulting wound often leads to death from infection and disease. Lampreys arrived in the Great Lakes in the mid-1800s via shipping canals. By the late 1930s, they had infiltrated nearly the entire system and, since then, have been at least partially responsible for the extinction of three whitefish species.
Federal officials launched a lamprey control program in the late 1950s in response to the devastation. The centerpiece became a pesticide, TFM, which kills larval sea lampreys. The population in the Great Lakes was reduced from about three million at its peak to approximately 350,000 today, and remains within the target range set by the Great Lakes Fishery Commission in four of the five lakes (Lake Superior is the exception.) Although no evidence has been found that TFM harms humans or other species, the commission is trying to reduce pesticide use wherever possible.
“We also want to have as many weapons as possible to control this beast, so that we don’t become overly reliant on any one technique,” says Marc Gaden of the Great Lakes Fishery Commission. In addition to the commission and Michigan State University, institutions collaborating on the pheromone experiments include the U.S. Fish and Wildlife Service, the U.S. Geological Survey and Canada’s Department of Fisheries and Oceans.
If this year’s large-scale field tests are as successful as earlier experiments, Gaden expects 3kPZS to begin complementing current control methods in more Great Lakes streams, wiping out larger numbers of lamprey while minimizing discharges of TFM. This should also substantially lower costs. The sea lamprey control program currently spends around $5 million a year on TFM. The cost of a gram of 3kPZS, however, has dropped from about $20,000, when Michigan State University got its first experimental batch four years ago, to just $1,250 today. As production increases towards potential full-scale implementation in 2012, the price could fall closer to $125 per gram, according to Michigan State’s Nicholas Johnson. For the 270 grams needed in the upper Great Lakes, that’s less than $34,000.
The prospect for other synthetic compounds in animal control, Gaden says, is limited only by our imagination. There’s even the possibility of a helpful kind of deceit: “You could use pheromones to guide salmon species around dams or up fish ladders,” says Johnson. Someday, he adds only half jokingly, they might even help anglers catch dinner. “I like to fish. If I could find a pheromone for walleye, I could do pretty well.”
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