Sea lamprey from a trap site on the Cheboygan River in northern Michigan. Image: U.S. Fish and Wildlife Service
By Kevin Duffy
For the first time researchers have combined the smell of death with the lure of sex in an attempt to better target the parasitic invader that has feasted on Great Lakes fish for decades.
Their target is the sea lamprey, an invasive species that with a toothy sucker-like mouth feasts on Great Lakes trout, salmon, sturgeon, walleye and whitefish.
Before lamprey were managed, they cost the Great Lakes 110 million fish annually. The cost of control is about $20 million a year, saving about 100 million fish annually, said Marc Gaden a spokesman for the Great Lakes Fishery Commission, the agency that manages sea lamprey.
They are controlled primarily with a selective pesticide that kills lamprey when it is dumped into rivers and streams.
Researchers know that lamprey are attracted by sex pheromones and repulsed by chemicals released by other lamprey that are dead or dying. So last summer, in an experiment near Mackinac City, Michigan, they tried to repel the fish from one half of a stream with the chemical released by dead lamprey, and attract them with the pheromone to a trap in the other half.
The idea is to concentrate them, making them easier to kill.
“The use of a repellant and attractant is a novel combination for use in a sea lamprey control program, something not typically used to control fish species,” said John Hume, the study’s lead author and research associate at Michigan State University.
Out of the lab and into the field
Until now, lamprey research was limited to lab settings, where controlled experiments revealed the lamprey’s extreme aversion to chemicals produced by decomposed lamprey when the same chemicals were dumped into a tank.
“This doesn’t occur naturally,” Hume said. “Decomposed lamprey are not dumped back in the water, but it’s conceivable that migrating lamprey grabbed by shoreline predators, like a heron or raccoon, will release the chemical compound intentionally or unintentionally.”
Downstream, the surviving lamprey can detect the chemical, avoiding it like a car would a series of traffic cones. Lamprey in this video don’t have the option of avoiding the chemical.
Dead, thawed lamprey larvae after a tough winter also signal this response, though researchers are still unsure what the chemical compound is, where in the animal it’s produced and its biological function.
“Laboratory trials indicated that the push worked, but this was a chance to test it in the field in combination with a pull, the sex pheromone, which does seem to attract them in the wild,” Hume said.
Researchers know more about the sex pheromone, which mimics the scent of good spawning habitat. An MSU laboratory synthesized one part — 3kPZS — that attracts mature females.
They tested it with the repellant to see if the chemicals’ combined effect could push lamprey away from exclusion areas and pull them into a trap, he said.
Mixed results
The result: The repellant pushed lamprey across the river faster, but the partial sex pheromone’s effect on capture wasn’t measureable.
“We didn’t see any additive effect,” Hume said. “It could be the result of stream conditions that year. It’s not the silver bullet control boards get excited about, and that’s part of the problem with field experiments.”
Despite trapping an unusually high percent of lamprey, Hume says the partial sex pheromone works best on sexually mature animals, which were not used in the study.
Future research will focus on teasing out these relationships, determining their natural function and working with new barriers and traps.
Meanwhile, researchers are continuing the fight against lamprey on a variety of fronts.
“We’re working toward an all tactics combined approach for the sea lamprey control program,” Hume said. “We have a barrier that blocks and redistributes them, we have the potential for many new improved traps, and we know push and pull cues have shown success in the lab.”
Nearly 1,000 barriers, including dams, in the Great Lakes basin are being incorporated into the sea lamprey control program, he said. They range from one to tens of meters above the surface of the water.
They control the invasion of sea lamprey that threaten local ecosystems and important commercial and recreational fisheries valued at $7 billion or more. But barriers also block native fish, including non-invasive native lamprey whose numbers have dropped worldwide.
Other researchers look to help lamprey pass
Ironically, elsewhere where the lamprey are native to the ecosystem, other researchers are trying to help them surmount such barriers. And the problems they have passing the fish, may help the researchers in the Great Lakes region learn to stop them.
“In the Pacific Northwest and across Europe researchers are finding ways to get native lamprey past barriers, and they’re failures inform us,” said Mike Wagner, coauthor of the push-pull experiment and aquatic behavioral ecologist at Michigan State.
Pacific lamprey, and other migratory lamprey including the Great Lakes invader, will move from a marine environment to freshwater before moving upstream for spawning. Barriers and dams obstruct sea lamprey passage but also restrict movement of native lamprey that can safely coexist with other species.
“We’re facing competing priorities,” Wagner said. “We want to restrict the distribution and population sizes of invasive species but also open up our river networks to be used again.”
Native lamprey support local ecosystems, and their young are thought to be an important food source for other native fish, like salmon. They’re also an important cultural resource to Native Americans in the Northwest, said Mary Moser, a research fishery biologist with the National Oceanic and Atmospheric Administration.
“Lamprey are valued as a food source, as well as for medicinal or ceremonial practices,” she said at a recent MSU conference on lamprey passage.
Chemical cues are also being investigated to help direct safe passage of native fish, but most fishways are designed for salmon and are hard for the native lamprey to navigate, Moser said.
Researchers hope to redesign the salmon-specific passes to support the movement of other native fish without removing the barriers that have successfully trapped sea lamprey from moving upstream.
Modified fishways can enable native fish to pass barriers by swimming, leaping, climbing or sucking their way up a series of low steps — a fishladder. The incline of steps and size of slots allow some species to pass through and others not.
“We’re mapping their decision-making in the lab,” Wagner said. “And in the field, with the addition of selective fish passage, we’re learning how to use chemical cues to guide sea lamprey away from high-quality streams and into traps.”