Chemical lures may hold key to controlling parasitic fish

TRAVERSE CITY, Mich. -- Disrupting the sea lamprey's mating call may be the key to limiting its numbers in the Great Lakes, where the eel-like parasite sucks the life out of sport fish, scientists say.

Male lampreys secrete a smelly bile acid during spawning season that draws ovulating females to nests, where they lay eggs for males to fertilize, Michigan State University researchers said.

The findings, which appeared recently in the journal Science, might lead to better trapping and sterilization programs in the Great Lakes, where the lamprey is a pariah. But it also could help produce more lampreys in places where they are considered a delicacy -- and it suggests that sex lures might be used to raise or reduce other fish populations.

"This research will help us produce new and more effective techniques for sea lamprey control," said Chris Goddard, executive secretary of the Great Lakes Fishery Commission, a U.S.-Canadian agency that helped fund the study.

The sea lamprey, native to the Atlantic Ocean, is believed to have invaded the Great Lakes through shipping channels in the early 20th century.

The jawless critter clamps its powerful disklike mouth onto the sides of fish, digs in with sharp teeth and sucks their body fluids. When satisfied, the lamprey releases its hapless host, which usually dies from the wound or a later infection.

'The worst disaster'

By the 1950s, lampreys had decimated lake trout and wreaked havoc with other popular species such as whitefish and sturgeon, Goddard said. Control programs have cut their numbers sharply in most of the Great Lakes, but they still do significant damage.

The study labels the lamprey invasion "arguably the worst ecological disaster ever to befall a large watershed."

After three to five years in the larval stage, the lamprey spends only a year to 18 months in its parasitic form before moving into streams to spawn and die. But the typical adult lives long enough to kill some 40 pounds of fish.

Scientists long suspected that males emitted a pheromone, or chemical "message" that guided females to nests, said fish physiologist Weiming Li, who led the research team. The Michigan State study confirmed the fact and helped explain how it happens.

Li's team captured sexually mature male lampreys and put them in containers where they could secrete the chemical, manufactured in their liver. Then, over two years, the scientists condensed the water until they had a pure sample of the compound.

Tested in a maze

The researchers used behavioral tests, including placing females in a maze, to show that the chemical is a sexual lure. They found it is potent enough, and produced in sufficient quantity, to attract females over long distances.

By analyzing its molecular structure, the researchers learned how to make synthetic versions of the chemical but still need to find ways to do so less expensively, Li said.

Scientists presently control lamprey by sterilization, placing barriers in streams to prevent spawning, and putting low concentrations of a pesticide into streams to kill larvae.

The newly discovered chemical could make it easier to capture lampreys for sterilization, Li said. Also, spawning grounds could be "flooded" with the chemical to confuse females, preventing them from finding nests the males have built.

"You could manipulate their movements into streams where you want them and keep them out of streams where you don't want them," said Dennis Lavis, supervisor of the U.S. Fish and Wildlife Service biological station at Ludington, which does lamprey control work.

"Any good pest control program needs a number of alternative methods, and this research offers some very exciting possibilities," Lavis said.