The sea lamprey, a serpentine fish that feasts on other fishes’ blood, nearly wiped out the Great Lakes’ native game fish populations in the 1940s. Now, researchers have characterized the primary components of the pheromone that the lamprey relies upon to find its spawning grounds. The work could provide a potent and inexpensive bait for lamprey traps.
Sea lampreys came to the Great Lakes from the Atlantic Ocean nearly a century ago. An expensive, 50-year program of vigilance has restored some native species, such as lake trout. The Great Lakes Fishery Commission will spend $15 million in 2005 to research and deploy lamprey-control methods, such as killing the larvae and trapping the adults.
Sea lampreys spend an extended larval stage of up to 20 years in freshwater streams and then migrate to a larger body of water. There, they latch on to one host fish after another as they mature for about a year. At sexual maturity, the parasites have only a few weeks to spawn before they die.
Scientists have speculated that the sea lampreys follow the scent of a pheromone to a nearby area suitable for spawning, though not to their own place of origin. Research in the 1980s revealed that adults preferably swim into streams that are already inhabited by lamprey larvae.
In the November Nature Chemical Biology, a team at the University of Minnesota describes the chemical structures of the three main compounds of the migratory pheromone. Two of them hadn’t previously been described by scientists. The group also reports synthesizing one of the three compounds.
The scientists began the search with 8,000 liters of water that had contained about 35,000 larvae growing in sand. The team concentrated the liquid down to “a gram or two of organic gunk,” says coauthor Thomas R. Hoye of the university’s Minneapolis campus.
The researchers separated individual compounds from the gunk and tested them in two ways. They hooked electrodes to a live lamprey’s olfactory tissue and measured the tissue’s response to the compound. The team also put adult lampreys in an underwater maze that gave them a choice of swimming to two areas.
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A compound made the cut as a possible attractant if it induced an olfactory response and if the lampreys consistently swam toward the maze area where the water contained diluted compound. The combination of tests established that the fish detected and were drawn to all three compounds, says coauthor Peter W. Sorensen of the university’s St. Paul campus.
“This mixture is the first migratory pheromone identified in a vertebrate,” notes the Minnesota team.
The group has succeeded in synthesizing the most potent of the three compounds and is now searching for a protocol that would produce one or all of them in quantities large enough to test as a lamprey-control method.
The new study “is a wonderful piece of science with potential for real application,” says Roger Bergstedt, a U.S. Geological Survey fishery biologist in Millersburg, Mich.