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Bile isnít Vile: Identifying the Scent that Lures Trout Home

Ben Thwaits studies the reaction of a trout in a fish tank.

Ben Thwaits tests the response of a rainbow trout to bile acids. Photo: David Hansen

If a Lake Superior Kamloops could talk, it still might not care to reveal what Sea Grant-funded graduate student Ben Thwaits has painstakingly found out: the smell of certain compounds found in fish excrement help adults of this strain of migratory trout find their way home.

Thwaits described his finding in April at the 28th annual meeting of the Association for Chemoreception Sciences in Sarasota, Fla. It builds on discoveries made about half a century ago by Arthur Hasler, a leading figure in 20th century freshwater ecology. Hasler discovered that migrating adult salmonids like trout locate their home streams for spawning by following distinctive odor trails remembered from their youth.

The precise identity of this odor has remained a mystery, however. Now, Thwaits, along with Peter Sorensen, professor with the University of Minnesota Department of Fisheries, Wildlife, and Conservation Biology, and Allen Mensinger, associate professor with the University of Minnesota Duluth Department of Biology, have found compelling evidence that the odor trail leading home is laced with bile acids, a distinctive class of steroids that is produced in the liver to aid with digestion.

There are different types of bile acids and they appear to be commonly released by stream fishes. Earlier work conducted in Sorensenís fish lab demonstrated that bile acids also function as attractants for migrating sea lamprey, a species that invaded the Great Lakes. Thwaits and Sorensenís research has revealed that Kamloops release a mixture of three bile acids: taurocholic acid, taurochenodeoxycholic acid, and cyprinol sulfate.

Adults detect these compounds with acute sensitivity and specificity. In fact, trout are so sensitive to the odor of these bile acids that they can detect them at concentrations of less than a thimble-full in a billion gallons of water. Thwaits and his colleagues also conducted classical conditioning experiments demonstrating that adult trout can remember the odor of bile acids for many months. Unique sets these digestive chemicals released in the feces and urine of stream-resident juvenile trout seem to be part of a natural home stream odor for returning adults.

Fisheries biologists and managers are realizing that a clearer understanding of the way some fish species detect and respond to specific odors may lead to new management strategies. For instance, bile acids might eventually be used to lure trout and salmon back to locations where they are now threatened. Bile acids could also be used to increase the rates at which stocked salmonids return to selected spawning streams. For sea lamprey, careful application of bile acids will likely soon become a means for reducing population size.

Bile acids probably don’t serve as pheromones in salmonid homing like they do for lampreys, explained Thwaits. Pheromones, by definition, are species-specific and elicit instinctive behavioral responses. The bile acids Thwaits and his colleagues identified are also released by other species, although in far smaller quantities. Even though the trout bile acids do not function as ‘classic’ pheromones, they do fulfill criteria to be considered an important part of learned home stream odor, said Thwaits.

By Sharon Moen
June 2006

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