Bile isn't Vile: Identifying the Scent that Lures Trout Home

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

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' findings build on work conducted half a century ago by Arthur Hasler, a pioneer in freshwater ecology who demonstrated that an ability to detect and remember smells enabled salmon to journey thousands of miles to spawn in the stream of their birth. The identity of this home stream odor has remained a mystery...until now.

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

(Left) Great Lakes trout migrating to their spawning grounds. Photo Credit: Jeff Gunderson.

(Right) Ben Thwaits tests the response of a rainbow trout to bile acids. Photo Credit: Dave Hanson

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, found compelling evidence that the odor trail leading home is laced with bile acids, a distinctive class of steroids produced in the liver to aid digestion.

molecular structure of three distinct bile acids that attract rainbow trout

Three bile acids attract rainbow trout. White = carbon, red = hydroxyls, blue = oxygen, yellow = sulfur

Stream-resident juveniles as well as adult trout excrete taurocholic acid, taurochenodeoxycholic acid, and cyprinol sulfate. Adult Kamloops detect these digestive 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.

"It's very interesting to us that rainbow trout can easily distinguish the odor of taurocholic acid from taurochenodeoxycholic acid and, once taught, can do so for at least 6 months," said Sorenson. The chemical difference between the two compounds is one hydroxyl molecule.

The bile acids released by Kamloops are also released by other species, although in far smaller quantities. Because they are not unique to rainbow trout, they are not pheromones, which by definition are species-specific compounds that elicit instinctive behavioral responses. Even though 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.

Other work conducted in Sorensen's fish lab demonstrated that bile acids function as pheromonal attractants for migrating sea lamprey. Careful application of bile acids will likely become a means for reducing the population size of this invasive species in the Great Lakes . Bile acids might eventually be used to restore trout and salmon to areas where populations have dwindled or to increase the rates at which stocked salmonids return to selected spawning streams. Since a variety of stream fish species release unique sets of bile acids in feces and urine, biologists are realizing that the way some species respond to odors may lead to new management strategies.

Bile isn't vile: Identifying the scent that lures trout home

Bile isn't vile:
Identifying the scent that lures trout home