Megan McPhee, research assistant professor at the University of Montana’s Flathead Lake Biological Station, steers us to an answer:
This behavior is best exemplified by salmon, which combine conventional open-water navigation and a keen sense of smell to find their way. Salmon can migrate out to sea to feed for several years before returning to spawn in the same stream, sometimes even the same section of stream, in which they were born. Other homing species probably use similar mechanisms, but few can match such precision.
How salmon return to the correct shoreline region is not completely understood. It appears they use some form of “map and compass” navigation based on information about position and direction of travel. This information most likely comes from a suite of environmental cues, including day length, the sun’s position and the polarization of light that results from its angle in the sky, the earth’s magnetic field, and water salinity and temperature gradients. Whatever the specific mechanism, as spawning time approaches, salmon have a seemingly inherited tendency to orient themselves toward the area of coastline where their natal waterway discharges.
We know in much more detail how they navigate after they find the river mouth. By the time salmon reach freshwater, they are guided largely by their sense of smell to the correct tributary. A series of experiments beginning in the 1950s demonstrated that young salmon become particularly sensitive to the unique chemical odors of their locale when they enter the smolt period (when they begin their downstream migration to the sea). Odors that the smolts experience during this time of heightened sensitivity are stored in the brain and become important direction-finding cues years later, when adults attempt to return to their home streams.
In one early experiment, salmon that were reared in one stream and then moved to a hatchery during the smolt stage returned to the hatchery, demonstrating the crucial role of imprinting during that transformative period of the fish’s life. More recent work has suggested young salmon may go through several periods of imprinting, including during hatching and while emerging from their gravel nests.
This multistage idea makes sense because many wild salmon spend more than a year in freshwater and have moved a considerable distance through complex surroundings by the time they become smolts. In contrast, hatchery salmon imprint to a much simpler environment, which may help explain why they tend to stray (return to the “wrong” stream) more frequently than wild fish do.
Editor's Note: This story was originally published in the December 2008 issue of Scientific American.