How Do Spawning Fish Find Their Way Back?
This is one question that has baffled me for quite some time (and this applies to homing pigeons as well, and err … cats). How is it that some fish, best exemplified by salmon, which can migrate out to sea to feed for several years, still manage to find their way back to the same stream, and sometimes even the same section of the stream in which they were born.
Though the exact manner in which they do it is still not completely clear, this article from Scientific American gives some pointers.
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.
By the time salmon reach freshwater, they are guided largely by their sense of smell to the correct tributary.
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.
