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Migration in Salmon

Salmon undergo a remarkable odyssey during their lives. Young salmon leave the freshwater streams in which they hatched, move thousands of kilometers into the open ocean, and then return as adults to spawn in their natal stream. The question of how they find their way back after many months and an enormous travel distance has intriqued scientists for decades.
Starting with nearly adult fish, which initiate their journey to freshwater, the migration can be broken down into two phases. The first is locating the coastline or general area of the freshwater source they are seeking. The second is the specific identification of the stream from which they migrated earlier in life.
While they can find the coastline without experiencing the outward swim, they cannot locate their natal stream without that experience. Thus it appears they learn some of the vector information on their outward migration. Salmon have magnetite and may be able to use geomagnetic information in orientation; such information might allow them to find a coastline in the absence of an orientational vector.
Consequently, as in honeybees and homing pigeons, salmon need a directional vector, and perhaps an expected travel distance, to achieve the coastline. Path integration could be used to determine their inward vector from their outward path. As an alternative to path integration, the directional vector may be genetically encoded. There is little evidence to help sort out the possible mechanisms for obtaining the travel vector.
Given that the salmon have do have a travel vector for long distance migration, they have several possible sources of compass information for orienting their travel. These include polarized light, the sun, and geomagnetic information. Ocean temperatures and currents may also provide information.
Once the salmon are close to their natal stream, they identify the appropriate freshwater source by olfaction. Young salmon, perhaps primed by the hormone thyroxine, imprint on a stream-specific chemical signature. This imprinting may take place during the transition from the parr to the smelt stage in development. Understanding the imprinting period is critical to developing strategies for restocking salmon streams from hatchery stock; this is an area of current research interest.

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