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Scientists do not know exactly how a salmon "remembers" the way back to its native stream after an ocean journey possibly lasting several years and covering several thousand miles. They agree, however, that salmon, like homing pigeons, appear to have an innate compass or "search recognition" mechanism that does not rely on the sun, moon, stars, or physical signs.
Some scientists theorize that the salmon's internal compass uses the tiny electrical voltages, generated by ocean currents, moving through the earth's magnetic field. Others believe that the salmon's homing mechanism may take its cues from the varying salinities (levels of saltiness) of the water or the specific smells encountered along the journey.
Sources: Frew, Timothy. Salmon, pp. 13-28; Netboy, Anthony. The Salmon, pp. 24-32.
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