Physiological Mechanisms of Olfactory Imprinting and Homing in Pacific Salmon

It is now widely accepted that specific odors of the natal stream are imprinted in the olfactory nerve systems of juvenile salmon during downstream migration and that adult salmon recall specific odors to recognize the natal stream during upstream migration. In my laboratory, several different physiological researches have been applied to clarify mechanisms of salmon olfactory imprinting and homing using four anadromous Pacific salmon (pink, chum, sockeye, and masu salmon) migrating from the North Pacific Ocean to Hokkaido, as well as lacustrine sockeye and masu salmon in Lake Toya, Hokkaido, Japan. In molecular biological researches, we examined gene expression levels of hormone in the brain-pituitary-thyroid axis during juvenile imprinting migration and in the brain-pituitary-gonad axis during adult homing migration, respectively. In behavioral researches using two-choice test tank (Y-maze), we compared whether artificial natal stream water (ANSW) prepared by the same composition of dissolved free amino acid in each natal stream have attractive effects on upstream selective movement among four Pacific salmon, and revealed that these salmon except for pink salmon showed significant selectivity to each ANSW of their natal stream. These findings are discussed in relation to physiological mechanisms of olfactory imprinting and homing in anadromous and lacustrine salmon.