Olfactory homing of chum salmon to stable compositions of amino acids in natal stream water.

Many attempts have been made to identify natal stream odors for salmon olfactory homing. It has recently been hypothesized that odors are dissolved free amino acids; however, it is unknown whether these odors change on a seasonal or annual basis. We analyzed dissolved free amino acid (DFAA) concentration and composition of water from the Teshio River in Hokkaido, Japan, where chum salmon (Oncorhynchus keta) returned for spawning, during juvenile downstream migration in spring and adult upstream migration in autumn with a 4-year difference. Among the 19 amino acids found in the Teshio River water, DFAA concentrations fluctuated largely, but 5-7 stable DFAA compositions (mole %) were found between the spring and autumn samples over a 4-year span. Two kinds of artificial stream water (ASW) were prepared using the same DFAA concentration in the Teshio River during the time of juvenile imprinting in spring (jASW) and adult homing in autumn (aASW), after a 4-year period. In behavioral experiments of upstream selective movement in a 2choice test tank, 4-year-old mature male chum salmon captured in the Teshio River showed significant preference for either jASW or aASW when compared to control water, but did not show any preference with respect to jASW or aASW. In electro-olfactogram experiments, adults were able to discriminate between jASW and aASW. Our findings demonstrate that the long-term stability of the DFAA compositions in natal streams may be crucial for olfactory homing in chum salmon.