Timing of imprinting to natural and artificial odors by coho salmon (Oncorhynchus kisutch)

Juvenile salmon learn odors associated with their home stream before seaward migration and use these odor memories for homing as adults. Experimental evidence has indicated that this olfactory imprinting occurs during a sensitive period associated with surges in plasma thyroxine levels during parr-smelt transformation; however, many natural migration patterns clearly indicate that home stream learning must occur before the sensitive period at parr-smelt transformation. To examine the timing of olfactory imprinting, we exposed hatchery-reared coho salmon (Oncorhynchus kisutch) to either natural odors or an artificial odorant (beta-phenylethyl alcohol) at specific developmental stages and tested their adult behavioral responses to these odors. Our results confirmed the importance of the parr-smelt transformation as a sensitive period for olfactory imprinting: only salmon exposed to beta-phenylethyl alcohol at the smelt stage demonstrated an increased attraction to this odorant as adults. We found little evidence that salmon learned homing clues prior to the smelt stage. Results further suggested that migration, and the associated stimuli, may also be important for optimal imprinting. Therefore, the absence of presmolt learning in this study may be due, in part, to the stable environment and restrictions on migration associated with hatchery rearing.