Experimental Evidence for Olfactory Imprinting by Sockeye Salmon at Embryonic and Smolt Stages

Abstract

AbstractAnadromous salmonids have an extraordinary ability to migrate back to their natal streams to spawn as adults, but the mechanisms underlying this ability are not completely known. Many experiments indicate that salmon imprint on natal odors at the smolt stage prior to seaward migration, but the life history and population genetics of some species, notably Sockeye Salmon Oncorhynchus nerka, suggest that imprinting also occurs during the period between hatching and emergence from the gravel as fry. To test the hypothesis that Sockeye Salmon imprint during this period, we exposed juveniles to a mixture of odorants during either the alevin or smolt stage. The smolt exposure group was further divided into different exposure durations (6 weeks, 1 week, and 1 d) to evaluate the duration of odor exposure needed for imprinting during that stage. Imprinting was assessed by testing fish as mature adults in two‐choice mazes containing unfamiliar water with and without the mixture of odorants. Fish exposed either as alevins or for 6 weeks as smolts both spent significantly more time in the odor‐scented arm than control fish (unexposed to the odors as juveniles). Fish exposed to odors for 1 week or 1 d as smolts showed similar but weaker responses. Concurrent measures of gill Na+/K+‐ATPase activity and plasma thyroxine confirmed that the fish exposed as smolts were undergoing parr–smolt transformation during exposure. We conclude that Sockeye Salmon imprinted as both alevins and smolts and that longer periods of odor exposure yielded greater behavioral responses to odors as adults, though specific times within the parr–smolt transformation period may be more sensitive to imprinting than others.Received June 21, 2016; accepted September 13, 2016 Published online December 2, 2016