The ability of salmon to home accurately to their natal stream to spawn has long intrigued biologists and has important consequences for the maintenance of population structure in these species. It is known that olfaction is crucial to homing, and that the transition from the freshwater to the marine environment (the parr–smolt transformation; PST) is a period of increased olfactory sensitivity and learning, resulting in a permanent memory of natal site odours that is retained, at least in part, in peripheral sensory neurones. These odours are then used as cues by sexually maturing fish on their homeward migration. We used quantitative polymerase chain reaction techniques to demonstrate transient increases in expression of odorant receptor transcripts (of up to fifty-fold over pre-PST levels) coincident with PST. Both olfactory (SORB) and vomeronasal receptors (SVRA and SVRC) are involved, which suggests that the fish learn both environmental odours and semiochemicals (pheromones). Receptor expression varies between families and changes over time indicating both genetic differences in odour stimuli and multiple periods of olfactory sensitivity. We suggest that changes in OR gene expression may have a role in homing behaviour and thus the maintenance of population structure in Atlantic salmon.
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