CHANGES IN THE OLFACTORY FUNCTION OF ATLANTIC SALMON, SALMO-SALAR, IN THE COURSE OF SMOLTIFICATION

Electrophysiological responses were recorded from the surface of the rostromedial and rostrolateral regions of the right olfactory bulb of Atlantic salmon, Salmo salar, during stimulation of the olfactory epithelium with different concentrations Of L-alanine and taurocholate. Fish exposed to taurocholate prior to testing were compared with nonexposed fish. The results revealed significant week-to-week differences in olfactory activity in nonexposed fish, with two peaks in activity of similar magnitude. The first peak occurred at the beginning of smoltification, when olfactory sensitivity reached its maximum (low response threshold). The second peak occurred after smoltification, when sensitivity was roughly 10 times lower (higher response threshold) than during the first peak. Fish exposed to taurocholate prior to testing did not exhibit these peaks in olfactory activity. Marked changes in the spatial distribution of the electrophysiological response to L-alanine and taurocholate occurred throughout the smoltification process in nonexposed fish. The two peaks in olfactory activity that we identified coincide with two peaks in odor learning (Morin et al. 1989. Can. J. Fish. Aquat. Sci. 46: 122-130, 131-136); the first coincides with a unique capacity for odor memory and is apparently a neural manifestation of a sensitive period for olfactory imprinting in Atlantic salmon.