Evidence of an Olfactory Imprinting Window in Embryonic Atlantic Salmon

Salmonid olfactory imprinting during the parr-to-smolt transformation (PST) is essential for successful homing to natal spawning sites years later by mature fish. However, in many species, juveniles may move away from spawning sites before the PST, yet home to the natal site as adults, indicating that there may be multiple imprinting windows. To test the hypothesis that olfactory learning can occur prior to the PST, we exposed Atlantic salmon (Salmo salar) to the imprinting odorant L-arginine at the alevin stage. Odour exposures were conducted in both brook and well water to test whether environmental characteristics of the rearing water affected endocrine and imprinting responses. Changes in thyroid hormone concentration differed over the course of development between water sources (p < .001). In brook water, fish experienced a plateau in thyroxine concentrations coinciding with the L-arginine exposure, but a continued decline was observed in those kept in well water. In subsequent experiments, migratory stage adults from the brook water-reared group exposed to L-arginine spent significantly more time than control fish (p < .04) in the L-arginine odour arm of a two-choice maze, whereas the well water-reared group's responses did not differ from those of the control group. Our findings provide physiological and behavioural evidence supporting the hypothesis that imprinting in Atlantic salmon can occur before the PST. These results indicate that the water source during development may affect the ability to imprint and may have implications for hatchery programmes that often incubate salmonids in well water during the alevin life stage.