Many animals undertaking long-distance migrations use Earth’s magnetic field as a “map” to assess their position for orientation. This phenomenon been particularly well-studied in salmonids using “magnetic displacement” experiments, in which animals are presented with magnetic field conditions that are characteristic of other geographic locations. However, whether use of magnetic map cues differs among populations of salmon has not been investigated. Here we show that nonanadromous and anadromous populations of Atlantic salmon (Salmo salar) raised under the same conditions within their native range differ in their response to magnetic displacements in the North Atlantic. The directions adopted by anadromous salmon juveniles to each of the magnetic displacements would support their migration from the eastern U.S. to western Greenland, had the fish actually been at those locations. In contrast, nonanadromous salmon did not appear to respond to the magnetic displacements. The findings are consistent with the hypothesis that the innate magnetic map of anadromous salmon is adapted to guide their marine migration.
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.
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