Mechanisms and timing of olfactory and magnetic imprinting and homing in Atlantic salmon (Salmo salar)
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Atlantic salmon were extirpated from Lake Champlain, Vermont nearly 200 years ago, and efforts to restore self-sustaining populations of wild-spawning salmon have been unsuccessful. We conducted three studies that may provide a better understanding of this species’ life history, to help guide Atlantic salmon restoration in Lake Champlain and serve as a model for salmon conservation elsewhere. Most salmon are anadromous, renowned for long-distance “homing” migrations from the ocean to their natal streams to spawn. However, the populations in Lake Champlain are “landlocked” (i.e. nonanadromous), featuring migrations to and from the lake instead of the sea. Salmon migrations can be divided into two distinct phases: a riverine phase, in which juvenile fish learn home stream odors (“imprint”) as they swim downstream and spawning adults follow those odors as they swim upstream; and an oceanic phase (lake phase, for nonanadromous populations) in which adult salmon migrate from distant feeding areas to the mouths of their natal rivers. We investigated dissolved free amino acids (DFAA) in salmon-bearing streams of New England to determine if DFAA could be an imprinting and homing odorant used by Atlantic salmon. While undetected in measurable concentrations in the water column of our study streams, DFAA were found in water from benthic layers. A subsequent experiment demonstrated that DFAA mixture compositions are different between salmon rivers while remaining similar between seasons within the same river. We also found evidence for an early imprinting period in Atlantic salmon, between hatching and emergence from their gravel nests – a period when wild fish first experience benthic water. The combined results suggest that hatchling salmon could imprint on benthic DFAA and later use this information to guide upstream migration as adults. Finally, a very different experiment was aimed at understanding the possible use of Earth’s magnetic field for long-distance navigation. In artificially manipulated magnetic fields, anadromous Atlantic salmon showed an innate preference for compass directions that in nature would steer them toward feeding grounds. However, non-anadromous fish showed no magnetic preference, raising the question of whether and how this sense may be employed on a scale relevant to lakes rather than oceans.