Otolith-inferred patterns of marine migration frequency in Nunavik Arctic charr.

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Alternative migratory tactics in salmonids reflect the large observed inter-individual variation in spatial behaviour which may range from strict freshwater residency to uninterrupted anadromy. In Salvelinus, sea migrations are performed during the ice-free period as freshwater over-wintering is thought to be obligatory due to physiological constraints. As a result, individuals can either perform a migration the next spring or remain in freshwater, as anadromy is generally considered facultative. In Arctic charr (S. alpinus), skipped migrations are known to occur but limited data are available about their frequencies within and among populations. Here, we used an otolith microchemistry approach relying on Strontium (88 Sr) to infer movements between freshwater and marine habitats, and annual oscillations in Zinc (64 Zn) to help with age identification. We determined the age-at-first-migration and the occurrence of subsequent annual migrations in two Nunavik Arctic charr populations sampled in Deception Bay (Salluit) and, river systems linked to Hopes Advance Bay (Aupaluk), northern Québec, Canada. The mode for age-at-first-migration was 4+ for both populations, although it exhibited large variation (range: 0+ to 8+). Skipped migrations constituted a rare event, as 97.7% and 95.6% of the examined Arctic charr at Salluit (n = 43, mean age = 10.3 ± 2.0 years) and Aupaluk (n = 45, mean age = 6.0 ± 1.9 years), respectively, were found to have performed uninterrupted annual migrations after initiation of the behaviour. The consistency of the annual migrations suggests that the tactic is sufficiently fitness-rewarding to be maintained under current environmental conditions. From a fisheries management perspective, these repeated migrations combined with low site fidelity may lead to large interannual variations in abundance at the local scale, which may represent a challenge for monitoring Arctic charr demographics on a river-by-river basis. This article is protected by copyright. All rights reserved.