Return migration to fresh water and overwintering locations used by sympatric anadromous populations of Arctic char (Salvelinus alpinus) and Dolly Varden (Salvelinus malma malma).

Anadromous Arctic char (Salvelinus alpinus) and northern form Dolly Varden (Salvelinus malma malma) are highly valued by Indigenous communities in northern Canada. Rapid, ongoing environmental changes are affecting populations of both of these iteroparous species, and understanding variability in life-history tactics is essential for assessment of current and future impacts. The Coppermine River, Nunavut, provides a unique opportunity to study anadromous life-history types of Dolly Varden and Arctic char that occur in sympatry in fresh water and have documented inter- and intra-specific variability in overwintering locations. We used acoustic telemetry and physical samples from community collection programs to infer that Arctic char overwintered in easily accessible habitats in the lower reaches of the Coppermine River, likely in non-spawning years, and that spawning likely occurs in rivers other than the Coppermine. Younger and smaller Dolly Varden overwintered in the river below a substantial obstacle (a large cascade), along with some larger adults that we suggest skipped spawning. Larger Dolly Varden, including potential spawners, migrated above the cascade and overwintered in the upper reaches of the Coppermine River. Arctic char exhibited distinct patterns of return migration to fresh water that were consistent with intent to overwinter below the cascade. Dolly Varden that overwintered above the cascade exhibited distinct patterns of return migration that were consistent with directed movements. Patterns of return migration to fresh water for Dolly Varden that overwintered below the cascade suggested some individuals intended to overwinter below the cascade, whereas others may have attempted to overwinter in the upper reaches of the river but failed to ascend the cascade. The patterns and variability we observed in overwintering location and return migration to fresh water have important implications for community-based management of the local fishery, provide insights into the ecology and adaptability of Arctic char and Dolly Varden, and highlight a need for additional information regarding spawning locations and spawning frequency in this system.