Evolutionary relationships between landlocked and anadromous Atlantic Salmon populations in the North Shore region of the Gulf of St. Lawrence

Atlantic Salmon ( Salmo salar) are typically anadromous; however, some individuals complete their life cycle in freshwater. A widely documented scenario suggests that this resident tactic arose independently in each river via isolation from anadromous Atlantic Salmon populations after the last ice age. Yet, origin of residency remains poorly studied in the North Shore region of the Gulf of St. Lawrence (Canada). To address this, we genotyped 189 resident and 196 anadromous individuals from five watersheds at 43 microsatellite markers. We found marked genetic differences between tactics within rivers, likely resulting from different levels of gene flow associated with geographic isolation, suggesting that residents may not have always been isolated as expected. Moreover, exploratory demographic inference analyses suggest that residents may have a common ancestral source, which does not support the strict independent development model typically expected. This pattern is rather more consistent with a two-wave colonization events and/or ancestral intracontinental gene flow. These findings bring nuances and complexity to views on the origin of residency in Atlantic Salmon, and are valuable for guiding conservation practices.