High Levels of Straying in Returning, Repeat-Spawning Atlantic Salmon Between Two Neighbouring Danish Rivers

Atlantic salmon (Salmo salar) are iconic for their homing capacity, with most populations migrating into the ocean and eventually returning to their natal river. However, straying is known to occur, with small portions of the population returning to different rivers. Here, we studied the seaward and return migration of Atlantic salmon kelts from wild and hatchery origin, in River Skjern, Denmark. Kelts were captured, tagged and released along River Skjern, or tagged at the local stocking facilities following gamete removal. Of the 199 tagged fish (134 wild and 65 stocked), 110 reached the ocean (55%). Wild-spawned kelts captured and released along the river were the most likely to survive (63 out of 97, 65.6%) while kelts tagged after stripping and reconditioning at the stocking facilities had the lowest survival rates (5 out of 25, 20%). Only 10 kelts were recorded returning after spending more than 1 year at sea (five wild and five stocked fish). While this constitutes a limited sample size, seven of the 10 returning fish (four wild, three stocked) entered a neighbouring river instead (i.e., strayed). This high straying may be due to (1) a weakening of homing ability with age, (2) the returning fish encountering the neighbouring river first during their southward return-migration from the Atlantic Ocean and mistakenly entering it given the similarity between the two river systems, (3) some fish actually originating from the neighbouring river, or (4) the influence of long-term hatchery-rearing practices, which are known to increase straying rates. Although the small sample size of repeat spawners limits firm conclusions, the observed high straying rates raise concerns for monitoring and management that depend on accurate adult return counts, particularly in systems with a history of stocking.