Stress‐linked morphological change associated with rearing techniques of hatchery‐reliant endemic landlocked Atlantic salmon ( <scp> <i>Salmo salar</i> </scp> m. <scp> <i>sebago</i> </scp> )

Domestication effects caused by hatchery rearing impact various traits in fishes. Lake Saimaa landlocked salmon (Salmo salar m. sebago) have been dependent on hatchery propagation for over 50 years. The population is therefore at risk of hatchery-induced phenotypes, which can be suboptimal in the wild, thus impacting the long-term viability of the already critically endangered population. To assess the impact of rearing techniques, one-summer-old landlocked salmon morphology and asymmetry (an indicator of stress) was compared between four different hatchery rearing backgrounds, with all fish originating from the same family groups. The first group was maintained under standard hatchery conditions, the second under enriched rearing conditions (varying water level, direction and velocity), the third in semi-natural hatchery conditions (outdoor stream ponds and natural prey) and the fourth group experienced wild conditions (stocked in the River Ala-Koitajoki as alevins in spring and electrofished in September). Fish were photographed from both left and right sides and morphology was compared between rearing types using classical linear measurements as well as geometric morphometrics. Results indicate that natural conditions produce more symmetrical fish that have longer pectoral fins than their hatchery conspecifics, whereas fish reared in semi-natural conditions show increased asymmetry. These results suggest that different rearing types elicit varying levels of stress, and that conservation efforts encouraging early-stage stocking in the wild could be recommended when natural reproduction is not possible.