The Saimaa landlocked salmon (Salmo salar m. sebago) in Finland is a critically endangered ecomorph due to the historical damming of rivers. The morph has been dependent on stockings of hatchery-reared juveniles for more than 40 years. Recently, various efforts have been taken to restore some of the earlier reproduction areas to return the natural life cycle. However, as the population has been reared in hatcheries over many generations and as the released fish generally suffer high post-release mortality, it is crucial to know whether domestication has affected the population’s fitness-related traits and to assess how the developmental environment affects salmon’s phenotype. In the present experiment, we studied the role of the early developmental environment in trait formation by comparing juveniles (0+ years) from three different backgrounds: 1) semi-wild fish from the River Ala-Koitajoki (stocked as alevins), 2) fish grown in standard hatchery rearing conditions and 3) fish grown in enriched hatchery rearing conditions. All the study fish originated from the same generation and had the same genetic background. We tested their capability of feeding on natural prey, critical swimming speed and studied their morphology. Our preliminary analyses indicate notable background-dependent variation in all the studied traits. Semi-wild fish consumed more natural prey items, had higher critical swimming speed and differed in their fin morphology, when compared to hatchery reared fish. The developmental environment seems to produce differences in key traits, but how this is linked to survival in the wild remains to be studied in the near future.