The environment can play an integral role on phenotypic development of an organism. In this thesis, I experimentally tested the influence of structural complexity on phenotypic development and post-release performance of juvenile Atlantic salmon (Salmo salar) when released into semi-natural and natural environments. From an applied perspective, I was interested in investigating alternative rearing strategies of juvenile Salmo salar with the goal of enhancing performance of fish when released into nature. In my first data chapter, I conducted an experiment to test the influence of environmental enrichment during incubation (i.e. addition of gravel to create a "complex" environment) on the phenotype and performance of juvenile salmon in a semi-natural environment. At the culmination of endogenous feeding (i.e. "emergence"), fish that incubated in a complex environment were heavier, in better body condition, fed more on novel prey, and took longer to reappear from a shelter after a simulated predator attack. In addition, when transferred from incubation environments to semi-natural stream channels, fish originating from the complex incubation environment expressed enhanced growth and survival. These findings suggest that Salmo salar are plastic to environmental factors during incubation and developmental differences could be contributing to performance in semi-natural stream channels. There were, however, no commensurate differences in brain volume with those observed in behaviour and performance, which may suggest that other developmental processes in the brain of juvenile Atlantic salmon are occurring that are not reflected in overall brain volume. In my second data chapter, I reared the fish from the preceding incubation study for 60 days after emergence in tanks with ("complex") or without cobble substrate ("simple"). I used a reciprocal approach to rearing and moved fish between complex and simple environments for two 30-day rearing periods to investigate if timing or duration of timing of exposure to habitat complexity influenced growth and condition in the hatchery and subsequent survival in semi-natural and wild environments. I found that fish reared in complex tanks exhibited higher body condition (i.e. heavier for a given length) for the first 30 days, however, differences generally faded during the second 30 days of hatchery rearing. Additionally, I observed rearing groups with smaller fish, on average, at release expressed higher growth rates in semi-natural stream channels and in the wild, such that sizes and conditions did not differ among groups at final recapture. Estimated survival after release into the wild (over ca. 260 days) was higher for fish incubated in a complex environment and reared in a simple environment for consecutive 30 d rearing periods (Complex>Simple>Simple, or "CSS") and those in SCC treatment than fish from 3 of the 7 other groups (CCC, CSC, SSC). There were no other differences in survival between treatment groups. Survival in the semi-natural stream channels was high (> 85%) and I did not detect differences between groups. The results presented in this thesis suggest that phenotypic traits of juvenile Atlantic salmon are sensitive to environmental factors early in development. Enriched environments containing gravel and cobble may serve to provide cover for juvenile fish during early life, allowing them to rest, hide from predators, metabolize, and allocate energy towards somatic growth and development. Such enrichment can provide fish an advantage when released into the wild, though effects may be complicated. As a rearing strategy for the release of salmon into nature to re-establish depleted populations, modifications are likely needed to simplify maintenance of rearing tanks, however this study serves as a benchmark for future efforts at alternative rearing techniques for enhancing performance of released juvenile Salmo salar in nature.