Survival, growth, and out-migration timing of reintroduced Atlantic salmon (Salmo salar) in Cobourg Brook, Ontario

Lake Ontario once hosted an abundant population of Atlantic salmon (Salmo salar), which died out prior to the 20th century due to human disturbances. A recovery effort has recently been established which uses an experimental approach by stocking three Atlantic salmon life stages into best-bet Lake Ontario tributaries. This thesis focused on the in-stream phase of the Atlantic salmon life cycle to evaluate the comparative growth, survival, and out-migration timing of stocked juvenile life stages. Fry and yearling Atlantic salmon were stocked as a paired-release introduction into Cobourg Brook, and evaluated for differences in survival, growth, and timing of migration/movement into Lake Ontario. In addition, fall fingerlings and age-2 Atlantic salmon were also stocked into the study site, and some insight into their performance was provided. Stocked yearlings exhibited poorer growth, but produced significantly more and larger out-migrants than salmon stocked as fry. Atlantic salmon stocked as fry displayed typical smolt behaviour and out-migrated during the predicted environmental smolt window for Cobourg Brook, whereas stocked yearlings out-migrated during atypical and environmentally suboptimal times. Older life stages appear to display greater short-term survival by avoiding high mortality events in the wild, however, maladaptive behaviors may develop due to increased time in captivity which may reduce overall fitness and subsequently their efficacy at re-establishing wild populations. This generalization may be valuable for future native species recovery programs when choosing a life stage to release into the wild. By integrating the new information provided in this thesis and re-evaluating information needs, the experimental approach employed in the current effort to re-establish Atlantic salmon populations in Lake Ontario tributaries would not only assist the repatriation of an single extirpated species, but also aid in the restoration of a highly valued ecosystem and help advance our knowledge of effective methods to reintroduce extirpated wildlife populations.