Minor shifts towards more natural conditions in captivity improve long-term survival among reintroduced Atlantic salmon

Elevating winter water temperatures is a common practice when rearing salmonids for supplementation or reintroduction. Doing so elevates developmental rates, producing larger juveniles with greater smolt-to-adult survival, but does not guarantee improved adult returns to stocked tributaries. To test whether more natural developmental conditions improve adult returns to stocked tributaries, three consecutive cohorts of yearling (age 1+) landlocked Atlantic salmon (Salmo salar) were released into two tributaries of Lake Champlain. Cohorts were reared under two winter thermal conditions (seasonal surface water and above-seasonal groundwater) and (or) two release times (early and normal). Relative to standard hatchery practices, modelled returns to experimental tributaries increased over replicate cohorts by 286% on average following exposure to seasonal rearing temperatures, but decreased by 89% on average when release dates were advanced. By utilizing cost-effective shifts towards hatchery rearing techniques that more closely resemble natural growth conditions, we demonstrate how hatchery programs may improve long-term survival and returns for fish species with complex life histories involved in supplementation and reintroduction programs.