Wild Chinook salmon (Oncorhynchus tshawytscha) and coho salmon (O. kisutch) from the Horsefly River in central British Columbia vary in adult migration timing, but often rear sympatrically as juveniles. These species provide an interesting opportunity to investigate adaptations to the environment in the juvenile life-history stage, which may differ from adaptations at the adult stage. Temperature preference did not differ significantly between Chinook salmon and coho salmon. Aerobic scope (difference between maximum and routine metabolic rates) was also similar between the two species, and neither showed a distinct temperature optimum for peak physiological performance. Coho salmon had a significantly higher upper thermal limit compared to Chinook salmon, although differences were small and may not be biologically meaningful. Thus, these two populations appear to be well adapted for their current environmental conditions rather than show any tendencies to be differentially suited to selective pressures they will experience as adults.
