Juvenile life history, migration, and habitat use of natural- versus hatchery-origin Chinook salmon

Objectives: Our overall objective was to directly compare the early life
history and migration patterns of hatchery and natural-origin fish and
determine when hatchery fish provide good surrogates for natural-origin
winter-run Chinook Salmon (Oncorhynchus tshawytscha) in the Sacramento
River system. Specifically, our objectives were to compare the juvenile
freshwater habitat use, residence time in freshwater habitats, and
size-at-age and relative growth of juveniles. Methods: Using otoliths
collected from adult carcasses, we reconstructed the freshwater life
history of hatchery and natural-origin fish through microstructural and
chemical analyses. We analyzed the otolith strontium isotope chemistry and
daily increment widths to characterize habitat use, migration timing, and
growth. Results: Following their hatchery rearing period, hatchery fish
had much shorter residence times (11 days, SD = 16 days) in the river than
natural-origin fish (129 days, SD = 37 days). Beyond the reduced duration
of river residence, hatchery fish occupied fewer areas of the watershed
during ocean outmigration. Despite these differences in outmigration
behaviors, adults from both groups reached saltwater during their
downstream outmigration at similar sizes (natural-origin: 633 µm, SD = 68
µm; hatchery-origin: 625 µm, SD = 63 µm) and ages (natural-origin: 161
days old, SD = 20 days; hatchery-origin: 165 days old, SD = 22 days) as
juvenile smolts. Conclusions: Considering differences in habitat use and
timing between hatchery and natural-origin fish is critical to avoiding
bias when applying data collected from hatchery fish to manage and
conserve natural populations. Explicitly considering the presence of
winter-run Chinook Salmon in habitats beyond the migratory corridor during
habitat management is needed to promote and enable the contribution of
multiple life histories for productivity, and resilience, and ultimately
recovery.