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

ABSTRACT 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 that were 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 d, SD = 16 d) in the river than natural-origin fish (129 d, SD = 37 d). Beyond the reduced duration of river residence, hatchery fish occupied fewer areas of the watershed during ocean out-migration. Despite these differences in out-migration behaviors, adults from both groups reached saltwater during their downstream out-migration at similar sizes (natural-origin fish: 633 µm, SD = 68 µm; hatchery-origin fish: 625 µm, SD = 63 µm) and ages (natural-origin fish: 161 d old, SD = 20 d; hatchery-origin fish: 165 d old, SD = 22 d) as juvenile smolts. Conclusions Considering the differences in habitat use and timing between hatchery- and natural-origin fish is critical to avoiding bias when applying data that are 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, resilience, and ultimately recovery.