Appraisal of hatchery-related effects on Pacific salmonids (Oncorhynchus spp.) is a necessary component of species conservation. For example, hatchery supplementation can influence species viability by
changing genetic, phenotypic and life-history diversity. We analyzed time series data for seven salmonid taxa from the Klamath River basin, California, to investigate trajectories of wild and hatchery adult populations.
Linear regression coupled with randomized permutations (n=99,999), two- tailed t tests, and Bayesian change point analysis were used to detect trends over time. Cross correlation was also used to evaluate relationships
between wild and hatchery populations. The taxa of interest were spring, fall, and late-fall Chinook Salmon (O. tshawytscha); Coho Salmon (O. kisutch); Coastal Cutthroat Trout (O. clarki clarki); and summer
and hybrid Steelhead Trout (O. mykiss). Significant decreases were detected for summer and hybrid Steelhead Trout. The proportion of wild fall Chinook has also significantly decreased concurrently with increases in hatchery returns. In comparison, returns of most Chinook and coho runs to the hatcheries, and fall Chinook strays to wild spawning areas from Iron Gate Hatchery have significantly increased since the 1970s.
Increases were also detected for wild late-fall Chinook and spring Chinook adults. However, both of these were significantly correlated with Chinook Salmon returns to Trinity River Hatchery, suggesting augmentation by
hatchery strays. Changes in abundances appeared related to changing ocean habitat conditions and hatchery practices. Our results suggest that anadromous salmonid populations in the Klamath River basin are becoming
increasingly dependent on hatchery propagation, a pattern that can threaten population persistence.