Abstract
Understanding factors influencing survival of Pacific salmonids (Oncorhynchus spp.) is essential to species conservation, because drivers of mortality can vary over multiple spatial and temporal scales. Although recent studies have evaluated the effects of climate, habitat quality, or resource management (e.g., hatchery operations) on salmonid recruitment and survival, a failure to look at multiple factors simultaneously leaves open questions about the relative importance of different factors. We analyzed the relationship between ten factors and survival (1980–2007) of four populations of salmonids with distinct life histories from two adjacent watersheds (Salmon and Scott rivers) in the Klamath River basin, California. The factors were ocean abundance, ocean harvest, hatchery releases, hatchery returns, Pacific Decadal Oscillation, North Pacific Gyre Oscillation, El Niño Southern Oscillation, snow depth, flow, and watershed disturbance. Permutation tests and linear mixed-effects models tested effects of factors on survival of each taxon. Potential factors affecting survival differed among taxa and between locations. Fall Chinook salmon O. tshawytscha survival trends appeared to be driven partially or entirely by hatchery practices. Trends in three taxa (Salmon River spring Chinook salmon, Scott River fall Chinook salmon; Salmon River summer steelhead trout O. mykiss) were also likely driven by factors subject to climatic forcing (ocean abundance, summer flow). Our findings underscore the importance of multiple factors in simultaneously driving population trends in widespread species such as anadromous salmonids. They also show that the suite of factors may differ among different taxa in the same location as well as among populations of the same taxa in different watersheds. In the Klamath basin, hatchery practices need to be reevaluated to protect wild salmonids.
Highlights
The existence of independent local populations within a region strengthens species resiliency, the ability of a species to replenish itself after high mortality events
Correlations and linear mixed-effects models Only two variables were significantly correlated with Salmon
A best model could not be determined between models made up solely of summer flow t or ocean abundance because the models differed in the length of Salmon River spring Chinook salmon Ocean abundance t Flow t Salmon River fall Chinook salmon Iron Gate Hatchery (IGH) returns t Salmon River summer steelhead Flow t Scott River fall Chinook salmon Ocean abundance t X IGH returns IGH returns t doi:10.1371/journal.pone.0098392.t002
Summary
The existence of independent local populations within a region strengthens species resiliency, the ability of a species to replenish itself after high mortality events. This is possible because local populations contribute genetic and phenotypic diversity, including diversity in life history that reflects local adaptation. In Pacific salmonids (Oncorhynchus spp.), such diversity in life-history traits helps to maintain populations [1] through spreading risk of extinction due to catastrophic events [2] or long periods of unfavorable conditions [3] among life-history variants. Studies suggest that populations with diverse life histories have greater stability in numbers [2] and probabilities of persistence [4], even when environmental conditions vary [5,6]. The ability of Pacific salmon to persist in changed environments in the future (e.g., because of climate change) requires protection of locally-adapted populations and their habitats [10,11,12]
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