Human-mediated hybridization threatens global biodiversity, but the fitness consequences of hybridization are poorly understood, especially in vertebrates. We used capture-recapture data from 5,249 individuals in three hybridizing populations of invasive rainbow trout (Oncorhynchus mykiss) and native cutthroat trout (O. clarkii) to quantify the effects of rainbow trout genetic admixture and environmental conditions on survival. Seasonal variation in environmental conditions interacted with individual admixture to influence seasonal survival. Overall, annual survival declined with admixture at the warmest site (juvenile hybrids with 25% non-native ancestry had 20.7% lower annual survival than non-hybridized cutthroat trout) but increased with admixture at the coldest site (juvenile hybrids with 25% non-native ancestry had 19.7% higher survival than non-hybridized cutthroat trout), providing evidence that fitness effects of hybridization vary in a context-dependent manner. Furthermore, higher survival for hybrids in cold streams and lower survival in warm streams is opposite of admixture-environment correlations, demonstrating that environmental gradients often used to infer patterns of hybridization may poorly predict variation in fitness across the landscape.

Individual-based studies measuring fitness-related traits in vertebrate taxon are rare. Most studies of human-mediated hybridization in wild populations quantify the spatial extent of non-native admixture or test for admixture-environment associations to infer factors affecting fitness in hybridizing populations. We used an extensive capture-recapture dataset to measure seasonal survival in hybridized populations that experience different environmental conditions. Our work shows that non-native admixture can affect survival differently among seasons and populations, and that inferences from individual-based studies can conflict with those from admixture-environment associations. For studies of non-native hybridization, this work highlights the importance of directly measuring fitness (survival and reproductive success), and at appropriate spatial and temporal scales to elucidate the variation in fitness differences (i.e., among populations and seasons).

Capture-recapture data were collected via backpack electrofishing and Mobile PIT surveys in July and October of each year. Migration was detected via migratory fish traps and fixed PIT antenna at the mouth of each creek. The environmental data were collected with HOBO dataloggers and multipass depletion estimates. See Strait et al. submitted for detailed methods.

The attached file contains all the information necessary to run the multstate models presented in Strait et al. submitted. The file is organized to run as as RStudio project. The file contains a README file that explains the structure, data, r scripts, jags models, and results, including definitions of data columns.