Invasive hybridization has unpredicted and variable effects on trout survival in contrasting environments

Invasive hybridization between native and introduced species is widespread, especially in fishes, yet its effects on fitness-related traits across environments remain poorly understood. We combined individual genetic admixture estimates with capture–recapture data to quantify how hybridization between native Westslope Cutthroat Trout (Oncorhynchus lewisi) and invasive Rainbow Trout (Oncorhynchus mykiss) influences survival. We measured individual survival of 5186 trout across three populations in contrasting environments. Non-native admixture significantly affected survival in two populations, with positive effects during summer and negative effects during winter. Prior research has shown hybridization increases with stream temperature; however, survival patterns did not follow this trend. In a warmer stream, native trout in all size-classes had ≥20% higher annual survival than hybrids, whereas in a cooler stream, juvenile native trout had ≥20% lower survival than hybrids. These findings indicate that hybridization effects on survival are complex and vary among populations, environments, and size-classes. Landscape patterns of hybridization may therefore not reflect local survival (or fitness) outcomes, emphasizing that measures fitness-related traits across populations and environments could elucidate the eco-evolutionary consequences of invasive hybridization.