Ecological and Genetic Impacts of Introduced Salmonids on Native Fishes in Riverscapes: Implications Under a Warming Climate

The invasion of nonnative fishes is often facilitated through a complex interaction of biotic and abiotic factors operating at multiple spatial and temporal scales within riverscapes.  Human-mediated hybridization can be a major consequence of nonnative species invasions and is a leading cause of biodiversity loss worldwide.  How hybridization affects ecological adaptations, what factors influence the spread of hybridization, and what level of hybridization is permissible pose difficult conservation questions with little empirical information to guide policy and management decisions.  This is particularly true for salmonids, where widespread introgression among nonnative and native taxa has often created hybrid swarms over extensive geographic areas resulting in genomic extinction. Interbreeding between westslope cutthroat trout (Oncorhynchus clarkii lewisi) with nonnative rainbow trout (O. mykiss) exemplifies the conservation challenges of interspecific hybridization.  We present over 30 years of research that has investigated the behavioral, ecological, and fitness consequences of hybridization between native westslope cutthroat trout and introduced rainbow trout and the spatiotemporal factors influencing the spread of hybridization.  Within this framework, we integrate our research with anticipated changes in regional climatic patterns to predict how and where climate change may affect the spatial extent and levels of introgression at multiple scales.  Our results suggest that hybrids are not only genetically different than westslope cutthroat trout but also have significantly reduced fitness and are ecologically different.  Furthermore, we found that hybridization is more likely to occur and spread in streams with warm water temperatures, increased land use disturbance, and near the main source of hybridization.  We conclude that hybridization is likely to continue to spread under a warming climate if hybrid populations with high amounts of rainbow trout admixture are not reduced or eliminated.  Extant aboriginal cutthroat trout are at greater conservation risk due to hybridization than previously thought and policies that protect hybridized populations may need reconsideration.