Local Habitat, Watershed, and Biotic Factors Influencing the Spread of Hybridization between Native Westslope Cutthroat Trout and Introduced Rainbow Trout

Abstract

The invasion of nonnative fishes in freshwater systems is often facilitated by the interaction of biotic and abiotic factors operating at multiple spatial and temporal scales. We evaluated the association of local habitat features (width, gradient, and elevation), watershed characteristics (mean and maximum summer water temperatures, the number of road crossings, and road density), and biotic factors (the distance to the source of hybridization and trout density) with the spread of hybridization between native westslope cutthroat trout Oncorhynchus clarkii lewisi and introduced rainbow trout O. mykiss in the upper Flathead River system in Montana and British Columbia. The presence of hybridization and the proportion of rainbow trout admixture were estimated using seven diagnostic microsatellite loci. We defined logistic and linear regression models including various combinations of spatial and environmental factors and used an information-theoretic approach to evaluate the relative plausibility of these models. Models combining measures of water temperature, disturbance, and source connectivity were the best-approximating ones for the presence of hybridization. Hybridization was positively associated with mean summer water temperature and the number of upstream road crossings and negatively associated with the distance to the main source of hybridization. The best-approximating models associated with the level of introgression among hybridized sites included measures of temperature, source connectivity, and the density of trout. The proportion of rainbow trout admixture was negatively related to the distance to the source and positively related to mean summer water temperature and density. Our results suggest that hybridization is more likely to occur and spread in streams with warm water temperatures, increased land use disturbance, and proximity to the main source of hybridization. However, habitat features alone may not limit the spread of hybridization; populations with high proportions of admixture and high densities may have to be reduced or eliminated.