Distribution of hybrid trout: Consequenses of site variability for native fish conservation

Hybridization is an important factor to consider for the conservation of many native stream fishes. Introgressive hybridization between introduced rainbow trout and native westslope cutthroat trout tends to result in hybrid swarms. Our primary objective was to determine the geographic distribution of cutthroat-rainbow hybrid trout invading populations of native westslope cutthroat trout in the Jocko River, Montana. We used a spatially explicit sampling approach to detect hybrids throughout a stream network. We assayed 997 fish from 31 sites (median n = 28 fish) at 5 codominant diagnostic microsatellite loci. We detected hybrids at 35 of 46 stream reaches sampled, indicating a spatially extensive distribution of hybrids throughout the basin. We observed little spatial autocorrelation using pairwise reach comparisons of magnitude of hybridization as a function of fluvial distance between reaches. Conversely, there were strong correlations between a magnitude of hybridization metric and distances from putative sources of hybrid dispersers. This indicates hybrid invasion patterns are predicable. We interpret the results of our hybrid invasion study in a conservation context using a theoretical framework describing the ecological and evolutionary consequences of hybrid swarms. The proliferation of hybrid swarms may lead to the loss of native genotypes (genomic extinction) and the formation of evolutionarily novel hybrid taxa that do not function in a manner ecologically equivalent to the native taxa (ecological extinction). In addition, hybrid swarms may not be viable during periods of environmental stress (demographic extinction).