Data from: Patterns of hybridization among cutthroat trout and rainbow trout in northern Rocky Mountain streams

Introgressive hybridization between native and introduced species is a
growing conservation concern. For native cutthroat trout and introduced
rainbow trout in western North America, this process is thought to lead to
the formation of hybrid swarms and the loss of monophyletic evolutionary
lineages. Previous studies of this phenomenon, however, indicated that
hybrid swarms were rare except when native and introduced forms of
cutthroat trout co-occurred. We used a panel of 86 diagnostic, single
nucleotide polymorphisms to evaluate the genetic composition of 3865 fish
captured in 188 locations on 129 streams distributed across western
Montana and northern Idaho. Although introgression was common and only 37%
of the sites were occupied solely by parental westslope cutthroat trout,
levels of hybridization were generally low. Of the 188 sites sampled, 73%
contained ≤5% rainbow trout alleles and 58% had ≤1% rainbow trout alleles.
Overall, 72% of specimens were nonadmixed westslope cutthroat trout, and
an additional 3.5% were nonadmixed rainbow trout. Samples from seven sites
met our criteria for hybrid swarms, that is, an absence of nonadmixed
individuals and a random distribution of alleles within the sample; most
(6/7) were associated with introgression by Yellowstone cutthroat trout.
In streams with multiple sites, upstream locations exhibited less
introgression than downstream locations. We conclude that although the
widespread introduction of nonnative trout within the historical range of
westslope cutthroat trout has increased the incidence of introgression,
sites containing nonadmixed populations of this taxon are common and
broadly distributed.