Hybridization between native and introduced species is noted is an important player in the decline of native species. Introgressive hybridization between native and introduced species is a growing conservation concern. In some areas, hybridization between westslope cutthroat trout and introduced species has led to populations in which all trout are hybridized; the native species has been, in essence, extirpated and replaced with a population of hybrids. For native westslope cutthroat trout and introduced rainbow trout in western North America, this process is thought to lead to the formation of “hybrid swarms”, populations where all of the individual trout are hybridized.
In evaluating the scope and trajectory of this process, it is necessary to sample broadly and to use methods that precisely evaluate the levels of hybridization. We developed new high-resolution genetic methods to assess levels of hybridization and sampled over 3800 fish to evaluate hybridization patterns. We found that many streams contained pure populations and that pure individuals made up the majority of fish in stream headwaters.
We used newly developed hybrid tests to measure levels of hybridization in 3865 fish captured in 188 locations on 129 streams distributed across western Montana and northern Idaho. Although introgression was common and only 37 percent of the sites were occupied solely by non-hybridized westslope cutthroat trout, levels of hybridization were generally low. Of the 188 sites sampled, 73 percent contained less than or equal to five percent rainbow trout alleles and 58 percent had less than or equal to one percent rainbow trout alleles. Overall, 72 percent of specimens showed no measurable levels of hybridization.
These patterns strengthened in the headwaters: in streams with multiple sites, upstream locations exhibited lower levels of hybridization 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 non-hybridized populations of this taxon are common and broadly distributed.
The question is why; given that hybridization spread rapidly within the lower reaches of these same rivers. Part of the answer lies in the resistance of these headwaters streams to warming; they have stayed cold even though the air around them has warmed significantly. These streams provide refuges not only for pure native cutthroat trout, but for a suite of cold-water dependent species.