Among the many issues associated with introductions of nonnative aquatic species is the likelihood of hybridization with native species. An oft-mentioned example in western North America involves cutthroat trout, a cold-water fish that was once much more abundant and widespread, and rainbow trout, a popular sportfish also indigenous to parts of the West but widely introduced elsewhere. Where cutthroat trout are native and rainbow trout introduced, hybrids between the two species often appear, levels of hybridization sometimes persist or grow, and the incidence of hybridization can spread. Managers worry that mixing of these species will inevitably and irreversibly lead to the formation of hybrid swarms in which all fish are hybrids and the genetic integrity of native populations is lost, undermining conservation efforts already challenged by warming stream temperatures associated with a changing climate.
Ecological differences between the species, however, may dictate the nature and extent of hybridization and point the way for strategic management. Although hybridization between rainbow trout and cutthroat trout is common, hybrid swarms between them are rare (McKelvey et al. 2016). Moreover, despite the presence of hybrid individuals in downstream areas, many headwater streams are composed entirely of genetically pure cutthroat trout. Surprisingly, these patterns are typical not only in areas where rainbow trout are introduced, but also where both rainbow trout and cutthroat trout are native. Collectively, this demonstrates that there is substantial genetic and environmental resistance to mixing between these species, regardless of whether they co-occurred naturally for millennia or over the last two centuries from human actions.
Knowing how environments might influence the degree and location of hybridization between these species represents a potentially powerful tool for managers. To address that need, we modeled how hybridization between westslope cutthroat trout and rainbow trout is influenced by stream characteristics that favor each species (Young et al. 2016). On this website, we describe that model, and provide high-resolution digital maps in user-friendly formats of the predictions of different levels of hybridization across the native range of westslope cutthroat trout in the Northern Rocky Mountains, representing both current conditions and those associated with warmer stream temperatures. Our goal is to help decision-makers gauge the potential for hybridization between cutthroat trout and rainbow trout when considering management strategies for conserving cutthroat trout.