Factors influencing the spread of hybridization between native westslope cutthroat and non-native rainbow trout.

Collaborator: Matthew Boyer (Montana Fish, Wildlife & Parks)

Radiotagged hybrid trout.Introductions of exotic species threaten the biodiversity of aquatic ecosystems throughout the world. Hybridization between introduced and native taxa can be a major consequence of species introductions, especially in circumstances where nonnative species hybridize with rare or endangered species, threatening their persistence. Consequently, an understanding of the genetic and ecological mechanisms that permit and constrain hybridization with nonnative species is critical for conservation and recovery of native species.

Hybridization with introduced salmonids is probably the greatest threat facing westslope cutthroat trout Oncorhynchus clarkii lewisi (WCT). Although theoretical and empirical studies predict that hybrids between WCT and rainbow trout O. mykiss (RBT) will experience outbreeding depression, the resulting fitness decline and loss of local adaptations may not be readily apparent. Furthermore, the unidirectional nature of hybridization can allow introgression, genes of one species blending with another, to spread even when hybrids have reduced fitness.

Although the upper Flathead River system in Montana is considered a regional stronghold for WCT, the long-term persistence of these populations is threatened by the continued spread of introgression with nonnative RBT. Previous studies indicate that increased straying by hybrids contributes to the spread of hybridization in the upper Flathead River. However, the genetic and ecological factors that determine invasion success and the establishment of future sources of hybridization remain poorly understood in the natural environment.

To examine these potential threats to WCT, the fundamental question of our research is: what factors influence successful invasion of WCT x RBT hybrids? Two non-mutually exclusive hypotheses to explain the increase of RBT introgression in the Flathead system are: (1) continued dispersal (i.e., straying) and gene flow from downstream source populations with high proportions of RBT mixing and (2) sufficient reproductive success and survival by hybrid fish and subsequent colonization of stream habitats. Our research project will test these theories by examining dispersal and several components of fitness between hybrids and non-hybridized fish. Further, we will examine the relationship between environmental factors and genetic and demographic characteristics of hybrid populations. The specific objectives are: (1) to compare the spawning dynamics and dispersal patterns of WCT, RBT, and hybrid trout in the upper Flathead River system; (2) to quantify the reproductive success and survival of WCT, RBT and hybrid trout using pedigree analysis based on microsatellite DNA markers in two recently invaded streams; and (3) to assess the environmental and ecological factors influencing the spatial and temporal distribution and abundance of hybridization in the upper Flathead River drainage, Montana and Canada. Results will be used to inform conservation and management programs for recovery of cutthroat trout.

For more information contact:
Clint Muhlfeld
406-888-7926
cmuhlfeld@usgs.gov