Evidence for Hybridization Between Native Yellowstone Cutthroat trout (Oncorhynchus clarki bouvieri) and Introduced Rainbow Trout (O. mykiss).

Interactions Between Introduced Rainbow Trout and Native Cutthroat Trout Populations. Although rainbow trout are native to western North America, they have been widely introduced into many watersheds where they were not historically found. Prior to the 1880's, Yellowstone cutthroat trout were the only trout in the upper Snake River drainage and the Yellowstone River drainage. Although other fish species have been introduced within this range, rainbow trout appear to be especially detrimental to native Yellowstone cutthroat trout populations because they are ecologically similar to cutthroat trout and can form fertile hybrid offspring. Our research focuses on determining the distribution of rainbow and hybrid trout within Yellowstone cutthroat trout populations of the South Fork of the Snake River. LINK to field identification of cutthroat, rainbow, and hybrid trout.	Above: Grey indicates the historic distribution of Yellowstone cutthroat trout. Blue indicates the extent of inland movement by rainbow trout. A large waterfall (Shoshone Falls) is a natural barrier to upriver movement by fish.
	Rainbow Trout in the South Fork of the Snake River Rainbow trout were regularly stocked into the South Fork of the Snake River until the mid 1980’s when fisheries biologists began to notice growing rainbow trout and hybrid trout populations while pure Yellowstone cutthroat were declining. Beginning in 2003 we have sampled trout from mainstem sites on the South Fork Snake River and from major tributary streams to determine where rainbow trout genotypes are spreading. We are looking for correlates between the spread of rainbow trout alleles and environmental characteristics.
Preliminary Findings Although genetic analyses are not complete, we have evidence that rainbow trout originally stocked in the mainstem South Fork of the Snake River are moving into some tributary streams. The links below demonstrate trout phenotypes we found in the South Fork of the Snake River and two neighboring tributaries, Pine Creek and Rainey Creek.
		Mainstem South Fork Snake River Pine Creek Rainey Creek

This work is supported by the National Science Foundation (Division of Environmental Biology, grant # 0212445 and 0211818).