I've been reading about the genetics of Cutthroat Trout, in particullar the Yellowstone Cutthroat Trout (YCT) and the Westslope Cutthroat Trout (WCT). As far as the State of Montana and Yellowstone National Park is concerned they qualify as genetically pure a YCT with less than 1% hybridization (usually mixed with WCT or Rainbow Trout); for WCT the variance is up to 20%. Why such a difference? Jim.

I am not a fish genetics expert (I work on mice) but from my reading of the scientific literature both trout have very substantial genetic variation. The papers below I think should be available to you and in both the reasons for the diversity are discussed.

http://www.researchgate.net/publication … iver_Basin

http://www.nrmsc.usgs.gov/files/norock/ … n_2011.pdf

This is really about management, conervation and "sub-specific" definitions. These fish are emblematic of the region and have high value for recreational fisheries so there is obviously a lot of interest in protecting them, and in trying to restore populations (e,g, by stocking from hatchery raised fish). However, cutthroat trout have a number of subspecies defined and the amount of introgression of (for example) WCT DNA into YCT fish that is "too much" is a decision for the authorities managing the Yellowstone fishery. So basically there is no magic number separating "pure" from "hybrid" - particularly since there is no biologically rigorous definition of a subspecies - and authorities are free to make and use their own labels and criteria.

The situation with rainbows and cutthroat is a bit different since they are clearly recognised as distinct species (O. mykiss and O. clarkii respectively). However, it is still the case that there is no formal definition of just how much introgression from another species is allowed before we stop calling a fish "pure".