The unknown moderator(s) perhaps, the dweeb(s) likes to rearrange..
Man-made barriers are expensive but workable. So are electrofishing, stream poisoning and ultimately stream closures specifically if the population slide continues it's current trend. Official population assessments are ongoing and there are more pure strain westslopes left than most here seem to be think but they are at the extreme periphery of their range and living conditions. IMO the population has declined for all the traditional reasons, industrial, agricultural and poor fishery mindsets but I believe I've seen an accelerated encroachment in certain waters since the wholesale spread of the catch and release "ethic", the extreme unwillingness of the vast majority of flyfishers to kill any fish what so ever and to hold all trout in equal respect. Hell, I was one of them for a time.