Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
This is the first of it's family I've seen, collected from a tiny, fishless stream in the Cascades. The three species of this genus all live in the Northwest and are predators that primarily eat stonefly nymphs Merritt R.W., Cummins, K.W., and Berg, M.B. (2019).
CharlieSawd on Feb 26, 2009February 26th, 2009, 8:39 am EST
I think it is Oncorhynchus clarki lewisi, the westslope cutthroat. Bonni's tend to have more of a yellow hue. There are resident westslopes in north western WY, and as you said you were fishing a high mountain lake, my guess is you were in this region.