One evening(dusk) two years ago at Merwin a guy went by us in a canoe, heading towards the dam with a fly rod. He was excitedly telling us how the kokanee would be coming to the surface after dark and that fly fishing for them was a blast!
He didn't say he would kill me if I told a couple of friends so there you have it!
Of course you have to be camping there with your boat at a dock or campsite to be able to be on the lake after dark.
BTW- at East Lake last year over the 4th we were trolling flies on the surface along the east shore at dusk- the kokes seemed to just flood into the <10ft water we were in- we caught kokes right and left this way right up to the one hour after sunset deadline.
Jim