I stumbled on a bit of river in Sweden a couple of summers ago that was full of rising whitefish. They would have a go at any bushy dry fly and the biggest fish I caught took a big mayfly imitation even though there were no danica to be seen. The local garage that sold me a permit were very helpful in terms of location and tactics. I'm sure I would have caught more if I had managed to 'match the hatch' but they really weren't that fussy!
I was in BC on a steelhead trip and was swinging one run with egg and nymph flies as suggested by the guide.
I hooked a fish and brought it in and I said "I caught a grayling " whereupon the guide said "we don't have grayling in BC, it's a whitefish ". I said "what's that?", his response was "s**t"!!