Fished Nimpo a couple of years ago and 120 fish might be caught in a couple of weeks of hard fishing. And, yes, the average size is something like 12-15 inches. Beautiful area, but it's a long way up there from WA state. You may want to try some of the Kamloops area lakes like Peterhope, Billy, or Tunkwa for something closer with better flyfishing.

Good luck and let us know how you do up there.
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TD