It seems to me that river otters have always been part of the trout streams south of the border, such as the great trout rivers is Colorado. I think their impact would be minor and perhaps even beneficial.
I feel your frustration Don, because the impact might be small, but as you're saying, it's just one more thing. However, I think the real issue rests with the 'Irrigators/Dam builders/ oil company dewaterers, OHVs [ quads/bike/etc.], cattle grazers, pipeliners, logging', not to mention the mud snails, dydimo, and whirling desease that seems to be popping up in trout streams.
I think shooting a few otters or pelicans would be the easiest of the problems to fix, but I also think the benefits would be negligable. The only critter we need to protect trout stream from is mankind.