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Recently I was swimming a jig and watched a fish swim up and bump it and swim off and my line did not move and I could not feel anything. If I wasn't watching I would have never known.
Scary isn't it?
I once watched a steelhead respond to a fly in an interesting way. The drift was on the money, and the steelie responded, but only by intercepting the drifting fly at the very tip of its snout, with mouth pretty much closed. It held the fly by the tufts of fuzz it was tied with, for only a moment, then let it go. That kind of response is essentially imperceptible. But the trout now knows that that fly is "not food".
We have a worthy adversary, don't we.