Somehow I missed this last post, thanks for the follow up. I couldn't help but think of the spawning shrimp, which aside from being such a big gawdy fly has been a 'closer' for me on recent outings even when stealthy little tan flies would fail. The only rationale I could place on this was (a) it is rarely used by anglers and (b) it's so juicy of a treat they can't ignore it.
I am wondering if there is an element of fish 'learning' what is typically a dangerous offering verses what has been consistently safe to eat? Certainly they learn which fish to eat and which to avoid over time. They learn which shadows to avoid (herons and ospreys) and which noises mean trouble. I would think that their ability to focus on the things they consume even exceeds their ability to be wary of other goings-on, which we can clearly see they do react to as they rooster tail off a flat.
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