As I like to say to my students when they ask a question---sounds like an experiment.

Years ago, in an animal behavior lab my class conducted an experiment that might be apropos to this question. We had an aquarium of full of shiners (minnows) that were acclimated to being fed by hand. As we lowered our fingers to the water the school of minnows would all congregate around our fingers with or without food.

We removed one minnow with a net and placed him/her in a small dish. Then we proceeded to harass the captured minnow for a time (no minnows were harmed in this experiments .) Next with an eye dropper we removed some water from the stressed minnow's dish. Then the eye-dropper was taken over to the aquarium with the other minnows who figured they were about to be fed and came to the surface. Instead of food they received one drop of the water from the stressed fish. The results were amazing and unequivocal. Every member of the school of minnows immediately scattered to all corners of the aquarium--instant flight response. Not only did they scatter, it was several days before we could get them to come back to being fed by hand.

It is certainly clear in an aquarium with schooling minnows--maybe these results have some application on the stream.

BW