I think that almost all of a fish's predators come from above ( excepting water snakes, larger fish etc.) & they get attuned to their environment. Any thing out of the ordinary, (Flash, or movement from above) will put them on edge.
Granted their brain is the size of a pea but they are survivors of evolution & Their eyes are on the top of the head so they're looking up. Like I said earlier I've seen Trout & salmon scoot away or feed off my boots but I've also seen them stay in the same area & act nervous & stop feeding. I have a shiny reel & after seeing the video I'm going to dull it up. I was also thinking of dyeing my vest a dark green or brown. This might be a little extreme but it couldn't hurt. I've fished some of Penn's limestone streams & there you definitely have to sneak up to the bank. & plan where you cast. I've seen fisherman kneeling on the bank casting from almost a prone position & they were the locals & they caught fish. I usually use a little stealth when entering a stream & look around first. I try to dry fly fish upstream but I've seen trout positioned sideways in the stream so that isn't a foolproof solution. I usually nymph or streamer fish downstream so a long line or stealth is needed. IMHO