If you have ever walked up to a trout stream and the fins just zip out of sight, you could have warned the gill breather by vibrations from your heavy foot falls or maybe you were busted by "Snells window".
Trout have some oddities. They typically have the same density as the water surrounding them. This is really convenient for trout survival considering that sound passes through air at roughly 770 mph, but passes through regular pure water at over 3000 mph. You make the wrong noise in a wet trout environment, you lose.
Banging oars or paddles on a kayak or canoe can send trout for cover at speeds you cannot cast quickly enough. Last time I checked my fly cast's don't go over 2000 mph, it pays to be quiet in and around trout water.
Trout are ectotherms. They acquire there temperature from the surrounding water. The term cold blooded (Poikilothermic) is misleading.
If the water is 38 degrees and the fish's metabolism is slow you may work harder for a trout. If for instance, the water temp is up to mid fifties they may come easier.
Also, deep water lakes can have fish in forty degree, 80 foot deep, low metabolic rate. In that same water body, fish higher in the water column, fish at a much warmer, higher, metabolic rate. One lake, one specie of rainbow trout and they become two different targets to fish for, based solely on temperature.
Trout have another phenomenon on there side and it's referred to as "Snells window". Trout looking around in their watery surroundings acquire an image with the proper light conditions that you, and any predator or prey, are much closer in view, than in actual reality.
Snells window if you're not careful as a fisherman has a double whammy. When I walk up to the stream the trout appear closer than they are. Watch where you let your body shadow fall to include the action of your rod casting.
Trout miss the fly sometimes not because you didn't set the hook, but they miss judged the distance. Trout have fed heavily on one hatch that occurs at seven, almost sunset pm. (Small little gnat lets say.) That same fish slaps at an eleven in the bright sunshine morning grasshopper and misses the first time. A second cast, and all is well.
If the trout can see you, before you see them or they appear closer than they really are, you better practice your casting and approach trout streams like you're going after a trophy buck.
Cloudy day, moonlight, bright sunshine all comes into play. Water clarity, turbidity, and currents add some fun to the mix as well. Tougher weather works in the fisherman's favor. Some lures that have internal sound producing features may scare fish one day and hook them the next. Glow in the dark or match the hatch is all part of the game.
The trout fishing game is not just the right food at the right time. They have a diet, but they will forget food if there sound spooked or the picture doesn't look right to them. Everyone knows about the occasional trout that hit the tin can lid but it's the exception more than the rule. They make mistakes but less often than you think.