A. You've broached a complicated subject; the subject of how a fish sees takes up over 10 pages in one text I have. I'll try to simplify it to get at what you are interested in.

A trout looking upward, we'll assume straight up, can see through a circular area that is bounded by the angle at which light is refracted back into the water from the surface. This angle is roughly 45o from the vertical and thus about 90o in total. The size of this circular window increases with depth, so a fish in 5' of water will have a larger window than one just under the surface.

That tells you how much of an area a trout can see through, but doesn't tell you how far to the sides it can see. You are familiar with the way that light rays bend in water; things aren't where they appear because of this refraction. The same thing happens in reverse for the fish. Light rays bend away from the edge of their circle of vision; thus, they can see objects that are outside of the "line of sight" from their eye to the edge of the circle.

Unfortunately, we can't see this circular window to gauge how far to stay back or down. The only rule of thumb is that the closer you are to the vertical above a fish, the more likely it is to see you and the lower and further back you stay, the better chance you will have of not being seen.

One other point you alluded to. Even when staying low and behind objects, don't forget that a fish can be spooked by senses other than sight. Most importantly here is the lateral line on the sides of fish. These nerve endings are sensitive to pressure changes resulting from such things as heavy walking or jostling of rocks on the stream bottom. These can send a fish away just as sure as a visual sighting.