How a trout sees an insect, or a fly, on the surface puzzled anglers for centuries, and 20th century authors spent a lot of time working on the problem. From the trout’s perspective, the surface of the water is a mirror, reflecting the stream bottom. However, there is a round “window” through the reflection, in which the trout can see above the surface, although this window gets smaller the closer the fish is to the surface. As an insect drifts toward a trout, the fish first sees those parts of the insect that sit on or break through the surface, as they interrupt the “mirror” effect of the surface. Then, when the insect enters the “window,” the trout can see the body and wings. By keeping the insect in the window, the trout can find its meal.
Note how this trout moves, in an attempt to keep the red damselfly in the window, which is quite small because the fish is so close to the surface.
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