Knowing how trout feed can help you catch more fish. Fast current and flies that emerge quickly — like caddisflies — or skitter along the surface cause trout to make loud, splashy rises. Classic rises make dimples in the water and leave behind a few bubbles, indicating feeding on mayfly duns and other flies riding above the surface film. Small dimples that leave no bubbles but sometimes include the dorsal and tail appearing usually indicate emergers or small flies dangling in the surface film. Bulges or swirls are the hardest to see and often indicate a fish feeding just beneath the surface.
As always, watch the fish before casting to the fish. Understanding what they are feeding on, by reading the rise, may help you catch that trout of a lifetime.
Copied from Midcurrents Techniques. See more at http://midcurrent.com/techniques/fly-fishing-strategy-tips/