In fly fishing, an eddy is an important component of a river, stream, or even lake environment. Insects can be trapped in the main current upstream of an an eddy. They can be caught there either as struggling nymphs or emergers, or as spent adults called spinners. As the main drift gets close, it collects these insects and when it reaches the eddy. At this stage the water is usually loaded with "pre-packaged" meals for hungry trout. Because trout food tends to collect in the lazy reverse currents of an eddy, fish often wait in this area for the next morsel to slide gently into their feeding lane.
Feeding trout love eddies and because of this fly anglers have learned to love them as well. An incredibly effective method for hooking large fish in good numbers during your day of fly fishing is to present a nymph or visible dry fly on a dead drift that passes through an eddy.