A large trout may need an undercut bank in three feet or more of water. A small trout may need only an overhanging rock in a couple inches of water or even just the rippled surface that fast water provides. The more different kinds or diversity of shelter in a stream or river, the more different kinds and sizes of fish are likely to be there. Hiding places are important to fish because being eaten by a predator is probably the most frequent cause of death in wild fish, especially among smaller ones. Predators, however, are very important in an ecosystem.
Most fish are both predators and prey.
The majority of fish live with the constant threat of being eaten by larger fish. Many of the physical characteristics of fish are evolutionary attempts to become the most efficient predator possible while making it harder for they themselves to be eaten. Because both predators and prey exist in nature, a balance must be achieved through the juggling of many different factors. The delicacy of this balance can be observed when humans upset a system by introducing a new species or changing the conditions of the stream. Because predators consume large amounts of the fishes also desired by humans, it is often assumed that reducing the number of predators will increase the number of fish available to fisheries.