<P> Fish are probably the best - known inhabitants of lotic systems . The ability of a fish species to live in flowing waters depends upon the speed at which it can swim and the duration that its speed can be maintained . This ability can vary greatly between species and is tied to the habitat in which it can survive . Continuous swimming expends a tremendous amount of energy and, therefore, fishes spend only short periods in full current . Instead, individuals remain close to the bottom or the banks, behind obstacles, and sheltered from the current, swimming in the current only to feed or change locations . Some species have adapted to living only on the system bottom, never venturing into the open water flow . These fishes are dorso - ventrally flattened to reduce flow resistance and often have eyes on top of their heads to observe what is happening above them . Some also have sensory barrels positioned under the head to assist in the testing of substratum (Brown 1987). </P> <P> Lotic systems typically connect to each other, forming a path to the ocean (spring → stream → river → ocean), and many fishes have life cycles that require stages in both fresh and salt water . Salmon, for example, are anadromous species that are born in freshwater but spend most of their adult life in the ocean, returning to fresh water only to spawn . Eels are catadromous species that do the opposite, living in freshwater as adults but migrating to the ocean to spawn . </P> <P> Other vertebrate taxa that inhabit lotic systems include amphibians, such as salamanders, reptiles (e.g. snakes, turtles, crocodiles and alligators) various bird species, and mammals (e.g., otters, beavers, hippos, and river dolphins). With the exception of a few species, these vertebrates are not tied to water as fishes are, and spend part of their time in terrestrial habitats . Many fish species are important as consumers and as prey species to the larger vertebrates mentioned above . </P> <P> Energy sources can be autochthonous or allochthonous . </P>

Where does most of the energy come from in a shaded section of a stream