The use of floating overhead cover by warmwater stream fishes

Increasing the amount of woody debris in streams has often increased population size of one or more focal species whereas clearing a stream of woody debris has often reduced populations. Alterations of the amount of woody debris change multiple aspects of the habitat simultaneously, so it is very difficult to know what particular stimulus or combination of stimuli evoked the changes in the fish populations. The purpose of this research was to alter habitat by the addition of overhead cover alone, and to see whether or not that single change would affect the distribution of stream-dwelling fishes. The addition of solid, floating cover objects 1.8 m by 2.4 m resulted in increased local populations of bluntnose minnow (Pimphales notatus), creek chub (Semotilus atromaculatus), and longear sunfish (Lepomis megalotis) five to six weeks later, but did not change the distribution of four other species: white sucker (Catostomus commersoni), green sunfish (Lepomis cyanellus), bluegill (Lepomis macrochirus), and largemouth bass (Micropterus salmoides). These results showed that for some species at least, changes in channel morphology, visual isolation, and amelioration of flow produced by adding three-dimensional complex woody debris were not essential for making locally attractive habitat changes in a warmwater stream. The effectiveness of overhead cover as used here would be expected to vary for different species and depend on such things as predation threat, flow, and food levels.