Habitat Partitioning in the Fish Community of a Southwestern River

Patterns of habitat partitioning and spatial overlap are docu- mented for the fishes of an environmentally unstable southwestern river. Notropis lutrensis (red shiner), Notropis girardi (Arkansas River shiner), Hybognathus placitus (plains minnow), and Gambusia affinis (mosquitofish) numerically dom- inated the fish fauna; few centrarchids or other large species were present. While the dominant species showed some spatial segregation at most times, their habitat use was transitory, changing as environmental conditions changed. The dominant species made wide use of the available habitat during mild environ- mental conditions and constricted habitat use when physico-chemical conditions were potentially stressful. Species associations were also transitory; no species pairs consistently exhibited strong spatial overlap. Habitat partitioning thus ap- pears less structured in this stream than in more stable environments that have been studied. The fish fauna was characterized by ecologic plasticity, with the most successful species showing marked habitat flexibility. The South Canadian River in central Oklahoma, like many rivers of the southern Great Plains, is wide and shallow, with seasonal extremes of discharge. Hefley (1937) described the stream thusly: Probably no more ecologically dynamic region exists: the seasonal, diurnal, and yearly fluctuations of meteorological factors are great and sudden; the course of the river changes with each succeeding rain and the shifting sand . . . is constantly being moved by wind and water. In such a stream, physico-chemical variables like temp- erature, salinity, and dissolved oxygen fluctuate widely, and may limit the fish fauna (Echelle et al. 1972; Matthews and Maness, in press), or strongly influence microhabitat selection by fishes (Mat-