Fish Species Diversity in Relation to Stream Order and Physicochemical Conditions in the Plum Creek Drainage Basin

Abstract

Water and fish samples were collected at 202 sampling stations throughout the Plum Creek drainage basin in south central Texas between January and April 1968. Fish species diversity (D) was analyzed to determine its relationship to stream order and physicochemical conditions. Fluctuations in physicochemical conditions decreased as stream order increased. Twenty-seven species of fishes were collected. Minimum, maximum and mean numbers of species per stream increased as stream order increased through the first four stream orders and decreased in the fifth-order stream. Mean D values decreased from first-order to secondorder streams, then increased through fourth-order streams; and decreased again in the fifth-order stream. Cumulative D values increased through third-order streams and decreased in fourthand fifth-order streams. In general, D values for adventitious streams correlated with trends in D values for the higher order stream into which the adventitious stream flowed. The decrease in number of species and in species diversity in the fifth-order stream may have been due to migration of fishes into lower order streams for spawning purposes and/or to escape abnormally high water in the fifth-order stream. Also, increased depth and obstacles, such as logs and barbed wire, may have reduced seining efficiency in the fifth-order stream. INTRODUCTION The use of stream classification in biological studies was proposed by Abell (1961). Several methods have been proposed for the classification of streams. Such classifications have been based on varying criteria such as slope and velocity (Huet, 1959); water source (Klugh, 1923); water velocity and nature of substrate (Pearse, 1939); size of drainage basin (Thompson and Hunt, 1930); algal associations (Margalef, 1960); habitat (Odum, 1959); temperature, altitude, rainfall and permanence of water (Usinger, 1963), and stream branching (Horton, 1945). Horton's method of stream classification was used in this study because it reduces the number of environmental variables. Horton designated the smallest unbranched tributary as a first-order stream. The confluence of two first-order streams formed a second-order stream, etc. The order number for any stream was not changed by the entrance of a smaller order stream. Several mathematical expressions or diversity indices have been proposed to measure community structure (Fisher, Corbett and Williams, 1943; Preston, 1948; Simpson, 1949; Margalef, 1951; I Present address: Zoology Department, Arizona State University, Tempe 85281.