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

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 (D̄) 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 D̄ values decreased from first-order to secondorder streams, then increased through fourth-order streams; and decreased again in the fifth-order stream. Cumulative D̄ values increased through third-order streams and decreased in fourth- and fifth-order streams. In general, D̄ values for adventitious streams correlated with trends in D̄ 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.