SEGREGATION BY SPECIES AND SIZE CLASSES OF RAINBOW-TROUT, SALMO-GAIRDNERI, AND SACRAMENTO SUCKER, CATOSTOMUS-OCCIDENTALIS, IN 3 CALIFORNIA STREAMS

The hypothesis that C. occidentalis compete with S. gairdneri for space in streams was examined by measuring microhabitat utilization of both species in 3 California streams. Two streams were similar in most respects except 1 contained only trout and 1 contained trout and a large population of suckers. The 3rd stream, formed by the union of the first 2, contained trout and a small population of suckers. The species overlapped in 5 of the 6 microhabitat variables measured: maximum depth, mean water column velocity, focal point velocity, surface water velocity, and substrate type. The species had strong vertical segregation; there was little overlap between species in focal point depth. Mean focal point velocities were also significantly different. Suckers roamed over and generally remained in contact with the bottom, while trout held position in the water column. Microhabitat utilization by trout in the stream without suckers was similar to that in the stream with a higher sucker density. Differences in microhabitat utilization by trout between the 3rd stream and the other 2 was attributed to the larger size of the 3rd stream. Both sucker and trout showed a similar within-species segregation of size classes; fish under 50 mm in length sought shallow water. Size-specific trends indicated ontogenic shifts in resource utilization which reduced overlap within species. Competition for space between trout and suckers was probably not a major factor regulating microhabitat utilization by trout, although the possibility that larger suckers may displace small trout needs further study.