CHANGES IN HABITAT AND MICROHABITAT PARTITIONING WITHIN AN ASSEMBLAGE OF STREAM FISHES IN RESPONSE TO PREDATION BY SACRAMENTO SQUAWFISH (PTYCHOCHEILUS-GRANDIS)

The Sacramento squawfish (Ptychocheilus grandis), a piscivorous cyprinid, was recently (ca. 1979) introduced into the Eel River, California, USA. We compared habitat and microhabitat use of resident fishes between areas where squawfish were present and absent at one location and between years before and after invasion by squawfish at a second location. The resident species showed a variety of responses to the presence of the predator. Juvenile rainbow trout (Oncorhynchus mykiss) and juvenile Sacramento suckers (Catostomus occidentalis) increased their use of fast-flowing riffles at both locations, in the presence of squawfish. Suckers also used significantly shallower depths within habitat types. Adult California roach (Lavinia symmetricus) decreased their use of run habitat and increased use of pools and riffles at the first site, but not at the second. When squawfish were present, juvenile roach and threespine stickleback (Gasterosteus aculeatus) were found in shallower water closer to the stream edge in all habitats. Spatial overlaps tended to be lower in the presence of squawfish. The introduction of squawfish has resulted in changes in the habitat and microhabitat use of the resident fish assemblage but no loss of species.