Biotic and abiotic factors affecting native stream fishes in the South Yuba River, Nevada County, California

• Main Author: Gard, Mark Freeman
• Format: Dissertation
• Language: English
• Subjects: aquatic sciences; ecology

The South Yuba River has a depleted native fish fauna, with five of the expected nine native fish species absent because of past human abuse of the system. Construction of a downstream dam excluded two native anadromous species, Pacific lamprey (Lampetra tridentata) and chinook salmon (Oncorhynchus tshawytscha) from the South Yuba River, while historic (late 1800s) sediment loadings, during the operation of a hydraulic mining site, were probably responsible for the extirpation of three native fish species, riffle sculpin (Cottus gulosus), California roach (Hesperoleucus symmetricus) and speckled dace (Rhinichthys osculus) from the South Yuba River. I examined the fluvial geomorphology, substrate particle size distribution, aquatic macroinvertebrates, and fish microhabitat use and abundance in the South Yuba River and tributaries to examine the effects of present sediment loadings, predation and competition by introduced smallmouth bass (Micropterus dolomieui), and elevated water temperatures. Present sediment loadings do not affect fluvial geomorphology, salmonid spawning gravels, aquatic macroinvertebrate populations, or fish survival and reproduction, but have reduced fish growth rates. Smallmouth bass predation is the most likely cause of reduced populations of Sacramento squawfish (Ptychocheilus grandis) and hardhead (Mylopharodon conocephalus), and the limited distribution of California roach, restricted to one tributary. Based on the results of microhabitat experiments, competition with juvenile smallmouth bass does not appear to contribute to predation by adult smallmouth bass on juvenile squawfish. Predation by native Sacramento squawfish is reduced by prey behaviors, including greater use of cover and shifts to shallower depths and lower velocities by juvenile squawfish, and by segregation of microhabitat use by species and size class. High water temperatures are the most likely factor affecting the current distribution of rainbow trout (Oncorhynchus mykiss). The South Yuba River native fish assemblage can be partially restored through a combination of increased summer flows (to decrease water temperatures), reintroduction of missing species, monitoring, and management of tributaries. Reductions of sediment loads and control of introduced species do not appear to be necessary at this time for conservation of the South Yuba River native fish assemblage.