Masters Thesis

Comparison of benthic invertebrate community structure and diet composition of steelhead (Oncorhynchus mykiss) in Dry Creek, California

Morphological changes in the Dry Creek (Sonoma County, California) associated with Warm Spring Dam, have reduced habitat availability for rearing fish, and potentially altered the community structure of benthic invertebrates that form the prey-base for juvenile salmonids. I described and compared the structure of benthic invertebrate assemblages and the diets of juvenile steelhead among four stream reaches of Dry Creek downstream of Warm Springs Dam. I hypothesized that if prey availability contributes to factors restricting the success of juvenile salmonids in mainstem Dry Creek, then diet composition should parallel observed differences in reach-specific relative condition and length of juvenile salmonids. Benthic invertebrate assemblages in Dry Creek displayed a longitudinal trend from Warm Springs Dam to the confluence with the Russian River; however, steelhead diet composition did not correspond with reach-specific benthic invertebrate assemblages as expected. Drift-foraging is likely an important feeding strategy for steelhead in mainstem Dry Creek. Steelhead condition and body length corresponded with reach-specific differences in steelhead diet composition. However, reach-specific differences in energetic cost associated with longitudinal differences in water temperature (water temperature was positively correlated with distance from the dam) may be a greater contributor to differences in steelhead size. The relatively high steelhead summer growth rates, in comparison with similar studies, may result from artificially-sustained summer flows in mainstem Dry Creek. Year-round flows in mainstem Dry Creek maintain stream connectivity during a period when non-regulated streams in Mediterranean climates typically become disconnected, therefore increasing food availability and foraging opportunities.