The Effects of Beaver on Trout in Sagehen Creek, California

DOCTORATE DISSERTATION: A study of the effects of beaver on trout was conducted on Sagehen Creek, Nevada County, California, from 1954 to 1957. The purpose of the inquiry was to investigate several ways in which beaver and their activities influence the three species of trout present in the creek. Accordingly, comparisons were made between sections of the natural stream and sections that had been ponded by beaver, in regard to: 1. physico-chemical conditions; 2. production of trout foods; 3. feeding habits of the trout; and 4. populations of trout and other fish. Further, the consequences of beaver dam removal on trout foods and populations were assessed, as well as the ways beaver dams affect trout spawning activities. The physical environment of the trout was altered measurably following the construction of beaver dams as the substrate changed from gravel and rubble to silt, water velocities decreased, aquatic and riparian vegetations were changed and winter ice conditions became less severe. Also, the depth and area of the aquatic habitat increased. Water temperature extremes lessened in the ponds, but water chemistry was little affected Although fewer different kinds of organisms were found to live in pond bottoms, much greater standing crops of bottom foods were living there than in the stream regardless of whether the comparisons were made on area or linear bases. Different bottom types were found to be decreasingly productive in terms of the bottom fauna in the following order: New beaver pond (most productive), old beaver pond, stream silt, rubble between beaver ponds, stream rubble and gravel All trout relied heavily on bottom faunas for food. In summer, however, drift organisms also became important. Trout living in the stream ate the stream fauna almost exclusively. Some pond trout were not as restricted in their feeding habits as rainbows living in the ponds depended largely on stream organisms for food. Pond brook and brown trout, on the other hand, were supported mainly by pond faunas. Foraging was not random; some organisms, though seemingly unavailable, were selected by trout in both habitats. The higher productivity of bottom fauna and the unique physical environment in the ponds were reflected in greater trout populations, these in turn resulted in a higher angler catch than in the stream. Sculpin populations were larger in stream than in pond areas. It was pointed out that sculpins and trout affect one another in several ways, but the net result of this interaction is unknown. A destructive flood occurred in Sagehen Creek in 1955 and resulted in a decreased fish population throughout the entire creek. This flood also removed most of the beaver dams in the rockslide colony Following dam removal, there was a substantial drop in the amount of trout food produced, and the tout populations dropped accordingly... The study concerned with the effects of beaver dams on trout spawning migrations indicates that the dams are not complete barriers to trout migration It is likely that the dams are partial barriers to movement in the fall, as concentrations of trout were observed below some dams at that season. However, there is no evidence that the barrier influence of the dams depresses the level of adult trout populations. It is concluded that beaver are of decided benefit to trout in Sagehen creek, but that caution should be exercised in applying the data and conclusion for this creek to other areas.