Annual and Spacial Variability of Fish Assemblages at Locations in the Lower San Joaquin River Drainage

Larry R. Brown

In:
American Fisheries Society, 31st Annual Conference, 1996, California-Nevada Chapter, Aquatic species loss and imperilment--Perspectives for our profession, Ventura, California, p. 20.

Abstract:
Assessments of annual and spatial variability of fish assemblages were conducted in the lower San Joaquin River drainage as part of the National Water Quality Assessment Program of the U.S. Geological Survey. Fish sampling was conducted at single stream reaches in the lower Stanislaus, Tuolumne, and Merced rivers in 1993, 1994, and 1995 to assess annual variability in fish assemblages. Two additional stream reaches were sampled at each location in 1995 to provide an assessment of spatial variability in fish assemblages. Two stream reaches on the San Joaquin River were sampled in 1993 and 1994 providing additional data on annual variability.

Similarity in assemblage composition, as measured by Jaccard Similarity Coefficients, which range from 0 (no species shared) to 1 (all species shared), was higher from 1993 to 1994 (mean=0.70, range=0.50-0.92, n=5) than from 1994 to 1995 (mean=0.43, range=0.35-0.50, n=3). The difference in similarities between 1994 and 1995 were due to the presence of additional species in 1995, including both native and introduced species. The presence of these species was likely related to high stream discharges prior to or during sampling. Mean similarity among reaches sampled in 1995 was 0.82 at the Stanislaus River (range-0.81-0.90, n=3), 0.80 at the Merced River (range=0.70-0.82, n=3) and 0.64 at the Tuolumne River (range=0.60-0.71, n=3). The lower assemblage similarity among reaches at the Tuolumne River compared to the other two rivers was primarily due to the presence or absence of rare species. These results suggest that annual variability is more important than spatial variability when assessing differences in fish assemblages at these sites.