A Comparison of Salmofly Density Upstream and Downstream of Ennis Reservoir

Abstract

Ennis Reservoir on the Madison River, southwest Montana, has been linked with changes in macroinvertebrate and fish assemblages. We tested the hypothesis that life history, distribution, and abundance of the salmonfly (Pteronarcys califomica Newport) differ upstream and downstream of Ennis Reservoir. We sampled larvae, shed exuviae, and discarded wings of salmonfiies in 1994 and 1995 upstream and downstream of the reservoir. Wolman pebble counts and estimation of substrate embeddedness were made at nine exuviae collection sites to determine habitat availability. Predation rate on adult salmonfties and crayfish densities also were examined. Salmonflies downstream of Ennis Reservoir were 6-8 X less abundant, significantly larger, and possibly required one less year as larvae to complete development compared to salmonflies upstream of the reservoir. Adult salmonfly abundance both upstream and downstream of Ennis Reservoir was 3.2 to 4.0 X higher in 1995 than 1994. Numbers of exuviae were correlated with substrate embeddedness suggesting that abundance was influenced by habitat availability. Predation accounted for 5 to 15 percent of adult salmonfly mortality. Crayfish were at least 6.6 X more abundant downstream of Ennis Reservoir.