Production of cutthroat trout (Salmo clarki) in relation to riparian vegetation in Bear Creek, Washington

Abstract

Removal of riparian vegetation can alter salmonid production in streams. This study examined relationships linking fish production with riparian vegetation, excluding those associated with physical disturbance of habitat.Bear Creek, a third-order stream on the Olympic Peninsula, was investigated before and after removal of riparian vegetation. Water temperature, aquatic insect production, and terrestrial insect fallout increased following streamside timber removal in a 1 km treatment zone. Temperatures averaged 2(DEGREES)C greater in the lower 500 meters of the treatment zone, but the upper 500 meters of the treatment zone did not change. Average annual biomass of aquatic insects in the upper 500 meters of the treatment zone was two times greater (6.1 gm('-2) compared to 3.0 gm('-2)) than biomass in the control zone. Biomass of aquatic insects increased in the lower treatment zone, as well. Insect fallout biomass (May-August) in the lower and upper treatment zones increased from 51 percent and 70 percent of the biomass in the control zone prior to treatment, to 64 percent and 102 percent of the biomass in the control zone following treatment.More than 70 percent of the food biomass consumed by all age groups of cutthroat trout was composed of aquatic insects, despite the greater abundance of insect fallout. Only during the summer low flow period (August) when aquatic production was low, did insect fallout become more important than aquatic insects in the trout diet. Changes in prey utilization were associated with temporal differences in prey availability, and prey availability may have limited the level of food consumption.Following canopy removal, mean weights of each age group of trout increased only in the lower treatment zone, but not always. Densities of age I and II trout increased only in the upper treatment zone. Differences in these responses to canopy removal were attributed to differences in food supply and temperature between the lower and upper treatment zones. Increased rates of food consumption were associated with increases in water temperature and food availability. This appeared to enhance physiological conditions for growth in the lower treatment zone. On the other hand, increases in trout density in the upper treatment zone were associated with increases in food availability without significant changes in water temperature. These results suggest temperature is a key factor determining how a trout population responds to canopy removal. (Abstract shortened with permission of author.)