Simulated Fine Woody Debris Accumulations in a Stream Increase Rainbow Trout Fry Abundance

Abstract

Habitat for young-of-the-year rainbow trout Oncorhynchus mykiss was enhanced in a fourth-order stream during August–October 1991 by the addition of wooden structures that simulated accumulations of fine woody debris (FWD). The experiment represented a two-factorial design with the presence or absence of FWD bundles and time since debris introduction as factors. Immediately after FWD placement, fry density, individual biomass, fry condition factor, and total fry biomass were similar in treated and untreated sites. As the experiment progressed, density and total fry biomass significantly increased at treated but not at untreated sites. Individual biomass and condition factor did not differ between treated and untreated areas, and they were affected only by time since FWD placement. Because individuals at treated and untreated sites were the same size, added FWD did not affect an individualˈs net rate of energy gain. Rather, we hypothesize that the FWD provided structurally complex habitat that acted as a refuge from predators and as sites from which foraging forays were staged. Adding FWD to a stream can increase carrying capacity for trout fry, and adult population density may increase as a result.