The Effects of Reduction in Trout Density on the Invertebrate Community of a Mountain Stream

Abstract

An experimental reduction in trout density was carried out for 4 yr to determine whether the numbers or species composition of aquatic invertebrates would be affected. In a Colorado stream, the standing crop of trout (mainly Salvelinus fontinalis) prior to this study was 4.86 g/m2, typical of infertile trout streams. Repeated electroshocking of a 1220 m long experimental section kept trout stocks at 10—25% of this initial value during the summers of 1975—1978. Density of invertebrates in the benthos (number of invertebrates per square metre) showed no consistent differences between the trout removal section and upstream and downstream control sections for the majority to taxa examined. Drift density (number of invertebrates per 1000 cubic metres) also failed to show an effect of the trout removal. Some taxa were significantly more abundant at one site than another, but this was attributable to changes along the stream gradient or differences in phenology among the sites rather than experimental reduction of trout. As trout stomach content analysis revealed very intensive grazing of a few taxa aquatic insects, their failure to increase in the trout removal section was unexpected. Statistical analysis showed that because of the variability of the system, a doubling or halving of numbers must occur to be detectable. Within these limits, it is concluded that removal of trout had no significant effect on the prey community, and two explanations are offered. Trout may actually consume only a small fraction of total prey, although this conclusion is limited by inability of current techniques to sample the benthos fully. It is also likely that because streams do not provide areas where fish are predictably absent, the invertebrate community is highly adapted to fish predation and so is not sensitive to manipulations in fish density.