Improved Grazing Management Increases Terrestrial Invertebrate Inputs that Feed Trout in Wyoming Rangeland Streams

Abstract

AbstractResearch in forest and grassland ecosystems worldwide indicates that terrestrial invertebrates can be a significant source of prey for fish, providing about 50% of their annual energy. We examined whether input of terrestrial invertebrates to rangeland streams in western Wyoming provides an important prey resource for brown trout Salmo trutta and brook trout Salvelinus fontinalis and how it is modified by livestock grazing. During summers of 2004 and 2005 we sampled falling invertebrate input and trout diets in five pairs of streams that had riparian zones under two different grazing systems: High‐density, short‐duration (HDSD) grazing versus season‐long (SL) grazing. The biomass of riparian vegetation and the input of terrestrial invertebrates were two to three times greater in reaches with riparian zones under HDSD than under SL grazing management. Likewise, the afternoon diets of individual trout in HDSD reaches had, on average, about twice as much terrestrial invertebrate biomass during summer than those of trout in SL reaches, but this was statistically significant only during late summer due to high variability. Overall, 57% of the afternoon diets of trout in reaches under both grazing systems consisted of terrestrial prey. Diel diet sampling during August 2005 showed that fish in HDSD reaches also had consumed more aquatic invertebrate prey, primarily at night. Total trout biomass in HDSD reaches was more than twice that in SL reaches. These results suggest that in rangeland streams of the western United States, both terrestrial and aquatic invertebrates are important prey resources for trout and that improved grazing management has the potential to influence fish populations through multiple food web pathways.