COLORADO PARKS & WILDLIFE

Past studies show that giant stoneflies (also known as salmonflies or willow flies), serve as a primary food source for trout in Colorado rivers where they occur. They live as aquatic larvae in rivers for 3-4 years before emerging as winged adults in June to mate and die. Their large and synchronous emergence produces some of the best fishing of the year and is eagerly anticipated by many anglers. The species' high densities and large size also make them important in the flow of energy and nutrients between aquatic and riparian systems. Although giant stoneflies can exist at extremely high numbers in certain locations, the density and range of this species has decreased in some places most likely in association with changes in stream flows and physical aquatic habitat. 

CPW completed a project to determine the influence of physical stream habitat on giant stonefly density in an effort to identify limiting factors and reasons for the species' decline. To do so, CPW researchers estimated stonefly density at 16 sites on three rivers; the Gunnison, the Colorado, and the Rio Grande and measured habitat variables at each site. 

The results of this study indicated that percent fine sediment, cobble embeddedness and average cobble size were the best predictors of stonefly density. Fine sediment is defined as sand, silt and clay particles less than 2 mm in diameter. As fine sediment levels increased, stonefly density decreased. The D50 is the median cobble size and as it increased so did stonefly density. Embeddedness is the extent that cobbles are submerged or buried by silt, stonefly densities were higher in areas with low embeddedness. Fine sediment was the single best predictor of the habitat variables and it explained 45% of the variability in stonefly density while a model with all of the top three variables explained about half of overall variability (50%), so further research is necessary to identify other environmental factors that could be influencing stonefly density. To support an average density of stoneflies (compared to sites in these three Colorado Rivers), a site would have approximately 5.1% fine sediment in the riffles while maximum stonefly densities could be expected with sites that have 0-2.5% fine sediment. 

The results of this study indicate that giant stoneflies prefer riffle habitat with low fine sediment, larger sized cobble and low levels of cobble embeddedness. This information will be used to inform management and restoration activities, as well as to identify sites for giant stonefly re-introductions. By maintaining and restoring giant stonefly populations, wildlife managers can protect an important part of native aquatic ecosystems and improve river fisheries for Colorado's anglers.