Use of Underwater Observations to Estimate Cutthroat Trout Abundance in the Yellowstone River

Abstract

The abundance of cutthroat trout (Salmo clarki) in four study areas of the Yellowstone River in Yellowstone National Park was estimated three times in 1981 by a team of observers equipped with snorkeling gear. Divers spaced themselves along lengths of PVC pipe to maintain individual counting lanes. Three replicate counts were normally made in the main channel and one count was conducted along the banks. Cutthroat trout generally were not disturbed by the presence of the divers and replicate counts were consistent. Population estimates were made for each study area and date, and confidence limits varied from the point estimate by an average of 24.5%. The technique appears to be applicable to a number of larger rivers with underwater visibilities in excess of 3 m. Factors that could affect both the precision and accuracy of the technique include water depth, fish behavior, underwater visibility, and the degree to which fish densities in areas counted are representative of densities in stratified habitat zones.