Comparison of Horizontal Acoustic and Purse-Seine Estimates of Salmonid Densities and Sizes in Eleven Wyoming Waters

Evaluations of stocking programs for rainbow trout Oncorhynchus mykiss and cutthroat trout O. clarki in reservoirs and lakes of the western United States are typically made by estimating angler catch rates and total catch of stocked trout. Because these estimates are derived through expensive creel surveys, evaluations are infrequent. Trout management could be improved if a rapid, reliable technique to assess population levels was developed. The objective of this research was to determine if horizontal acoustic estimates of salmonid densities and mean lengths were correlated with data collected simultaneously with a 9.1-m-deep purse seine. During daylight hours of summer (June–September) trout were found in offshore surface waters and typically accounted for greater than 90% of the fish captured there. Salmonids seldom accounted for more than 10% of nearshore trap-net catches. This spatial segregation of salmonids offshore and nonsalmonids nearshore simplified interpretation of acoustic data. Side-looking acoustic and purse-seine estimates of salmonid densities were correlated (r = 0.87, N = 14) across the range of densities encountered (0.4–81.2 fish/ha). Salmonids captured offshore were typically large (>200 mm standard length), suggesting that newly stocked trout were underrepresented by purse-seine and acoustics samples. Side-looking estimates of mean salmonid lengths, transformed from target strengths, were within 50 mm of mean lengths captured by purse seine in 9 of 11 cases. Both purse seining and horizontal acoustics have strengths and weaknesses as sampling techniques, but when used together, the two methods provide invaluable information about salmonid densities, lengths, and body conditions. With good weather, a crew of six people could estimate trout numbers in a small impoundment (<1,500 ha) in 1–2 d. Data collection techniques described in this paper are likely to be effective when sampling salmonid populations in many waters of western North America.