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

Abstract

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.

Assessment of the pelagic fish populations using CEN multi-mesh gillnets: consequences for the characterization of the fish communities

“…In many studies dealing with lake fisheries, the CPUE resulting from gillnetting is often used as an indicator of biomass or abundance of fish populations (Yule, 2000). However, the correlation between the CPUE from gillnets and abundance based on other sampling techniques, such as active fishing gear or echo-sounding, has not always been confirmed (Hansson and Rudstam, 1995;Peltonen et al, 1999).…”

Paddlefish as Potential Acoustic Targets for Abundance Estimates

“…Species composition is an important consideration in acoustic surveys. Yule (2000) found similar densities of salmonids in acoustic surveys and purseseine hauls in Wyoming lakes and reservoirs, where purse seine bycatch was generally less than 10%. Cryer (1996) reported a good match of 10 acoustic surveys of rainbow trout Oncorhynchus mykiss from acoustic counts in Lake Taupo, New Zealand, with expectations from a range of published empirical models and a recent creel survey.…”

“…Generalized equations to estimate fish lengths from TS have been developed for the side, dorsal, and any aspects (Love 1969(Love , 1971(Love , 1977. These equations are widely used, and have proven useful for a variety of species (Burczynski et al 1987;Brandt et al 1991;Yule 2000;Warner et al 2002). However, the empirical, species-specific TS-length relations more closely estimate true fish size due to the complexity of acoustic backscatter (Foote 1987;MacLennan and Simmonds 1992;Fleischer et al 1997).…”

Banks Lake Fishery Evaluation Project Annual Report : Fiscal Year 2008 (March 1, 2008 to February 1, 2009).

“…The horizontal transducer was offset by 7° and sampled fish targets from 1.5-to 8 m below the surface. Data within 16 m of the horizontal transducer were excluded from analysis due to the narrow beam width and potential boat avoidance by fish in the near field (Mous and Kemper 1996;Yule 2000). The vertical transducer data were analyzed from 8 m below the transducer (9 m subsurface) to within 1 m of the bottom of the reservoir.…”

“…Species-specific abundance estimates were calculated by multiplying the species composition of various size classes by the acoustic abundance estimates for the corresponding sizes. We applied the length frequency from the vertical transducer to the horizontal data because fish target echoes in horizontal aspect do not relate to fish length as they do in vertical aspect (Kubecka 1994;Yule 2000). The assumption that fish species composition and size distribution was the same from 1.5-to 8 m (horizontal acoustics) and from 8-to 25 m was validated with netting data.…”