Application of Dual-Beam Acoustic Survey Techniques to Limnetic Populations of juvenile Sockeye Salmon (Oncorhynchus nerka)

Abstract

Two hydroacoustic surveys of Cultus Lake, British Columbia, were conducted in July 1983 and February 1984 to estimate the size and distribution of the juvenile sockeye salmon (Oncorhynchus nerka) population. The surveys demonstrated the application of a combined dual-beam/echo integration technique for obtaining accurate abundance estimates and other quantitative data. The dual-beam system measured in situ the target strengths (and backscattering cross section) of individual fish. For each survey the mean backscattering cross section of the sampled population was used to scale the integrator outputs to absolute fish density and abundance estimates. Based on previously derived empirical formulas, the mean target strengths compared well with the mean lengths of fish captured by trawl. For this monospecies population with a dominant single-size group of fish, the variance in integrator outputs was due almost entirely to the spatial distribution of fish and not to the variance in measured backscattering cross sections. The confidence intervals of biomass estimates depend on autocorrelation between consecutive acoustic samples, which is related to the spatial distribution of the surveyed population. Trawl catches indicated that about 95% of the fish in the lake were juvenile sockeye salmon. Population estimates made by hydroacoustics were consistent with other available biological data.