Measurements and analyses are presented of the backscattering of 420-kHz sound by 43 individual animals of representative zooplanktonic and micronektonic taxa. Direct measurements of an individual's target strength were made with a commercial dual-beam sonar system in an enclosure filled with filtered seawater deployed off a dock at Friday Harbor, Washington. The dependence of target stengths upon individual length, wet weight, and dry weight was investigated. In addition, the target strength and statistical variations of echo amplitude due to variations in shape and orientation of the organism were compared with acoustic scattering models involving different shapes (the general shapes of the sphere, and straight and uniformly bent finite cylinders were used along with attempts to take into account roughness). It was found that: ( 1 ) backscattering cross sections are proportional to volume of the organisms rather than area as would be predicted by a sphere scattering model, (2) mean target strength based on average backscattering cross section is best described by the bent cylinder model whose modal series solution is truncated, and (3) the fluctuations of the echo amplitudes are well described by the Rice probability density function whose shape parameter is related to the randomly rough straight cylinder model. These extensive studies showed conclusively that the elongated animals scattered sound more like elongated targets than spherical ones, thus demonstrating the need for models more sophisticated than the spherical ones routinely used to date. The data and model analyses provide a basis for devising future acoustical data acquisition and processing techniques for bioacoustical field studies.
High-density demersal layers of krill have been detected in the submarine canyons off Georges Bank by means of a high-frequency, dual-beam bioacoustical technique. Krill densities in these demersal layers were observed to be two to three orders of magnitude greater than the highest densities observed in water-column scattering layers. Such abundances may help explain the unusually high squid and demersal fish production estimates attributed to the Georges Bank ecosystem.
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.
A high-frequency, dual-beam acoustical technique for directly estimating the size distributions within zooplankton assemblages has been developed. In combination with data from echo integration, acoustical size distributions can be used to apportion zooplankton numerical density and biomass concentration into different size classes. Two major conclusions can be drawn from calibration experiments and preliminary field studies using the technique: the backscattering intensities from individual zooplankters increase proportionally with the cubes of their lengths; and the combination of dual-beam and echo-integration techniques has the potential to produce rapid, high-resolution, size-specific data on zooplankton distributions in the water column.In the late 1940s Johnson (1948) proposed that the ocean's sound-scattering layers were likely to be biological in origin. At the same time, he suggested that such sound scattering or reverberation need not be treated strictly as noise, but could be used as a valuable tool in marine ecology. The subsequent development and refinement of acoustical techniques for analyzing lish distributions has received considerable attention (Venema 1985). In fact, such techniques have become the standard sampling procedures for many fishery assessment survey programs around the world (ICES in press). Lagging behind this work in fishery AcknowledgmentsWe thank D. Willows and the staff at Friday Harbor Laboratories for putting up with inconveniences and strange behavior on the dock associated with this project. In addition, we thank the captain and crew of the RV Seward Johnson and sub crew of the Johnson Sea Link for making our cruise scientifically productive and enjoyable. Special thanks are extended to M. Youngbluth for inviting us to participate on his cruise, as well as to N. Hairston, Jr., A. Morin, and two anonymous reviewers for reviewing early versions of the manuscript.
Hydroacoustic surveys with dual‐beam processing and echo integration techniques were conducted on Lake Oahe, South Dakota, in the summers of 1983‐1985 to assess the abundance and distribution of rainbow smelt (Osmerus mordax). Rainbow smelt are the primary coldwater forage fish for the developing salmonid fishery in this large Missouri River impoundment. Numerical estimates of pelagic fishes were 98,180,000 ± 5,550,000 (95% confidence limits), 95,430,000 ± 28,600,000, and 106,697,000 ± 21,880,000 for 1983, 1984, and 1985, respectively. Over 98% of the fish collected by midwater trawling and gillnetting during the surveys were rainbow smelt, suggesting that most of the fish sampled acoustically were also rainbow smelt. Biomass estimates for sampled portions of Lake Oahe were 157,100 kg (2.8 kg/hectare), 477,100 kg (8.5 kg/hectare), and 202,700 kg (3.6 kg/hectare) for 1983, 1984, and 1985, respectively. The accuracy of these biomass estimates is unknown because of the indirect method of calculating them. The geographical and vertical distributions of rainbow smelt were temperature dependent. Rainbow smelt occupied the colder water (5‐14° C of the hypolimnion in stratified areas. Only about one‐third of Lake Oahe's 152,000 surface hectares (at maximum pool) contained suitable coldwater habitat in the summer. Rainbow smelt made diel vertical migrations that were also temperature‐limited. During the day most rainbow smelt were within several meters of the reservoir bottom but moved upward to the thermocline at night. Some age‐0 rainbow smelt moved slightly into the epilimnion. The consistency of the acoustic estimates of rainbow smelt abundance over the 3‐year period indicated that the developing salmonid stocks have not yet significantly reduced the abundance of rainbow smelt.
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