Acoustic and in situ measurements of freshwater amphipods (Jesogammarus annandalei) in Lake Biwa, Japan

During a 23-d study of Lake Biwa, Japan (starting 23 August 1993), multifrequency inverted echo-sounder measurements of nocturnal scattering layers in the metaand epilimnion were performed at a single location. Direct samples from within this scattering layer indicated that it was composed of Jesogammarus annandulei (Crustacea: Amphipoda), with mean adult length of 8.3 mm and population densities from 4 to 50 per ml. Estimates of the scattering cross-section For individual amphipods were extracted from echo-amplitude probability distributions combined with volume scattering strength from a 198kHz sonar. Total scattering cross-sections for adult amphipods at 88, 118, and 198 kHz were estimated as 4.3-10.9 X 10d8 m2, 8.7t1.2 X 10e8 m2, and 2.8LO.5 X 1O-7 m*. These cross-section measurements were found to be consistent with a fluid cylinder acoustic scattering model with a 1.2-mm radius and a 9.6-mm length. The acoustically derived population densities, sizes, and length-to-radius ratio were consistent with in situ amphipod samples. The amphipods exhibited a clear nocturnal migration into the lower thermocline, concentrating at depths of 15-25 m beginning after sunset (near 1830 hours local time) each day. Population densities (lo-min averaged) showed maxima of lo-30 per m’ near 2000 hours, with densities decreasing rapidly toward midnight and disappearing by approximately 0430 hours. High-frequency, quantitative hydroacoustic techniques are effective tools for rapid, high-resolution surveys of both fish and zooplankton. Compared with conventional net-trawl techniques, acoustic systems have the advantages of high spatial and temporal resolution as well as potentially wide areal coverage when used from ships, Also, acoustic sampling does not have the problem of net or pump intake avoidance by larger animals. When the identity of acoustic targets is known, multifrequency sonar techniques can be used to extract population density vs. size distributions for both fish and zooplankton (e.g. Greene et al. 1989; Greenlaw 1979; Napp et al. 1993). The development of these bioacoustic surveying techniques has required an understanding of the detailed physics of acoustic scattering and development of accurate theoretical and empirical scattering models (e.g. Stanton 1989; Wiebe et al. 1990). In this study we demonstrate that these marine bioacoustic techniques and models can be readily transferred to a lake environment. Our bioacoustic results were collected as part of a multidisciplinary limnological study of Lake Biwa, Japan, from 21 August to 15 September 1993. This collaboration was Acknowledgments M.V.T. was funded by the Canada-Japan Science and Technology Fund, in collaboration with David Farmer of 1.0-S. and his sponsors. The SeaScan instrument development was funded by grants from the U.S. Office of Naval Research. Svein Vagle of I.O.S. assisted with the calibration of the SeaScan sonars. Field research and laboratory analysis was supported by a grant from the Japanese Ministry of Education, Science, and Culture (No. 05760158). WC arc also indebted to the many BITEX collaborators, and especially Michio Kumagai of the Lake Biwa Research Institute, who provided comparison temperature, water property, and meteorological data. called the Lake Biwa Transport Experiment (BITEX). The goal of this experiment was to understand the physical and biochemical ecosystems in the lake, thereby enabling more effective water quality management. Lake Biwa is the largest lake in Japan, with a surface area of 670.5 km2 and a maximum depth of 104 m. This lake is the water source for -14 million people in the surrounding prefecture and the KyotoOsaka-Kobe area and also supports a modest fishery of some 5,000 t per annum. The lake has two major basins (north and south) separated by a 1.35~km narrows. The north basin is the larger, with a length along a northeast-southwest axis of 48 km and a maximum width of 23 km. The bioacoustic study was in the pelagic zone near the southern end of the north basin. The sonar systems used in these experiments were originally set up for study of small air bubbles created by breaking surface waves; therefore, such systems were not optimal for investigation of crustaceans. Despite this limitation, 150 h of sonar measurements in the range 29-198 kHz were collected. These data were complemented by coincident measurements of phytoand zooplankton populations, water thermal structure, biochemical properties, and surface meteorology made by other BITEX participants. In this study we first needed to identify and separate the acoustic signatures of the various fish and zooplankton species. In general, the acoustic scattering data included varying contributions from juvenile and adult fish and several species of crustaceans (e.g. copepods, branchiopods, and amphipods). For amphipods, which follow well-known vertical migration patterns and thus are easily distinguished from other scatterers, both the frequency-dependence and echo-amplitude statistical variations of the acoustic backscatter provided estimates of mean sizes. Direct sampling at several times