ACOUSTIC AND IN-SITU MEASUREMENTS OF FRESH-WATER AMPHIPODS (JESOGAMMARUS-ANNANDALEI) IN LAKE BIWA, JAPAN

During a 23-d study of Lake Biwa, Japan (starting 23 August 1993), mul tifrequency inverted echo-sounder measurements of nocturnal scattering layers in the meta- and epilimnion were performed at a single locatio n. Direct samples from within this scattering layer indicated that it was composed of Jesogammarus annandalei (Crustacea: Amphipoda), with m ean adult length of 8.3 mm and population densities from 4 to 50 per m (3). Estimates of the scattering cross-section for individual amphipod s were extracted from echo-amplitude probability distributions combine d with volume scattering strength from a 198-kHz sonar. Total scatteri ng cross-sections for adult amphipods at 88, 118, and 198 kHz were est imated as 4.3 +/- 0.9 x 10(-8) m(2), 8.7 +/- 1.2 x 10(-8) m(2), and 2. 8 +/- 0.5 x 10(-7) m(2). These cross-section measurements were found t o be consistent with a fluid cylinder acoustic scattering model with a 1.2-mm radius and a 9.6-mm length. The acoustically derived populatio n densities, sizes, and length-to-radius ratio were consistent with in situ amphipod samples. The amphipods exhibited a clear nocturnal migr ation into the lower thermocline, concentrating at depths of 15-25 m b eginning after sunset (near 1830 hours local time) each day. Populatio n densities (10-min averaged) showed maxima of 10-30 per m(3) near 200 0 hours, with densities decreasing rapidly toward midnight and disappe aring by approximately 0430 hours.