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

Abstract

During a 23-d study of Lake Biwa, Japan (starting 23 August 1993), multifrequency inverted echo-sounder measurements of nocturnal scattering layers in the meta- and epilimnion were performed at a single location. Direct samples from within this scattering layer indicated that it was composed of Jesogammarus annandalei (Crustacea: Amphipoda), with mean adult length of 8.3 mm and population densities from 4 to 50 per m3. Estimates of the scattering cross-section for individual amphipods were extracted from echo-amplitude probability distributions combined with volume scattering strength from a 198-kHz sonar. Total scattering cross-sections for adult amphipods at 88, 118, and 198 kHz were estimated as 4.3 ± 0.9 x 10-8 m2, 8.7 ± 1.2 x 10-8 m2, and 2.8 ± 0.5 x 10-7 m2. 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 (10-min averaged) showed maxima of 10-30 per m3 near 2000 hours, with densities decreasing rapidly toward midnight and disappearing by approximately 0430 hours.