Mv. Trevorrow et Y. Tanaka, ACOUSTIC AND IN-SITU MEASUREMENTS OF FRESH-WATER AMPHIPODS (JESOGAMMARUS-ANNANDALEI) IN LAKE BIWA, JAPAN, Limnology and oceanography, 42(1), 1997, pp. 121-132
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.