Material properties of North
Atlantic cod eggs and early-stage larvae and their influence on acoustic
scattering
Dezhang Chu, P.H. Wiebe, N.J. Copley, G.L. Lawson,
Velmurugu Puvanendran-2003
ICES Journal of Marine Science, 60(3): 435-707
Abstract:
To study the acoustic signatures of Atlantic cod (Gadus
morhua) at different biological stages from eggs to early-stage larvae
(<37 days post-hatch), we conducted a series of experiments to estimate
their sound-speed and density contrasts. A laboratory version of the
"Acoustic Properties of Zooplankton" system was used. Sound speed
was estimated by means of travel time between two transducers using a
broadband acoustic signal (~300–600 kHz). Density was estimated using
a dual-density method in which two fluids of different densities were
employed. It was found that the density contrasts of cod eggs and
early-stage larvae were nearly all slightly less than unity (0.969–0.998),
while the effective sound-speed contrasts were only slightly greater than
unity (1.017–1.024) for eggs and yolk-sac stage larvae (<5 days
post-hatch), and increased significantly (>1.130) for larvae older than
16 days. This change in sound-speed contrast reflected the transition of the
swimbladder from an uninflated state to an inflated state. The regression
relation between estimated target strength at 500 kHz and larval length
in centimetres was found to be TS=176.1 log10L-82.4 (dB). The
inflation ratio of the swimbladder for early-stage larvae was an exponential
function of time. The predicted period of time until full swimbladder
inflation was 43.3 days.
(Woods Hole Oceanographic Institution, Woods Hole, MA
02543, USA, e-mail: dchu@whoi.edu)