Material properties of North Atlantic cod eggs and early-stage larvae and their influence on acoustic scattering

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 (similar to300-600 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 swim-bladder 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 log(10)L - 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. (C) 2003 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.