Mac Pro

Abstract

When evaluation of biomass is made by integrating sonar signals, large systematical errors can occur, especially for subsurface layers within a depth range of 0-50 m. Such errors can be due to the observed fact that the acoustical backscattering target strength of a sea animal's wake can be larger than that of the animal itself. In this paper some examples are given. Investigation of this effect by using special processing procedure (taking into account temporal dynamics of target strength, spectral characteristics of scattered signals) has shown that a wake area moves in the water in the opposite direction compared to the animal movement direction. It was also shown that the target strength of the wake area has almost a constant value during 1.5-2 min. From our point of view the most likely hypothesis to be drawn from these facts is that the scattering of sound in the wake area is due to microbubbles. Bubbles are forced to stay in the vortex area of an animal's wake and after the decay of the vortex they rise up quickly. The number of bubbles in the wake of sea animals such as squid is not large and can be about 10. The target strength of a wake is about TS approximately -15 dB, for example, for squids at depths of more than 10 m. It is connected probably with the decrease of pressure in the wake area when squid eject water. At this moment a diffusion takes place of soluble gas from water to the microbubble nuclei which consistently exist. Besides, it was shown that in all cases of high target strength of wake area there was a high concentration of phytoplankton in the water.

Cite this document ^(BETA)