Sources of acoustic backscattering at 87.5 kHz

Abstract

Sources of acoustic backscattered energy at 87.5 kHz were identified using three independent approaches. The first approach combined the Johnson fluid sphere scattering model [R. K. Johnson, J. Acoust. Soc. Am. 61, 375–377 (1977)] with a hypothesized distribution of scatters. From these models the percentage of total scattering due to organisms of a particular size range was determined. The second approach inferred scatterer abundance from probability distributions of acoustic scattering strength for insonified volumes of different sizes. The third approach consisted of comparing levels and patterns of scattering strength observed with a sonar to biological samples taken in the same area at the same time. All three approaches indicated that the sources of backscattered acoustic energy came from large zooplankton and small nekton. However, the first approach underestimated the importance of large scatterers at night. Day scattering was dominated by large copepods and small euphausiids. When small fish were present, they dominated the scattering. Night samples were dominated by mesopelagic fish, squid, and euphausiids. The fish and squid usually dominated the scattering early in the evening while euphausiids tended to dominate the scattering later at night.