Modeling target strength of individual herring (Clupea harengus) at any aspect as a function of pressure and frequency

Abstract

Fish target strength is primarily dependent on the physical dimensions of the fish, the acoustic frequency, and the orientation of the fish. In traditional vertically observing echosounder surveys, fish are insonified in the dorsal aspect with fairly limited tilt angle variation. In oblique-angled sonar surveys, however, fish may be insonified at other aspects. For herring, target strength is also depth dependent as they cannot refill the swimbladder at depth. Understanding the depth dependent target strength from several insonification angles is thus required for quantitative measurements with sonar. The dataset used in this study consists of seven herring, imaged using magnetic resonance imaging (MRI). The herring were placed in a pressure chamber inside the MRI, and subjected to different pressures corresponding to water depths of 0, 20, 40, and 60 m. Images were acquired of each specimen at each pressure. The swimbladders were segmented and 3D models of the swimbladders for each fish and pressure were constructed. These models were then used for computing the directivity pattern of the swimbladder at any angle as a function of frequency using the finite element method. Modeling results are also compared with measured dorsal and side aspect TS at different depths. The broadband backscattering (30-200 kHz) is evaluated for the potential for species and size discrimination and the effect including the fish body is discussed.