Target strength measurements and modeling of walleye pollock and pacific hake

Abstract

This paper reports a laboratory target strength (TS) measurement and theoretical modelling of walleye pollock and Pacific hake. The measurements are performed in a tank as a function of fish tilt angle, which is controlled in one-degree steps. A precision echo sounder is used and sphere calibrations are conducted before or after the measurement. A specialized soft X-ray imaging system is used to map the fish swimbladder. The X-ray images are digitized to obtain the fish morphological parameters. Based on these parameters, theoretical TS of the fish is calculated using the approximate deformed cylinder model and the general prolate spheroid model. An advantage of using these models lies in their simplicity and the fact that few parameters are required in the model computation. Calculations indicate a negligible contribution of the fish body to the total scattering in the wide range of tilt angle. The theoretical TS values are compared with the measured values. Agreement between measured and theoretical TS is reasonably good for near normal incidence, but deteriorates for larger tilt angles. The level of agreement is discussed. Obtained maximum and average TS values are nearly equal to or a little bit smaller than the published data. Using the deformed cylinder model, broad band TS characteristics are generated. Our results highlight the usefulness of the simple method to predict fish TS using the deformed cylinder model associated with X-ray imaging.