Target strength measurements and modeling of walleye pollock and Pacific hake

This paper reports a laboratory target strength (TS) measurement and theore tical modelling of walleye pollock and Pacific hake. The measurements are p erformed 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 im aging system is used to map the fish swimbladder. The X-ray images are digi tized to obtain the fish morphological parameters. Based on these parameter s, theoretical TS of the fish is calculated using the approximate deformed cylinder model and the general prolate spheroid model. An advantage of usin g these models lies in their simplicity and the fact that few parameters ar e required in the model computation. Calculations indicate a negligible con tribution of the fish body to the total scattering in the wide range of til t angle. The theoretical TS values are compared with the measured values. A greement between measured and theoretical TS is reasonably good for near no rmal incidence, but deteriorates for larger tilt angles. The level of agree ment is discussed. Obtained maximum and average TS values are nearly equal to or a little bit smaller than the published data. Using the deformed cyli nder model, broad band TS characteristics are generated. Our results highli ght the usefulness of the simple method to predict fish TS using the deform ed cylinder model associated with X-ray imaging.