Estimation of the variation in target-strength of fish under pressure condition

This paper reports a newly developed measurement system of fish swimbladder shape under pressure condition and theoretical modeling of bluegill (Lepomis macrochirus) under pressure condition. A pressure tank with a pressure device is used in combination with the existing soft X-ray machine. Observed swimbladder images taken from the lateral aspect are digitized to obtain the fish morphological parameters. Based on these parameters, target strength is calculated using deformed cylinder scattering model. Swimbladder images indicate that the swimbladder is compressed from the ventral side to the backbone of the fish. This fact supports that the swimbladder length is less sensitive to the pressure and the swimbladder height is sensitive to the pressure. Maximum TS in the calculated TS pattern at 59 m depth becomes 5.3 dB lower than one at the atmosphere condition at the frequency of 38 kHz. Averaged TS, assuming normal distribution of tilt angle with mean of -5.0 degrees (head down) and standard deviation of 15.0 degrees, becomes 4.8 dB lower.