In situ fish target strength measurements compared with x-ray images of swimbladder

Abstract

The swimbladder is considered to be responsible for most of the fish acoustic backscattering energy. An air-filled swimbladder can contribute up to 90% of backscattered sound. This paper discusses the results of in situ Target Strength (TS) measurement of Selar boops (Oxeye scad) and Megalaspis cordyla (Torpedo scad) compared with X-ray images of fish body and its swimbladder. In situ fish TS measurement has been deployed by using Furuno FQ-80 Scientific Echo Sounder. The deployment using net cage was placed in the bottom of vessel KK Senangin II. The measured results show that the TS of Megalaspis cordyla is higher than Selar boops. Laterally and dorsally X-ray imaged of fish has been deployed. The actual and ratio of swimbladder length, width, volume, and surface area to fish body had been measured and calculated. Tilt angle of swimbladder to horizontal fish length was measured as well. Ratio of swimbladder's length and surface area to fish body for Megalaspis cordyla is higher than Selar boops. The swimbladder's tilt angle to horizontal body length of Megalaspis cordyla is less than Selar boops. The X-ray images of swimbladder support the result from in situ measurements, where Megalaspis cordyla gives higher TS than Selar boops.