Depth dependence of swim bladder target strength for one finlet scad, round scad and Indian mackerel

Abstract

Target Strength (TS) is a main parameter for fish detection of abundance, size, species, population, and distribution. Fish stock assessment is necessary to provide advice optimum exploitation of fishery resources. The objectives of this research are to investigate in situ fish TS measurement at the sea, relationship between fish depth dependence and TS, and to investigate X-ray images of fish’s swim bladder and compared with fish TS. In situ TS measurement using FQ 80 echo sounder has been deployed for Selar boops, Megalaspis cordyla, Alapes djedaba, and Indian mackerel at South China Sea, exactly near from Redang and Kapas Island, Terengganu, Malaysia. FQ 80 included in the Research Vessel of KK Senangin II which transducer placed 2.8 meter from water surface. Fish put into net underwater which placed in the below of vessel. Ping-to-ping of TS value, depth, and position of fish targeted can be viewed from echogram by Top View of TS using FQ 80 Analyzer. TS of every fish species is different although at the same size and the same range or depth from transducer. For example, Megalaspis cordyla produce higher TS than Selar boops which have more length and weight. Depth dependence of every fish species influences to the backscattering cross section and TS. This influence varies based on specific-species. X-ray images of fish’s swim bladder had been deployed at Health Centre of Universiti Teknologi Malaysia. Length, width, volume, and surface area of swim bladder had been measured. Tilt angle of swim bladder to fish length has been measured. Physical properties of swim bladder from X-ray images show positive correlation with measured of in situ results. Fish with bigger swim bladder and less tilt angle produce higher TS.