Fig. 1

Diagram of the acoustic-mechanical apparatus used to measure cuttlebone target strength (TS) versus length (L _b ), and tilt angle (θ). The apparatus comprise: a rectangular (1.2 width x 1.8 length x 1.2 m height), acrylic, saltwater tank; 70 and 120 kHz echosounders (Simrad ES60 and EY500, respectively); two split-beam transducers (Simrad ES70-11 and ES120-7 F, respectively); and a DC servo-motor system for controlling the tilt angle of cuttlebone. The cuttlebone was slowly rotated for the angle range of −90° (head-down orientation) to 90° (head-up orientation) at a fixed speed of 0.167 rpm and stably maintained by tying monofilament lines between both ends of cuttlebone and two horizontal bars for suspending