Target strength and behavior of swimming juvenile southern bluefin tuna in a cage estimated by echo‐trace analysis

Abstract

Echo‐trace analysis (ETA) was applied to echoes from juvenile southern bluefin tuna (Thunnus maccoyii, SBT) swimming in a cage. The experiments were conducted offshore Esperance in Western Australia, using a split‐beam echosounder operating at 70 kHz. The dimensions of a cylindrical cage were 7 m in diameter, 20 m in depth, and 8.5 cm mesh size of a net. A transducer was set at the center of a cage. The samples of SBT were caught by trolling near the study site. The age classes were 1 or 2 years old that were ranging 45 to 82 cm in fork length. Only one SBT was put into a cage at a time in order to obtain single echoes from one fish. As the swimbladder of juvenile SBT was not gas‐filled in this stage, its target strength (TS) was much lower than TS of a same‐size fish with gas‐filled swimbladder. Because of low TS values, there were some difficulties to obtain high quality echo‐traces continuously. So, preselections of high signal‐to‐noise and noninterference echoes from the echogram were needed before applying ETA. Swimming velocities and TS patterns in dorsal aspect were estimated by ETA.