Abstract
To provide target strength (TS) information for estimating the body length of yellowfin tuna Thunnus al- bacares and its abundance around fish aggregating devices, TS was measured ex situ and in situ. In the ex situ TS measurements, two cameras synchronized with a 200 kHz echosounder were used to obtain the precise orientation of the yellowfin tuna under free swimming conditions. The ex situ TS (dB re 1 m 2 )-fork length (FL, cm) regression was: TS = 27.06 log (FL) - 85.04. Ex situ TS was found to reach its maximum in the tilt angle range of -15 to -20 after excluding TS samples with insignificant correlation to the tilt angle. The angle between the vertebra and the swim bladder was approximately 25 according to X-ray images, supporting the above tilt range. The relationship between the swim bladder volume (VSB, ml) and the fork length was: VSB = 0.000213 FL 3 . The results from the in situ TS measurements indicated that the tilt angle was highly concentrated between -10 and 15. The results from a calculation using the ex situ TS-FL equation with the fork length from biological sampling agreed strongly with the average in situ TS.
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