Abstract
Acoustic target strengths (TS) of myctophid fish Diaphus theta were measured in situ at 70 kHz off the east coast of Hokkaido by tethering an acoustic-optical system (the Japanese Quantitative Echosounder and Stereo-video Camera System or J-QUESTχ) at a depth of 150 m from a research vessel. Fish length distributions were obtained from daytime hauls taken with a frame trawl after the TS measurement. Measured average TS (＜TS＞) and a standard length (SL) in cm gave the empirical length-average TS relationship,＜TS＞=20logSL-70.6, assuming that the linear TS is proportional to the square of the SL. A laboratory TS measurement and theoretical modeling of D. theta support the estimated length-average TS relationship.
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