In situ and ex situ target strength measurement of mesopelagic lanternfish, diaphus theta (family myctophidae)

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-QUESTx) 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 () and a standard length (SL) in cm gave the empirical length - average TS relationship, = 20logSL70.6, 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.