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Abstract

A 26-hr acoustic observation was carried out on board of the “Ocean Research II” in the Kuroshio waters at 123°09.027'E, 25°05.019'N from August 4 to 5, 2007. Acoustic volume scattering strengths (SVs) were collected by a scientific echo sounder (EK500, 38 kHz) and diel variations of sound scattering layer were analyzed. The maximum SV of surface scattering layers (SSL) around 20~80 m was observed after dawn, and mean SVs of SSL in nighttime was about -68 dB, 8 dB higher than that of daytime. Two deep scattering layers (DSL) were found in daytime at depth of 400~500 m and 600~700 m, which resulted in mean SVs were about -72 dB and -78 dB, respectively. Two clear diel vertical movements were also detected from these two DSL. A part of these two DSL started to ascend at dusk with a speed of 2.9 (±2.1) and 3.8 (±2.8) cm/s, and stayed at 10 to 120 m at nighttime, while another part of these DSL remained in the deep layer during daytime. These two DSL started to descend at dawn with a speed of 2.0 (±1.1) to 4.0 (±3.8) cm/s, and then stayed at depth of 400~500 m and 600~700 m during daytime, respectively.

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