DIEL CHANGE IN ACOUSTIC CHARACTERISTICS AND ZOOPLANKTON COMPOSITION OF THE SOUND SCATTERING LAYER IN I-LAN BAY IN NORTHEASTERN TAIWAN

Abstract

A 36-h acoustic observation of the I-Lan Bay was conducted on board the Ocean Research II from June 2 to 4, 2013. The acoustic volume backscattering strength (SV) was measured using a scientific echo sounder (EK500, 38 and 120 kHz) to determine the acoustic characteristics of the deep scattering layers (DSLs) before and after the vertical movement. Two DSLs were found during the daytime at a depth of 80-130 m (upper DSL, UD) and 130-230 m (lower DSL, LD), with mean SVs of approximately -78 dB (UD) and -77 dB (LD) at 38 kHz and -65 dB (UD) and −60 dB (LD, 130-200 m) at 120 kHz. Two clear diel vertical movements (DVMs) were detected for both DSLs, which started to ascend at dusk at a speed of 1.04 cm/s (UD) and 1.92 cm/s (LD) and remained at 10-80 m at nighttime. These 2 DSLs started to descend at dawn with a speed of 0.59 cm/s (UD) and 1.38 cm/s (LD) and then remained at depths of 80-130 m (UD) and 130-230 m (LD) during the daytime. The habited depth of both DSLs may influenced by the change of water temperature. The dominant species of DSLs in abundance and biomass were copepods and decapods, respectively. The vertical nighttime distribution of the dominant scatterer of smaller and larger decapods was discussed in this paper.