Abstract:Using the hydroacoustic method with a 38 kHz scientific echo sounding system, the diel vertical movement of the deep scattering layer (DSL) in I-Lan Bay, northeastern Taiwan, was studied on April 13-15 and June 12-14, 1997. The DSL started to descend about 30 minutes before sun set at a speed of 1.4-1.7 m/min and resided at 180 to 280 m in the daytime. It started to ascend ca. 30 min before sun rise at a speed of 1.5-1.9 m/min and stayed at 10 to 150 m at night. Plankton samples were simultaneously collected by an IKMT sampler to determine the species composition. The dominant species (ca.>1.5cm) were Sergia lucens and Pasiphaea japonica, which made up 89.3% of the catch. The mean packing density during the daytime was about 3 times higher than that at night.