Diel vertical movement of the deep scattering layer on the continental slope of I-Lan Bay, Taiwan

Using the hydroacoustic method with a 38 kHz scientific echo sounding syste m, the diel vertical movement of the deep scattering layer (DSL) in I-Lan B ay, northeastern Taiwan, was studied on April 13 similar to 15 and June 12- 14, 1997. The DSL started to descend about 30 minutes before sun set at a s peed of 1.4-1.7 m/min and resided at 180 to 280 m in the daytime. It starte d to ascend ca. 30 min before sun rise at a speed of 1.5-1.9 m/min and stay ed at 10 to 150 m at night. Plankton samples were simultaneously collected by an IKMT sampler to determine the species composition. The dominant speci es (ca. >1.5 cm) were Sergia lucens and Phasphaea japonica, which made up 8 9.3% of the catch. The mean packing density during the daytime was about 3 times higher than that at night.