1Graduate School of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido 041-8611, 2Graduate School of Agricultural and Life Sciences, The University of Tokyo, Bunkyo, Tokyo 113-8657, 3Tottori Prefectural Fisheries Experiment Station, Sakaiminato, Tottori 684-0046, Japan
We examined threshold-setting methods to identify Maurolicus japonicus SL (Scattering Layer) in the Sea of Japan. Acoustic survey data were collected in several months (May, June, Aug., Sep., and Oct.) from 2000 to 2003 around the Oki Islands, southwest of the Sea of Japan. The results showed the following threshold-setting method to be appropriate: to set the value as the lower limit SV threshold obtained by subtracting 10 dB from the averaged maximum SV of the water column. The depth range extracted by this method matched the existence of M. japonicus conrmed by the bottom and midwater trawl sampling. This method corresponds to the change of packing density by seeking the threshold baseline from the M. japonicus SL.