Acoustic Distinction Between Pointhead Flounder and Juvenile Walleye Pollock by Echo Shape and Frequency Characteristics

Abstract

This study addresses the methods for distinguishing the pointhead flounder Cleisthenes pinetorum from the juvenile walleye pollock Gadus chalcogrammus in and near Funka Bay, Hokkaido, Japan. Acoustics data of th Gadus chalcogrammus e fish were monitored using a Simrad EK60 (38, 120, 200 kHz) split-beam echo sounder, and biological samples were collected using a rod for the pointhead flounder and a frame-type midwater trawl for the juvenile walleye pollock. The pointhead flounder schools presented a patch shaped echo on the echograms and showed strong scattering at all three frequencies, whereas the distribution patterns of the juvenile walleye pollock schools were layered. The volume backscattering strength (SV) of the target schools extracted from the echograms showed that the pointhead flounder presented a higher SV at high frequency, which is consistent with the early surveys of other bladderless fish. In contrast, the juvenile walleye pollock showed higher SV at low frequency, which also agreed with the early surveys.