Acoustic identification of krill and copepods using frequency differences of volume backscattering strength around Funka Bay, Hokkaido, Japan

Abstract

In and around Funka Bay in southwest Hokkaido, Japan, krill and copepods are the dominant species of zooplankton. In this area, they are important prey for walleye pollock Gadus chalcogrammus and other marine animals. In this study, we examined the possibility of identifying krill and copepods based on differences in volume backscattering strength at different frequencies. Acoustic surveys were conducted in and around Funka Bay in March 2012 aboard the T/S Ushio-maru using a Simrad EK60 quantitative echo sounder. The acoustic data were collected at 38, 120 and 200 kHz. Biological samples were collected by a ring net with vertical towing. To identify the krill and copepods, differences in volume backscattering strength at three frequencies were examined. Krill and copepods showed stronger response at 120 and 200 kHz than at 38 kHz. The results showed that the differences in volume backscattering strength between 120 and 200 kHz (ΔSV200-120kHz) were -1.5 to 1.3 dB for krill and 2.2-3.7 dB for copepods. Based on these differences between frequencies, we were able to identify krill and copepods.