Diet changes in vertical distribution patterns of zooplankton and walleye pollock (Theragra chalcogramma) off the Pacific coast of eastern Hokkaido, Japan estimated by the volume back scattering strength (Sv) difference method

In this study, we tried to accurately discriminate between zooplankton and walleye pollock using a quantitative echosounder at two frequencies (38 and 120 kHz). To estimate the abundance of walleye pollock and zooplankton (mainly Euphausia pacifica), an acoustic and trawling survey with bongo net sampling and CTD (X-CTD) was conducted during June 2001 off the Pacific coast of eastern Hokkaido, Japan. Neocalanus copepods and E. pacifica were predominant in almost all the net samples. One Size class of Neocalanus copepods (mean TL +/- SD = 2.0 +/- 1.1 mm, N = 1 044 080) and two size classes of E. pacifica (mean TL +/- SD = 6.1 +/- 2.2 mm, N = 13,099; mean TL +/- SD = 15.4 +/- 3.6 mm, N = 7628) were collected. The distribution patterns of these two classes and of walleye pollock were identified using the volume back scattering strength (Sv) difference method. On the continental shelf, the walleye pollock schools (small walleye pollock: mainly age 1 and 2) overlapped with almost all zooplankton layers during both day (near the sea bottom and mid-water) and night (mid-water and sea surface). Near and offshore of the continental shelf break, during day, the walleye pollock schools (large walleye pollock: mainly older than age 3) overlapped with almost all zooplankton layers near the sea bottom, but during night, the large walleye pollock schools (which were mainly distributed near the sea bottom) showed little overlap with most zooplankton layers (which were mainly distributed between mid-water and sea surface). This result suggests differences in feeding strategies between small and large walleye.