2014 Volume 78 Issue 2 Pages 97-103
The diel vertical migration of walleye pollock Theragra chalcogramma late larvae and early juveniles (12–32mm in body length) was examined by the acoustic method using a quantitative echosounder and net towing on two transectsin the northern Japan Sea around Hokkaido in April. Larvae and juveniles of walleye pollock were distributed at 40–80m and 20–50m depth layers during the daytime and the nighttime, respectively. They hardly migrated to the acoustic deadzone, such as near the surface and the seabed area, which allows examining the distribution pattern by acousticdata logging. The total backscattering volume (NASC: m2 · NM-2) of them for each transect was 1.1- or 1.4-times higher during the nighttime than the daytime. It would be caused by diel differences in distributional concentration of euphausiids and spatial overlap of euphausiids and pollock larvae and juveniles. During the daytime, euphausiids aggregated in swarms at the middle layer; these swarms were easily distinguishable from “pollock echograms” and we removed these swarms from the echo integrations. On the other hand, euphausiids were diffused and they overlapped with the distribution of walleye pollock at night, so it was difficult to discriminate clearly between these two organisms. Therefore, acoustic surveys to estimate the biomass of walleye pollock larvae and juveniles should be carried out in daytime.
