計量魚群探知機でみた噴火湾の来遊するスケトウダラ産卵群の分布と海洋環境

Abstract

The distribution and behavior of so-called spawing walleye pollock Theragra chalcogramma around Funka Bay were investigated during the acoustic survey by the R/V “Ushio Maru” in late 1993. Three types of echo, namely, near surface echo patches, near bottom echo patches and the sound scattering layer (SSL) in the middle layer, were recorded on the echogram of the dual frequency quantitative echo sounder. Hydrographic data by CTD observation showed that cold and less salty water mass below 5°C and 33.5 layers on the bottom of the shelf of Oshima Peninsula then formed a water front along the boundaries to surrounded the warmer and more salty waters. By comparing the hydrographic and acoustic dana, the sound scattering layer appeared along the water front and demersal echo patches were found in great numbers in the cold water mass, Activity of fishing boats and hydrographic conditions in this area suggested that the demersal echo patches were the schools of walleye pollock which were migrating for spawning and the sound scattering layer consisted of zooplankton aggregation in the water front. Distribution of the echograms indicated that the height of fish schools were 1 to 4 meters and the SV of the echo were about -65 to -60 dB. The density of the schools of walleye pollock along the transects were 5.8 in November to 8.1 shoals/nautical mile in December, respectively.