Study on acoustical estimation of distribution and abundance of isada krill Euphsusia 
          pacifica Hansen, off the Sanriku and off the Jhoban, northern Japan

 Kazushi Miyashita*1, Ichiro Aoki*2, Tadashi Asami*3, Hideki Mori*4, and Kenji Taki*5

		*1 Japan Marine Fishery Resources Research Center (JAMARC)
		*2 The University of Tokyo
		*3 Tokyo University of Fisheries
		*4 Hokkaido University
		*5 Tohoku National Fisheries Research Institute

     In this study,  we examined acoustical estimation  of distribution and abundance 
of  isada krill  Euphausia pacifica  Hansen using  two-frequency  algorithm with  the 
theoretical scattering model. In April 1997, the acoustic survey and midwater trawling 
were conducted in  the daytime off the Sanriku coast  and Jhoban coast, northeastern 
Japan. The  acoustic data, collected  with a two-frequency quantitative  echo sounder 
system (38kHz and 120 kHz), were used for analysis.
     In  this survey  are,  midwater trawling  showed that  isada  krill and  juvenile 
walleye  pollock Theragra  chalcogramma were  mainly distributed.  There were  two 
patterns  of  frequency  characteristics  in  back-scattering  strengths  (SV)  at  two 
frequencies: one  was the case when  the SV difference  (SV120-SV38) between two 
frequencies  was relatively  large (10dB<!bSV<15dB),  and the  other  was relatively 
small (!bSV<2dB).  The former was isada  krill and latter was  walleye pollock. The 
biomass density of  isada krill was very high in  the area off the Sanriku  coast, and 
relatively low in the area off the Jhoban coast.