Precise fish target strength pattern measurement by the control method
                                     in an indoor tank

   K. Sawada, Y. Takao, T. Okumura, J. Sakai, K. Shirakaihara and Y. Miyanohana


                     Ebidai, Hasaki, Kashima, Ibaraki 314-04, Japan


     The dorsal-aspect  target strength of tethered  fish, ayu (Plecoglossus  altivelis) 
with open swimbladder is measured as a function of fish tilt angle, ranging from -50 
dB (head-down aspect) and  50 dB (head-up aspect) at interval  1 in an indoor tank.  
Operational frequency is 70 kHz.  Echo data are stored at each tilt angle on the hard 
disk built in an oscilloscope. 
      The preciseness  of the  measurement system  is confirmed  in advance  using 
artificial targets, whose shapes are prolate spheroid, and which are made of expanded 
polystyrene (EPS).   Since  the boundary  condition of a  submerged target  made of 
EPS could be considered nearly  equal to that of the swimbladder in water, three TS 
patterns are compared  between alive ayu, swimbladder model made  of EPS, and the 
soft prolate spheroidal scattering theory model.
       This  swimbladder model  is  manufactured  on the  basis  of  a alive  ayu's 
swimbladder shape  obtained from  the soft  X-ray images analysis  system.   These 
patterns agree well.  From these results we can conclude that swimbladder is a main 
body  of  the  backscattering  from  fish  and that  EPS  target  is  useful  for  the 
experimental  verification  for  theoretically   approximated  scattering  models  of  a 
submerged target with complicated shape.