Preliminary results of acoustic survey data analysis for juvenile southern bluefin tuna 
                            (Thunnus maccoyii) (1996-1997)

            T. Nishida, T. Inagaki, K. Miyashita, H. Kishino, and J.L.Laake

                     Ocean Research Institute, University of Tokyo
                        Minamitai, Nakano-ku, Tokyo 164, Japan

     This paper summarizes the  results of the 1996 and 1997  SBT acoustic surveys 
conducted in the water off Southwest Australia during January-February.
     The main purpose  of this survey is to estimate  density and biomass of Age 1 
SBT(southern  bluefin tuna).  Scanning sonar,  scientific echo  sounder  are primarily 
used to  search SBT schools  for day  and night. Eye-ball  search is also  conducted 
during  day time.  in this  paper, density  and biomass  are estimated  based on  the 
information from sonar  and echo sounder.  Randomly selected  zig zag transect lines 
are used for the  survey, which cover the area homogeneously.   Search range of the 
sonar is  600m in both sides  from the transect  line. Majority of  SBT schools were 
found by the sonar.
     Experienced sonar  specialists can  distinguish schools  between SBT  and baits 
and  also  estimate biomass  of  the  SBT  schools.   Species  have  been  identified 
occasionally by fishing  such as troll and  pole. which confirms that sonar  specialists 
judgement are correct. In addition, estimates and detection of species groups by sonar 
specialists have been  repeatedly verified in the real fisheries.   Thus their judgement 
and estimate  in the acoustic  survey are likely  reliable.  Line  transect methods are 
applied  to estimate  density  and  biomass.   "DLSTANCE"(software)  is utilized  to 
conduct the  line transect method. Necessary  information are analyzed  and following 
estimation  are obtained  : density  (ton/km2) (1996:0.3679,  1997:0.3861), biomass(ton) 
(1996:5,226, 1997:5,635)  and biomass(million  of fish) (1996:1.70,  1997:1.84). Based  on 
these estimates,  it is concluded  that age 1  SBT abundance slightly  increased from 
1996 to 1997.