Target strength measurement of encaged juvenile fish

		Kohji Iida*1, Tohru Mukai*1, and DooJin Hwang*2



	*1: Hokkaido University, Faculty of Fisheries, Japan
	*2: National Research Institute of Fisheries Engineering, Japan


     Target strengths of  juveniles of six species  at five frequencies were measured 
experimentally using  fish cage for the  purpose of estimating  recruit abundance and 
distinguishing them from adults in acoustic survey.
     Measurements were  conducted on the barge,  which was constructed in  fishing 
port for the experiments. The  juveniles used for measurements were walleye pollock, 
cod, herring, sea smelt, goldeye rockfish, purple puffer, which were caught by set net 
or carried from artificial breeding  tanks. Body lengths of juveniles were ranged from 
50 to 100 mm  and they were classified into three size classes  in every species. The 
specimens were accommodated  into the cubical cage  of 0.125m^3, which is  made of 
lace cloths. The echo  signals from encaged fish at 28, 39,  50, 98, and 198 kHz with 
increasing the density  of fish exponentially were  recorded into a data recorder.  The 
echo signals  were digitized  later and processed  by squared integration  to calculate 
the target strength of encaged fish.
     Results show that (1)The backscattering  strength of encaged juveniles indicates 
linear  density dependence  in  all size  classes  in all  species,  (2)Normalized target 
strength (TScm)  of juveniles indicates  -75 to -65  dB, which is  about 5 dB  lower 
than  that  of  adults,   and  (3)Normalized  target  strength  shows  the  frequency 
dependence that TScm below L/lambda=3D6 is 5dB higher than over L/lambda=3D6.