FISHERY BULLETIN: VOL. 74, NO. 2 



NMFS, U.S. Navy, and CF&G. Two of the distribu- 

 tions are "absolute" target strength in decibels 

 and three are relative measurements, i.e., the 

 calibration coefficient was not included in the cal- 

 culations. The range of peak target strengths ob- 

 served in any one sample varies from 28 to 34 dB. 

 The two distributions of absolute target strength 

 were obtained with the same sonar unit aboard the 

 David Starr Jordan. The value of the calibration 

 coefficient was recomputed after hydrophone 



calibration between cruises and remained con- 

 stant. As such, the favorable comparison between 

 the samples may be deceptive. The CF&G data 

 were obtained and processed in a similar fashion 

 using a 38-kHz sounder. 



The theoretical target strength of a fish school 

 has been discussed by Weston (1967) and Uretsky 

 (1963). Modeling a fish school as a two dimen- 

 sional array of bubbles in a liquid, both Weston 

 and Uretsky predicted a sharp drop in response 



30- 



20 



10 



NOVEMBER 

 n=l98 

 )J = -l.ldB 

 s= 8.8dB 

 range= 35 dB 



JULY-AUGUST 

 n=l78 

 x = -9.lldB 

 s= 6.62 dB 

 range = 32 dB 



' -13 -7-1 5 II 17 

 RELATIVE TARGET STRENGTH (dB) 



10 



-21 -17 -13 -9-5-1 3 7 1 

 ABSOLUTE TARGET STRENGTH (dB) 



40- 



30- 



20- 



10- 



-40 -30 -20 -10 



RELATIVE TARGET STRENGTH (dB) 



I 6. FEB., OCT, a DEC. 

 ^"'~ n=ll7 



x= 14.2 

 s= 8.2 

 range= 36 dB 



16- 



DECEMBER 

 n=l09 

 x=-9.ll 

 s= 5.79 



ranges 28 dB 



J L. 



I'll I i_ 



15 



10 



>- 



o 



3 

 O 

 UJ 

 (T 

 U. 



UJ 



> 



5 :f 



UJ 



-20 -16 -12 -8 -4 



ABSOLUTE TARGET STRENGTH (dB) 



10 



10 Figure 4. — Distributions of five samples of peak school 

 target strengths; a and b are from NMFS data, c and d 

 are from Navy data, and e is from CF&G data. 



-7 -1 5 II 17 23 

 RELATIVE TARGET STRENGTH (dB) 



29 



286 



