The plot in part (A) of figure 7 shows a -27 

 db target strength near the back portion of the 

 fish. This area is located along the abscissa 

 of the graph at about 0°. In the same area, the 

 return in part (B) was markedly reduced when 

 the bladder was punctured and flooded. Byway 

 of comparison, the higher frequency patterns 

 in parts (C) and (D) show no appreciable dif- 

 ferences. The influence of the swim bladder 

 at the lower frequencies might be due to 

 lessened attenuation by the flesh at those 

 frequencies and a consequent increase in the 

 response from the swim bladder. For fish of 

 the size of the yellowtail tested, and assuming 

 typical flesh attenuation to be in the order of 

 0.6 db/cm. at 280 kHz, a difference of some 

 7 db can be attributed to flesh attenuation 

 (Goldman and Heuter, 1957). This increased 

 attenuation at the higher frequencies could 

 mask the influence of the swim bladder. It is 

 also conceivable that some misalignment oc- 

 curred during the repeated aspect angles at 

 this high frequency. 



A similar experiment was run with yellowfin 

 tuna ( Thunnus albacares ) at a frequency of 

 40 kHz. In this test, the fish was suspended 

 both in normal swimnning position and nose up, 

 tail down. A typical pattern for the yellowfin 

 tuna is given in figure 8. Figure 9 shows a 

 series of directivity patterns for the 73-cm,- 

 long yellowfin tuna for 20 kHz and 40 kHz. A 

 comparison is made between an intact fish and 

 the same fish with its swim bladder punctured 

 and flooded. For this observation (head up, 

 tail down) the back portion of the fish was 



DIHECTIVITr PATTIBN 

 ♦ = 270 



YELIOWFIN TUNA 

 IFfoirnJ 73 em long 





* = 270 Rol 



Figure 8. — Typical 40 kHz directivity pattern for a yellow- 

 fin tuna suspended In the normal swimming position. 



9 • 90' 



ROTATE * 



73 cm LONG 



A 20 kHi 



B: 20 kHz - BLADDER PUNCTURED & FLOODED 



C: 40 kH! 



D 40 kHi - BLADDER PUNCTURED & FLOODED 



Figure 9. — Directivity pattern for a yellowfin tuna sus- 

 pended nose up, tall down and rotated about the long 

 axis. Tests were made with and without the swim bladder 

 Intact. 



viewed. At 20 kHz, difference in response be- 

 tween the intact fish and the fish after the 

 swim bladder had been flooded was not appre- 

 ciable. At 40 kHz, however, the return de- 

 creased significantly when the swim bladder 

 was flooded. With the above exceptions, all 

 the tests showed that in general the target 

 strength of yellowfin tuna was comparable 

 to that of skipjack tuna and yellowtail of ap- 

 proximately the same size. 



Additional test were made with largemouth 

 bass ( Micropterus salmoides ) and white crappie 

 ( Pomoxis annularis ), both alive and dead. When 

 the bass and crappie were alive, the gill 

 movement was readily observable on the plots 

 of the directivity pattern. Difference between 

 target strength in live and dead fish was scant 

 or nil. The target strengths were 15 db lower 

 than those of the larger tunas due to the smaller 

 size of the fish (largemouth bass 40 cm,, white 

 crappie 32 cm,). Deflating the bladders de- 

 creased the target strength 3 to 7 db, 



RELATION OF PRESENT TO EARLIER 

 FINDINGS 



It is evident that the target strength of a 

 fish depends largely on its size. Only in some 

 aspects, especially when the fish is viewed 

 from above, does the swim bladder contribute 

 significantly to the strength of the echo. The 

 contribution of the swim bladder rapidly de- 

 creases with increased frequency, apparently 

 because of an increase in attenuation by flesh 



25 



