66 em FROZEN 



Figure 5. — Rectilinear plotof directivity patterns of 40 kHz 

 (left side only) of four skipjack tuna. See text for ex- 

 planation. 



270 290 310 330 350 10 



BACK 0° 

 d - 90° ROTATE ♦ 



70 cm LONG - 40 kHi. 

 60 cm LONG - 40 kHs. 

 60 cm LONG - 280 kHz. 

 70 cm LONG -280 kHi. 



anatomical parts of a skipjack tuna were esti- 

 mated by the progressive dissection of a 7 2 -cm. 

 specimen. First a reference pattern was run; 

 then patterns with the opercle and preopercle 

 removed from one side--from both sides--the 

 corselet removed- -pectoral and pelvic gridles 

 removed--head, tail, and viscera removed-- 

 flesh from one side removed- -and flesh from 

 both sides removed. These tests indicated that 

 any large cross-sectional area of any part of 

 the fish showed a substantial echo with respect 

 to the target strength of the intact fish. At 

 280 kHz no significant difference existed in the 

 beam aspect target strength when only the skel- 

 eton of the skipjack tuna was used minus head 

 and tail, compared to an intact fish. 



Fish With Swim Bladders 



A frozen yellowtail, 80 cm. long, was used in 

 a sTies of tests on fish with swim bladders, 

 f igure 7 shows a series of directivitypatterns 

 from the 80 -cm. -long yellowtail suspended nose 

 up, tail down, so that the back and sides of the 

 fish could be examined. The top two directivity 

 patterns are for 50 kHz--first with the fish 

 intact and second with the swim bladder punc- 

 tured and flooded. The bottom two patterns are 

 for 280 kHz under like conditions. The 50 kHz 

 frequency was selected because the effect of 

 the swim bladder for this specimen appeared 

 more pronounced there than at any other fre- 

 quency. 



Figure 6.— Directivity patterns for two skipjack tuna 

 suspended nose up, tail down and rotated about the long 

 axis. 



The irregular nature of the directivity pat- 

 terns showed the fish to be a highly complex 

 acoustic target. The surface of a fish and 

 various internal structures both are believed 

 to contribute to the reflectivity of the whole 

 animal. The relative contribution of various 



A 50 kH>, 



B 50 kHz - BlADDEfi PUNCTURED & FLOODED 



C 280 kH< 



D 280 kHz - BLADDER PUNCTURED & FLOODED 



Figure 7. — Directivity pattern for a yellowtail suspended 

 nose up, tail down and rotated about the long axis. Tests 

 were made with and without the swim bladder intact. 



24 



