acoustic reflections which had too great am- 

 plitude, especially at 280 kHz, for the types 

 of tests anticipated. Monofilament nylon af- 

 forded an improvement of some 15 db over 

 the wire rigging at 280 kHz and was used 

 throughout the tests. 



TESTS 



Previous studies (Harden-Jones and Pearce, 

 1958; Midttun and Hoff, 1962) show that the 

 swim bladder in fishes is an important con- 

 tributor to their acoustic cross-section. Since 

 skipjack tuna (Katsuwonus pelamis ) lack a 

 swim bladder yet are of considerable com- 

 mercial importance, they were tested for com- 

 parison with fishes such as the yellowtail 

 ( Seriola dorsalis ) that have swim bladders. 



Fish Without Swim Bladders 



To illustrate the nature of the directivity 

 patterns obtained, two figures are shown for a 

 skipjack tuna. This tuna was 60 cm. long, 

 weighed 5.5 kg., and initially had been chilled 

 but not frozen for 48 hours. Measurements 

 were made at two frequencies --40 kHz and 

 280 kHz. Figures 3 and 4 show patterns ob- 

 tained when the dorsal fin was in the vertical 

 plane and the body rotated in the horizontal 



SKIPJACK ICh.lladI 



12 poundi 60 cm long 



DItlCTIVITT MTTIRN 



DIRiCTIVITY PATTERN 



♦ = 270' 



SKIPJACK iCh.lledl 



12 poundv 60 cm 



F.eqcency 40 liMc ♦=270" Bolole 6 Depth 3 90 m.leti 



Tetr D.ilonce 6 m.rc. Tempefoiu-e 2rC Scoie lOdboftH 



Figure 3. — Directivity pattern for a 60 cm. skipjack tuna 

 rotated in the horizontal plane. The test frequency was 

 40 kHz. For this and following figures the target range 

 was 6 m., the temperature 21° C, and the test depth 

 3.9 m. Each ring represents a 10 db decrement from the 

 outer -20 db ring. 



Figure 4. — Directivity pattern for a 60 cm. skipjack tuna 

 rotated In the horizontal plane. The test frequency was 

 280 kHz. For this and following figures the target range 

 was 6 m., the temperature 21° C., and the test depths 

 3.9 m. Each ring represents a 10 db decrement from the 

 outer -20 db ring. 



plane. These observations indicated that the 

 target strengths of the skipjack tuna tested 

 were essentially the same at the frequencies 

 used. The structure of the beam patterns be- 

 came more spiked at the higher frequency. 



One can calculate the target strength of the 

 fish when viewed from the side by assuming 

 its effective acoustic target area to be equal 



to its projected area. Using Tg = 10 log (r^) 



in which A = the scattering cross -section or 

 effective target area, the calculated target 

 strength is about -23 db at the beam aspect. 

 This value is of the same order of magnitude 

 as the value of about -20 db obtained by the 

 measurements. 



Figure 5 shows several measurements of 

 different skipjack tuna at afrequencyof 40kHz 

 orientated and rotated as before. Only the left 

 side is shown. The 66- and 72-cm, fish were 

 frozen. Their patterns indicate a greater 

 amount of directivity, perhaps because they 

 were stiffer than the 60- and 70-cm, fish. 



Figure 6 shows the target strengths of the 

 skipjack tuna when each fish was supported 

 nose up, tail down, and rotated about its long 

 axis. The figure gives the directivity patterns 

 in views of the back and sides of the fish. 

 Measurements were made at 40 kHz and 280 

 kHz. A slight increase in target strength is 

 seen for the larger fish. 



23 



