with rise in frequency. The observations of 

 Harden-Jones and Pearce (1958) and Midttun 

 and Hoff (1962) showed an increase in target 

 strength and directionally with fish size up to 

 about 70 cm. At this point the thickness of the 

 tissue between the swim bladder and the sur- 

 face of the back of the fish is sufficiently great 

 to begin to nnask the contribution of the bladder. 

 The earlier results are combined with some of 

 ours in figure 10. The abscissa is in units of 

 20 log fish length to put the contribution of size 

 into logarithmic units. Target strengths of 

 both side and back aspect are given for our 

 larger fishes. In open water these species will 

 usually be detected at some slant range and 

 their target strength will lie somewhere be- 

 tween the values given for side and top. The 

 decreases in target strength due to flooding 

 of the swim bladder are indicated by the 

 vertical arrows. 



LENGTH dF FISH (cm.) 



40 50 60 70 80 90 100 



32 31 36 38 



20 LOG LENGTH 



SKIPJACK Tu^A, SIDE ASPECT 

 rELLOWFlN TUNA, SIDE ASPECT 

 YELLOWTAIL. BACK ASPECT 

 YELLOWFIN TUNA, BACK ASPECT 

 CRAPPIE, 

 BASS. 



SKIPJACK TU 

 YELLOWTAIL. 

 YELLOWTAIL. (C 

 YELLOWFIN TU 

 CRAPPIE, 

 BASS, 



lA, BACK ASPECT 



SIDE ASPECT 



Ibockl, BLADDER FLOODED 



:h|, BLADDER FLOODEC 



1 perch] JONES a PEARCE 1959 



2 COO, BACK ASPECT 



3 COALFISH, BACK ASPECT 



MIDTTUN a HOFF 



Figure 10. — Comparison of target strength measurements 

 made in this experiment with those obtained by other in- 

 vestigators. All measurements were made between 20 

 and 40 kHz. 



SUMMARY 



1. The acoustic target strength for skipjack 

 and yellowfin tunas, yellowtail, white crappie, 

 and largemouth bass were measured for dif- 

 ferent orientations of the fish at frequencies 

 from 20 kHz to 280 kHz. 



2. The differences in target strength among 

 living, dead, chilled, and frozen fish were 

 negligible, 



3. The contribution of the swim bladder to 

 target strength decreased as size of the fish 

 and test frequency increased. 



4. With the exception of the contribution of 

 the swim bladder, target strength was inde- 

 pendent of frequency. 



ACKNOWLEDGMENTS 



J. Roshon and A. Huntly of the U.S. Navy 

 Electronics Laboratory, San Diego, provided 

 technical assistance. Personnel of the Bureau 

 of Commercial Fisheries Biological Labora- 

 tory, Honolulu, and the Bureau of Commercial 

 Fisheries Fishery-Oceanography Center, La 

 JoUa, assisted by supplying test specimens. 



LITERATURE CITED 



CUSHING, D, H. 



1964. Cons. Perma. Int. Explor. Mer, Con- 

 tributions to Symposium 1963 on the 

 Measurement of Abundance of Fish 

 Stocks. J. A. Gulland[ed.], 34: 190-195. 



GOLDMAN, D, E., and T. F. HEUTER, 



1957, Errata: Tabular data of velocity and 

 absorption of high frequency sound in 

 mammalian tissues. J. Acoust. Soc. of 

 Amer. 29: 655, 



HARDEN-JONES, F. R,, and G. PEARCE. 



1958. Acoustic reflection experiments with 

 perch ( Perca fluviatilis Linn.) to de- 

 termine the proportion of the echo re- 

 turned by the swim bladder. J. Exp. 

 Biol., 35: 437-450, 



MIDTTUN, L., and I. HOFF. 



1962, Measurements of the reflection of 

 sound by fish. Rep, Norw, Fish, Mar, 

 Invest. 13(3): 18 pp. 



MS. #1997 



26 



GPO 888.770 



