Acoustic Target Strength of Several Species of Fish 



By 



H. W. VOLBERG^ 



Vice President-Manager 



Straza Electronics 



Division of Straza Industries 



El Cajon, Calif. 92021 



ABSTRACT 



To design fish-finding sonar equipment it is necessary to have information about 

 target strengths of fish. This study was made principally to determine the target 

 strength of tunas at several acoustic frequencies. In addition, measurements were 

 made on other living, dead, fresh, and frozen fresh-water and salt-water fishes, 

 Sonne without swim bladders. 



INTRODUCTION 



The use of sound waves to locate objects in 

 the sea has been refined over the last few 

 decades. Most of the impetus for the develop- 

 ment of equipment and techniques came from 

 the military. Peaceful application, especially of 

 scanning sonar equipment, often lagged far be- 

 hind the potential of the military capabilities. 

 Many of the features necessary for sonar sys- 

 tem design are available from the military; 

 however, the most important one for fishery 

 work, the target strength of a fish, was not 

 available from the military. 



The problems involved in fishery work are 

 complicated by such variables as the presence 

 or absence of a swim bladder in the fish, the 

 behavior of the species (schooling or solitary), 

 the size of the individuals, the expected orien- 

 tation of the fish to the sound beam (vertical or 

 horizontal echo ranging), and the relation of 

 fish target strength to the sound frequency 

 used. Recently, European experiments have 

 yielded data on several species. These in- 

 vestigations (Harden-Jones and Pearce, 1958; 

 Midttun and Hoff, 1962) were chiefly concerned 

 with determining size/target strength relation 

 and the contribution of the swim bladder to 

 target strength. Gushing (1964) combined the 

 other European data with his to illustrate a 

 fish length-target strength relation applicable 

 for middle frequencies (near 30 kHz), The 



species reported have swim bladders; some of 

 Cushing's fish had artificial swim bladders 

 placed inside their bodies. 



The primary demand in American fishery 

 sonar work comes from the groups working on 

 the fast-swimming, often pelagic species such 

 as scombroids and salmonoids. Differences in 

 target strength between side and dorsal aspect 

 ire of interest as is, for example, the differ- 

 ence in target strength between a yellowfin 

 tuna ( Thunnus albacares ), a species with a 

 swin bladder, and a skipjack tuna ( Katsuwonus 

 pelamis ), a species without one. 



In 1963, Straza Electronics was invited to 

 submit a proposal for a high- resolution sonar 

 systenn for fishery research. To calculate 

 energy requirements and receiver sensitivity 

 values for the proposed system it was neces- 

 sary to know something about the target char- 

 acteristics of the species to be studied (chiefly 

 tunas). Also, the system envisioned would be 

 multifrequency, using frequency modulated 

 bands, one of several tens of kilo Hertz and 

 one of several hundreds of kilo Hertz. This 

 report summarizes the tests nnade to deter- 

 mine target strengths for several species of 

 fish at various frequencies and for various 

 positions of orientation. Differences of 

 target strength between living, fresh, and 

 frozen specimens and the contribution of 

 the swim bladder to the sound reflective 

 properties of some fishes also were investi- 

 gated. 



^This work was sponsored and funded in 1963 by Straza 

 Electronics, with the cooperation of the U.S. Navy Elec- 

 tronics Laboratory and the Bureau of Commercial Fish- 

 eries. 



2 Present address: Consultant In Acoustics, 5729 Gen- 

 natte Avenue, La Mesa, Calif. 92041. 



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