LOVE: MEASUREMENTS OF I-ISII TARGET STRENGTH 



malacopterygians, the more primitive teleosts; 

 the crappies and seatrout are acanthoi)teryg'ians, 

 the more advanced teleosts; and the mummi- 

 choo;s, killifish, and silversides belong to inter- 

 mediate orders which have characteristics of 

 l)oth groups (Berg, 1947; Bertin and Arambourg, 

 1958). In general, the malacopterygians have 

 physostomous swim bladdei's, osseous bone tis- 

 sue, intermuscular bones, comparatively many 

 vertebrae, fins without siiines, and cycloid scales. 

 In general, the acanthopterygians have physo- 

 clistous swim bladders, osteoid bone tissue, no 

 intermuscular bones, comjiaratively few verte- 

 brae, fins with spines, and ctenoid scales. Con- 

 sidering that the swim bladder, bones, and 

 possibly scales of a fish contribute to its acoustic 

 cross-section, it is obvious that the malacopter- 

 ygians and the acanthopterygians have signifi- 

 cant structural differences in components which 

 have been shown to be acoustically important. 

 Why the malacojiterygians and one intermediate 

 species display the characteristic minimum in 



CRAPPIES 



02 



001 



005 



002 



00005 



GOLDFISH 



FiGl'RE 7. — Average measured dorsal-aspect acoustic 

 cross-sections for different species of fish. (From Love, 

 1971.) 



(t/L- near L X = 10, or why the acanthopter- 

 ygians and the other two intermediate species 

 have no distinctive cr/L- vs. L/\ curve cannot 

 be answered, given the present limited knowl- 

 edge of echo-formation by fish. 



Although these differences cannot be presently 

 exjjlained it seems probable that if there were a 

 geograjjhic area in which two species with about 

 the same size and habits predominated, and if 

 one species were a Clupeiform and the other a 

 Perciform, a ship with a wide-band sonar could 

 differentiate between individuals of each species 

 by examining their target strength vs. frequency 

 curves. This hypothetical example indicates how 

 very limited the present capability to identify 

 fish by determining target strength is. Hope- 

 fully, more measurements at many frequencies, 

 with dissection and removal of various compo- 

 nents of the fish, and more sophisticated model- 

 ing techniques will exjilain the features of the 

 target strength vs. frequency curves for indi- 

 viduals of a few species. This could then lead 

 to the prediction of curves for other species, 

 which in turn could greatly increase the ability 

 to differentiate between individuals of different 

 species. This information could then be applied 

 to the difl'erentiation of schools of different spe- 

 cies, although it is to be expected that the man- 

 ner of distribution of the fish in the school will 

 cause significant differences between the target 

 strength vs. frequency curve obtained for the 

 school and the average curve for the individuals 

 in the school. 



SUMMARY 



Some of the more important results of mea- 

 surements of fish target strength to date are: 

 ( 1 ) it has been determined that practically every 

 case of interest to the fishing industry is in the 

 region of interference effects, (2) the major con- 

 tributors to the target strength of a fish in this 

 region have been determined and their acoustic 

 impedances measured, (3) the variations of tar- 

 get strength with aspect for an individual fish 

 have been examined, (4) estimates of the dorsal- 

 aspect and maximum side-aspect target strength 

 of an individual fish have been made, (5) there 



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