LOVE: MEASUREMENTS OF FISH TARGET STRENGTH 



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Figure 1. — Typical target strength measurement system 

 electronics. 



neous material and had the same shape, it would 

 take a comprehensive measurement program to 

 determine the target strength of a fish at all 

 aspects and frequencies because of the complex 

 shape of the body. Since fish are definitely not 

 homogeneous and different species of fish have 

 different shapes and internal structures, the 

 problem of determining the target strengths for 

 all species becomes immense. Considering the 

 differences among individuals of the same spe- 

 cies due to age, sex, condition of health, etc., it is 

 obvious that an experimental program to pre- 

 dict completely the target strength of any given 

 fish is impossible. Since the complete determi- 

 nation of the target strength of all fish is im- 

 possible, the most that can be hoped for is that 

 experimental techniques will eventually lead to 

 empirical results which can be generalized to 

 apply to any species of importance. 



RESULTS OF MEASUREMENTS OF 

 INDIVIDUAL FISH 



Most of the early experiments conducted dur- 

 ing the 1950's investigated a specific situation. 

 For example, if a researcher had an echo sounder 

 which operated at a given frequency, he would 



measure the dorsal-aspect target strengths of a 

 number of fish of the commercial species found 

 in his geographic area, in order to obtain an 

 average dorsal-aspect target strength vs. fish 

 size for those species. This information was 

 valuable to anyone designing or using an echo 

 sounder of the same frequency to find these 

 species, but it was of minimal value to anyone 

 else. 



A second technique used is just an extension 

 of the earlier technique. With it, different spe- 

 cies of fish have been examined at many aspects 

 and/or frequencies in attempts to determine how 

 target strength varies with fish size, species, 

 aspect, and frequency. Figure 2 shows some 

 typical results of this technique. The results 

 are from a live 21-cm black crapjne which was 

 rotated about its dorsiventral axis and insonified 

 with frequencies of 30 kHz and 130 kHz. It 

 is seen that the maximum target strength occurs 

 very near the side aspect, where the insonified 

 area is a maximum. At 130 kHz the number of 

 lobes in the pattern is substantially greater, and 



 50dB -40118 ^*^-30dB 



Figure 2. — Target strength of a 21-cm black crappie 

 versus aspect, (a) 30 kHz. (b) l.'iO kHz. (From Love, 

 1969.) 



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