LOVE: MEASUREMENTS OF FISH TARGET STRENGTH 



on how the component is approximated and on 

 the limits of that ai^proximation. Hence the 

 curves are a function of whether the swim blad- 

 der is approximated by a spherical bubble or a 

 rigid cylinder; what the limits of the geometric 

 and Rayleigh scattering regions are for the body 

 and the backbone; and whether the body is 

 approximated by an ellipsoid or a plane in the 

 geometric region. These curves indicate that 

 the swim bladder jiredominates over most of the 

 given L/\ range, but that the body and backbone 

 become significant at the higher L/X's. It is 

 apparent that Haslett's measurements of the 

 acoustic cross-sections of sticklebacks and gup- 

 pies vary widely and do not follow any of his 

 curves. This variability is not fully explained 

 by any of his experiments on fish or models. 

 Obviously, the nature of the echo-formation pro- 

 cess is quite complex if Haslett cannot explain 

 this variability after such comprehensive work. 



In attempting to quantify fish resources the 

 objects of interest are usually fish schools rather 

 than individual fish. When the target strength 

 of a school is measured, the question to be an- 

 swered is "What is the average size and number 

 of fish required to produce this target strength?" 

 The answer will depend on the average target 

 strength of the fish in the school, and any vari- 

 ations among the individuals will be of minor 

 importance. If a forward-looking sonar is used 

 for quantification, the minimum size and number 

 of fish required to produce a given target 

 strength will occur at the aspect for which the 

 target strength of an individual fish is a max- 

 imum. This aspect will be near the side aspect 

 of the fish. Thus, average values for the max- 

 imum side-aspect target strength of an individual 

 fish are important for quantification of fish 

 schools with a forward-looking sonar. If a down- 

 ward-looking sonar, or echo sounder, is used for 

 quantification, average values for the dorsal- 

 aspect target strength of an individual fish are 

 important. 



For these reasons the author has made max- 

 imum side-aspect (Love, 1969) and dorsal-aspect 

 (Love, 1971) target strength measurements as 

 a function of fish size, species, and frequency. 

 The measurements were made in the laboratory, 

 and hollow rubber spheres were used as refer- 



ence targets for calibration. It was found that 

 the magnitude of the variation in target strength 

 for one species was of the same order as it was 

 for all species. Therefore the data for all spe- 

 cies were combined with all other available per- 

 tinent data and a regression line was calculated 

 for each aspect using the method of least squares. 

 Figure 4 shows all the dorsal aspect data. The 

 data were obtained using fish from Ifi families 

 in 8 different orders: Clupeiformes, Cyprini- 

 formes. Gasterosteiformes, Cyprinodontiformes, 

 Mugiliformes, Gadiformes, Beryciformes, and 

 Perciformes. The fish ranged in length from 

 about 1 cm to 1 m. Some had swim bladders 

 while others did not. Insonifying frequencies 

 ranged from 8 kHz to 1480 kHz. Note that the 

 parameters used here are a/X- and L/\, which 

 diff"er slightly from those used by Haslett. The 

 equation for the regression line calculated from 

 these data is 



o-A^ = 0.041 (L/X)i94, (9) 



and the dorsal-as]iect target strength is 



r„ = 19.4 log L + 0.6 log \ — 24.9 (10) 



Equation (10) is for T,, at 1 m and L and \ in 

 meters and is valid in the range 0.7 ^ L X ^ 90. 

 Figure 5 shows all the maximum side-aspect 

 data. The data were obtained using fish from 

 13 families in 7 different orders: Cypriniformes, 

 Gasterosteiformes, Cyprinodontiformes, Gadi- 

 formes, Beryciformes, Perciformes, and Pleuro- 

 nectiformes. Fish size and acoustic frequency 

 ranges were approximately the same as those for 

 dorsal aspect. The equation for the regression 

 line calculated from these data is 



0-/X2 = 0.064 (L/\y\ (11) 



and the maximum side-aspect target strength is 



Ts = 22.8 log L — 2.8 log X — 22.9 (12) 



Equation (12) is valid in the range 1 ^ L/X 

 :^ 130, and again Ts is at 1 m and L and X are in 

 meters. 



Figure 6 is a nomogram solving equations 

 (10) and (12), given the acoustic frequency, /, 

 in kHz, and the fish length, L, in cm. 



709 



