MEASUREMENTS OF FISH TARGET STRENGTH: A REVIEW 



Richard H. Love' 

 ABSTRACT 



The concept of target strength and its application to the quantitative assessment of fishery resources 

 are discussed. Methods of determining the echo characteristics of fish are reviewed and a number of 

 results presented. Among the more important of these results are: (1) practically every case of in- 

 terest to the fishing industry is in an acoustic region in which the target strength varies widely with 

 fish size and aspect and acoustic frequency, (2) the major contributors to target strength in this region 

 have been determined to be the swim bladder, flesh, and skeleton, and (3) the average maximum side- 

 aspect and dorsal-aspect target strength of an individual fish have been determined for this region. 



Quantitative assessment of fishery resources is 

 a difficult task, and many groups have turned to 

 acoustic techniques to conduct assessment sur- 

 veys. Present acoustic techniques can give an 

 estimate of the number and dimensions of fish 

 schools in a geographic area and, by estimating 

 the density of the fish in a school, the number of 

 individual fish in the area can be approximated. 

 Measurements of fish target strength are being 

 made by various investigators in an effort to en- 

 able the direct acoustic estimation of the number 

 and size of individuals in a school and to enable 

 the direct identification of those individuals by 

 acoustic methods. This paper discusses the 

 concept of target strength and its application 

 to the quantification and/or identification of fish 

 schools, reviews target strength measurement 

 techniques, and discusses some results which 

 have been obtained utilizing these techniques. 



TARGET STRENGTH 



Active sonars project acoustic energy into the 

 water in an effort to detect objects by the echoes 

 they return, the intensity of the echo depending 

 on the proportion of the sound reflected back to 

 the receiver. The target strength of the echo- 

 producing object is a quantitative measure of its 

 reflecting characteristics and is defined as 



10 log 



a)' 



(1) 



' U.S. Naval Oceanographic Office, Washington, D.C. 

 20390. 



Manuscript accepted April 1971 



FISHERY BULLETIN: VOL. 69. NO. 4, 1971. 



where h is the intensity of the sound striking 

 the target and /i is the intensity of the reflected 

 sound measured at 1 m from the acoustic center 

 of the target. If h is the intensity of the re- 

 flected sound measured at some distance r from 

 the target, then, assuming that the sound spreads 

 spherically, and there are no losses, h will be 

 directly proportional to h: 



Ir 



V47r/ ?■= 



(2) 



o- is defined as the acoustic cross-section of the 

 target, and inr- is the spherical surface area 

 through which all the incident energy is reflected, 

 cr depends on the size, shape, and orientation 

 of the target, and, in general, will vary with the 

 angle between the incident direction and the 

 direction of the receiver. In all present fish- 

 eries work, this angle is zero and therefore in 

 this paper cr will be the acoustic cross-section 

 of a target for the case in which the source and 

 receiver are located at the same point. 



By letting r in equation (2) be equal to 1 m 

 and combining equations (1) and (2), 



10 log 



fe) 



(3) 



Now if Is is the intensity of the projected sound 

 1 m from the source, 



h = ll 



and therefore, 



(4) 



703 



