Sonar Target Classification Experiments with a 

 Continuous-Transmission Doppler Sonar 



By 



FRANK J. HESTER, Fishery Biologist 



Bureau of Commercial Fisheries Fishery-Oceanography Center 

 La Jolla, Calif. 92037 



ABSTRACT 



A continuous -trans mission sonar with very fine echo frequency discrimination 

 was designed and constructed to study Doppler effects caused by the motion of fish 

 as they relate to fish size and swimming characteristics. Although the equipment 

 performed as theory predicted, difficulties with sea noise and trouble in maintaining 

 contact with fish schools showed that commercial application of this approach is 

 unsuitable without considerable additional development work. These problems and 

 some results are discussed, and notes on target-strength measurements of several 

 species of fishes are included in this report. 



INTRODUCTION 



In 1961 the Inter-American Tropical Tuna 

 Commission recognized that the stock of 

 yellowfin tuna in the eastern tropical Pacific 

 was in danger of being overfished and pro- 

 posed a catch quota to conserve the resource. 

 This quota could be increased if fishing for 

 small yellowfin tuna (fish less than 20 to 25 

 pounds [10 kg.]) could be controlled (Inter- 

 American Tropical Tuna Commission, I960). 

 It is difficult however to determine the size of 

 the fish before it is caught, and the attempt 

 has never been made. In addition, it is of value 

 to the fishermen to know approximately the 

 size of the fish before setting their nets. This 

 information decreases the chance of catching 

 fish so small that they "gill" in the net meshes. 



Recognizing the economic and managenaent 

 benefits to be gained fronn determining the size 

 of a fish before it is caught, the Tuna Re- 

 sources Laboratory of the Bureau of Com- 

 mercial Fisheries began a study to develop a 

 shipboard sonar that could locate and classify 

 fish schools. This study began in 1963 and 

 ended in 1968. The work was done chiefly 

 through contracts with industry. Straza In- 

 dustries,^ now Ametek/Straza, was the prime 

 contractor, and the bulk of this report is taken 

 from their statements of progress. For that 

 reason Frank Hester has preferred to be 

 listed as Editor. 



^Use of a trade name does not Imply endorsement by 

 the Bureau of Commercial Fisheries. 



FISH ECHOES IN RELATION TO SONAR 

 DETECTION 



One of the first items of concern was what 

 general categories could be used to classify 

 fish. Table 1 gives the general categories for 

 the various means that might be used to detect 

 and identify various single fish specimens. 

 This table does not represent all of the possible 

 means but does include the obvious techniques 

 that might be used for the detection and identi- 

 fication of fish. The visual observations column 

 of the table includes movement in two cate- 

 gories: velocity and body motion, i.e. with 

 respect to the mediunn and with respect to the 

 body of the fish. The acoustic observations 

 column is included only to show that many 

 categories of noise may be produced in the sea 

 from animal life. These sounds can interfere 

 with acoustic observations, even with ultra- 

 sonic equipment. In the frequency range 50 

 to 60 kHz (kilo Hertz); this problem is not 

 serious but sea organisms can produce 

 certain noises that interfere with measure- 

 ments . 



Table 1 indicates three separate categories 

 under the active acoustic heading. The first of 

 these categories, the static echo return or 

 target strength of a fish, were measured on a 

 limited number of specimens at the U.S. 

 NTCF (Navy Transducer Calibration Facility) 

 at Sweetwater Dam, San Diego, Calif., to de- 

 termine the target strength and directivity 

 patterns from various specimens. This work, 

 which was necessary for specifying the design 



