frequencies were displayed as many spikes of 

 lower amplitude. 



In practice we felt that the sonar 

 information--f i s h swimming speed from 

 matching the ship speed to the speed of the 

 school and Doppler shifts caused by tail beats 

 and body motion- -would result in some approx- 

 imation of fish size and type. (For more 

 detailed analysis of these relations and some 

 more examples of results see Hester, 1967.) 



Results of Shipboard Tests 



Over a 2-year period we tested our equip- 

 ment under a variety of conditions on several 

 species of fish. We encountered two major 

 difficulties to the obtaining of Doppler records 

 from fish schools. 



The first difficulty was sea-noise Doppler 

 effect from surface reflections. Theover-the- 

 side transducer array was 10 feet (3.0 m.) 



below the surface. This shallow depth combined 

 with the 10° vertical beam width of the hydro- 

 phone allows surface reflections to be detected 

 readily. To receive a Doppler recordfrom fish, 

 sea conditions must be extrennely favorable-- 

 no wind ripples or short-period swell. Such 

 conditions do occur, particularly in the early 

 morning, off the Pacific coast, but the oc- 

 currence of fish schools and "workable" con- 

 ditions together are rare. It was soon apparent 

 that commercial applications of Doppler equip- 

 ment would require a more complex signal- 

 processing procedure than was originally ex- 

 pected. 



The second major difficulty arose fronn the 

 irregular behavior of the large schooling 

 fishes such as tunas. These fishes, because 

 of their streannlined shape, are detectable 

 with our sonar only when viewed from the 

 side (fig. 14), When schooling, scombroids 

 usually travel at a good rate (3 to 5 knots) 



Figure 14. --Target echo video from a fresh 492-mm. black skipjack ( Euthynnus Uneatus ) suspended 5 m. from the 

 sonar dome. The sonar gain was set to saturate the receiver for the target's side aspect (a). The echo video is 

 markedly reduced when the target is rotated head (b) or tail (c) to the beam. 



17 



