inp to the central Pacific. Several lines of scientific 

 investijration have led to the formulation of this hypothesis. 

 They all point to the probability that there exists in the 

 central Pacific a very large population of skipjack tuna of 

 which the only central Pacific fishery, that in Hawaii, takes 

 a vcrv small amount. 





Sonar Investigations 



Conducting the basic scientific studies that are required 

 to bring this great resource into production, the Laboratory 

 in Honolulu has equipped one of its research vessels, the 

 Townncnd Cromiccll, with a complex, sensitive, and power- 

 ful electronic device, a CTFM (continuous-transmission, 

 frequency-modulated) sonar, to study the movements of 

 tunas in the water. The sonar emits a sound signal whose 

 reflection by a solid object, such as a tuna or a tuna school, 

 indicates to the operator the distance and direction of the 

 object from the ship. In principle, the sonar resembles 

 radar, but where the radar signal is an electromagnetic 

 wave, the sonar uses ultrasonic pressure waves. 



romplementintr the sonar on the Toimsoid CroniwcU is 

 a 1-1-channel electronic device which records the informa- 

 tion i)rovided by the sonar. These data are automatically 

 converted for analysis on large computers. Thus a tuna 

 becomes, in succession, an echo picked up by the .sonar 

 (appearing on the sonar screen as a point of light), a 

 nuniljer in analog form on magnetic tape, a number in 

 digital form on another magnetic tape, and eventually 

 Arabic numerals on a computer printout. 



With information such as this scientists at the Lab- 

 oratory in Honolulu will be able to determine the ways 

 tunas move about in the ocean. At present, most knowledge 

 depends on sightings of fish when they ascend to the sur- 

 face in pursuit of prey. How long the central Pacific 

 schools remain at the surface, to what depths they descend, 

 whether they always maintain a schooling formation at 



FIGURE 13. When the sonor oboord the TOWNSEND CROMWELL 

 is used in the active modefobovel, beams of underwoter sound ore 

 reflected bock to the ship from the body of the fish. When the sonar 

 is used in the possive mode (belowt, a sound-producing device, which 

 has been fed to the fish, emits a signal every second, and from this 

 signal the movements of the fish can be followed. A combination of 

 the modes will permit gathering more information on fish behavior. 



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