BIRD FLOCKS 



CONVENTIONAL 

 PULSED SONAR 



FREQUENCY 

 MODULATED SONAR 



FIGL'RE 7. The new continuous-transmission, frequency- 

 modulated sonar to be installed aboard the Toirnsend 

 Cromwell will allow scientists to track individual fish and 

 fish schotds underwater. To locati* tuna schools the com- 

 mercial fleet now depends heavily on sighting bird flocks. 



which are associated with the schools (left). Conventional 

 pulsed sonar (center) locates schools but cannot keep them 

 under constant surveillance, as can the frequency-modulated 

 sonar (right), which opens up wide areas of the sea to 

 the scientists. 



Although the observations just described tell us how fast 

 individual fish swim, they say nothing about the speed of 

 schools as units. What these speeds are, whether the fish 

 school at night when visual contact may be lost, what 

 depths they may prefer — these are unknown factors at 

 present. They may not remain so. In spring 1966, the 

 Townsend Cromwell will take to sea for the first tests of a 

 new sonar system that is now being constructed in California 

 for the Laboratory in Honolulu and that will be installed on 

 the vessel early in 1966. 



Unknown to the fish, long beams of unheard sound will 

 play upon creatures almost a mile away from the ship and 

 far below it. The echoes of these beams, received by sensi- 

 tive electronic equipment aboard the vessel, will give scien- 



tists a new dimension to their picture of life in the open 

 ocean. 



The sonar will be a continuous-transmission, frequency- 

 modulated equipment operating in two ranges of frequencies 

 (fig. 7). This sophisticated gear was chosen instead of the 

 more conventional pulsed sonar because it allows a target 

 to be kept under constant surveillance. Its use lessens the 

 probability of "losing" a target. The equipment will have a 

 range of about 1,000 feet on individual tunas and perhaps 

 5,000 feet on a target of suitable strength — for example, a 

 school. It will be capable of high resolution, distinguishing 

 between fishes 7 inches apart at 300 feet. 



And a still more exciting prospect looms ahead — the 

 possibility of the construction of a nuclear submarine 



