July 1966 



COMMERCIAL FISHERIES REVIEW 



21 



dri ) were caught. Ten weather stations and 

 10 bathythermograph stations we re made. 



Note: See Commercial Fislieries Review , June 1966 p, 11, 

 >}e :!< i'fi :^< i^ 



NEW SONAR GEAR INSTALLED 

 ON RESEARCH VESSEL 

 "TOWNSEND CROMWELL": 



A research vessel of the U. S. Bureau of 

 Commercial Fisheries Biological Laboratory 

 in Honolulu, Hawaii, is undergoing acceptance 

 trials of special electronic e qu ipm e nt de- 

 signed to reveal facets of fish behavior never 

 known before. 



Biologists agree that the central Pacific 

 Ocean holds uncaught stores of tuna, one of 

 the most valuable of the food fishes. A re- 

 cent estimate says that at least 150,000 tons 

 of a single species, the skipjack tuna, could 

 be taken without harming the stock. That 

 would about equal the entire U. S. catch of all 

 species of tuna, and the tuna landings in dol- 

 lar value are the third most important in the 

 country. 



But these tuna inhabit an area of some 30 

 million square miles, about 10 times the size 

 of the continental United States. They are on- 

 ly rarely seen. Where and how they can be 

 located depends on aspects of tuna behavior 

 scientists have been unable to study until now. 



The Bureau has installed on the research 

 vessel Townsend Cromwell , operated by its 

 laboratory in Honolulu, a new device that is 

 expected to provide scientists with fresh clues 

 for the finding and catching of the subsurface 

 tunas. The device is a continuous -transmis- 

 sion, frequency -modulated (CTFM) sonar. 



Sonar (from SOund NAvigation and Rang- 

 ing) is a method of locating underwater ob- 

 jects by the sound they reflect. It is the un- 

 derwater equivalent of the more familiar ra- 

 dar, but whereas radar uses radio waves, 

 sonar uses sound waves, for sound waves 

 travel great distances under water and radio 

 waves do not. 



The sonar consists of two principal ele- 

 ments. The first of these is a set of retract- 

 able transducers located in the bottom of the 

 vessel. When in use, they are extended 5 feet 

 beneath the keel. The sonar emits a continu- 

 ous beam of high-frequency sound. Whenthis 

 sound strikes an object with a density differ- 

 ent from water, a fish, for example, an echo 



is returned. The second principal element 

 of the sonar is the console, which is a large 

 metal cabinet installed in the "sonar room" 

 of the Townsend Cromwell . When an echo is 

 returned to the transducer, it appears as a 

 point of light on a cathode ray tube (like the 

 screen of a television set); simultaneously it 

 also produces a sound which can be heard 

 through earphones. The operator can deter- 

 mine how far the object is from the ship, in 

 what direction, and approximately how deep 

 it is. Normally, searching is done automati- 

 cally. In the search mode, he can detect skip- 

 jack tuna schools as far as 2,000 feet from 

 the ship; a high resolution mode provides for 

 a more detailed examination of objects within 

 400 feet of the ship. 



The transducer can sweep a complete cir- 

 cle or it can play on a small segment of a 

 circle, depending on how the operator sets 

 the controls. It can operate in any position 

 from the sound beam directed horizontally to 

 it directed vertically. Thus it can search a 

 complete underwater hemisphere whose di- 

 ameter is 0.8 mile. This can be done in a 

 matter of seconds 



Most sonars now in use send out a single 

 pulse of sound and wait for it to be reflected 

 before emitting another. The Townsend Crom - 

 well sonar emits a continuous beam of sound 

 and there is a continuous return of echoes 

 from targets to the vessel. This means that 

 the sonar can keep track of a fast -moving fish 

 or school. This feature is called Continuous 

 Transmission. The Townsend Cromwell sonar 

 is frequency modulated, like FM radio. For 

 these reasons, the sonar is often referred to 

 as a CTFM sonar. 



Installed early in April 1966 (see photo- 

 graphs on page 22), the sonar was being "de- 

 bugged" and undergoing tests before accept- 

 ance. If it operates according to standards, 

 its first use will be to study the composition 

 of tuna schools near the island of Oahu, site 

 of a large surface tuna fishery. It is the in- 

 tention of the scientists operatingthe sonar to 

 determine how tightly the subsurface tunas 

 are schooled, to what depths they go, their 

 speed, and how far they range. Such infor- 

 mation can then form the basis of studies and 

 experiments and gear with which to catch the 

 fish. 



