Yet from the experimental work to date, one would think 

 that the tuna, which is demonstrably far from deaf, is com- 

 pletely silent. A biological sound produced by its own kind 

 might well be of importance in the behavior of tunas, but 

 whether such a sound is produced is uncertain. 



Maintaining Swimming Depth 



Visitors to the Kewalo facility, watching the tunas con- 

 tinuously circling the tanks, are likely to ask two questions: 

 "Do they always swim in the same direction?" and "Don't 

 they ever stop swimming?" The answer to both questions 

 is "No." The fish do change their direction of swimming. 

 And never do they cease to swim, although normally they 

 swim only slowly. 



For swimming, to the tuna, is far more than a method of 

 getting from one place to another. It is as vital to the fish 

 as breathing is to a man. If the tuna stopped swimming, it 

 would suffocate. And since the density of its firm body is 

 slightly greater than that of sea water, the fish would also 

 sink. 



The Kewalo facility allows exact studies of the swimming 

 speed of tunas as it is related to the needs for ventilation 

 of the gills and the maintenance of hydrostatic equilibrium. 



John J. Magnuson has found that the little tunny circles 

 the tank at 0.75 meter second (about 1.5 m.p.h.) both day 

 and night. When the fish were deprived of food for several 

 days, their speed declined to 0.55 m. sec. (about 1.1 m.p.h.). 

 The little tunny feeds by day. If the search for food played a 

 pi'edominant role in establishing swimming speed, one would 

 expect the fish to swim slower by night; and certainly one 

 would expect that the creature starved for several days and 

 questing for food would swim faster than the satiated 

 one. The little tunny, however, show^ed neither behavior. 

 Whether the minimum speeds reached were chiefly related 

 to a single function — gill ventilation or hydrodynamic lift — 

 Magnuson is not yet sure. For some tunas, there is evi- 

 dence that the requirements for gill ventilation are .some- 



30 



20 



O 





•10 



-20 





50 



100 



200 300 400 500 



CYCLES PER SECOND 



1000 



FIGl RE 5. Thf >rlIo»*Jin hear^ brwl sound> that arc near 

 ."jOO r.p.s.. as is shown by the dip of the hearing curve at 

 that freqiienry. Sounds of this frequenrv are common in 

 the sea. An example is the sound pr«»duced h> a school :>f 

 small fish swimming. 



what less than those for hydrodynamic lift, which means 

 that the fish would sink before it would suffocate. 



Magnuson's research, summarized very briefly here, has 

 turned up one interesting if probably not too important 

 bit of information : when he began to measure body density, 

 he discovered that the little tunny in the laboratory tanks 

 were slightly less dense than those fresh from the sea, most 

 likely because of the presence of more lipids in their flesh — 

 the creatures seemed to be getting fat on their shoreside 

 diet. 



The Fastest Fish 



So far as this capability has been measured, the tuna 

 seems to be one of the fastest fish. It ranks with the cheetah. 



