'1 



52FTV LITTLE 



TUNNY 



SKIPJACK 



FUil'RE 4. At a constant brightness, a ,^ellowfin sees 

 details of an .olijcct better than a skipjack nnrl a skipjack 

 belter than a little tunny. 



How Well Do Tunas Hear? 



All man's ingenuity has failed to greatly improve his 

 ability to see underwater, yet with the aid of relatively 

 simple instruments, he can hear across oceans. Sound travels 

 fast and far in the sea, obeying physical laws that are well 

 defined. The explosion of a 1-pound charge of dynamite 

 off the Hawaiian Islands has been picked up by hydrophones 

 on the Caliiornia coast. 



The first quantitative measurements of the hearing capa- 

 city of a fish were made in the 1920's. The creature was a 

 goldfish, and it responded to frequencies between .32 and 

 2,752 c.p.s. (cycles per second). That is, it could hear sounds 

 so low that men could scarcely hear them but was unable to 

 hear high-pitched sounds common in the human experience. 



Despite the worldwide interest on the part of scientists 

 and the fishing industry in the use of underwater sound to 

 attract or repel food fishes, little quantitative research has 

 been performed on the hearing abilities of the creatures. 

 One rea.son may be that such knowledge is only slowly 

 acquired because it is dependent on training the animals to 

 respond to signals that offer punishment or reward. During 

 the past months, Robert T. B. Iversen has recorded several 

 hearing curves for yellowfin tuna. No such curves are avail- 

 able for any other member of the scombrids — the large and 

 commercially important family to which the tunas belong. 



Iversen has shown that the yellowfin tuna hears well at 

 frequencies from about 100 to 2,000 c.p.s. Its hearing is 

 most acute between 350 and 800 c.p.s. (fig. 5). Many sounds 

 in the sea that might be expected to have biological 

 significance for tunas are contained within that range. 

 Examples are the sounds made by small fish swimming 

 and by schools of squid. Iversen has been measuring audi- 

 tory "thresholds," the minimum intensity at which the 

 yellowfin tuna can hear a sound of a particular frequency. 

 He has not yet ventured into other aspects of hearing, such 

 as directional orientation, the ability to locate the source 

 of a sound. 



Although tunas respond to sound, there is only the 

 slightest evidence that they themselves produce sound. That 

 fish make sounds men can hear was demonstrated to science 

 101 years ago. (Fishermen had known it for untold 

 millennia). Since the advent of sensitive hydrophones, 

 scientists have learned that the sea is quite a noisy place. 

 Many laboratories possess tape recordings of the hissings, 

 grunts, squeaks, and moans that contribute to the totality 

 of audible sound in the ocean. The living sources of some 

 of these have been identified. The carangid Trachurus 

 tiachurus Linnaeus, for example, is known to make "sounds 

 like those produced by running the fingers along the teeth 

 of a comb." 



