SHARKS AND UNDERWATER SOUND 407 



off to the right or left and also headed out of view. These responses usually 

 occurred when sharks were within 5 m of the source. Sharks farther from the 

 source generally moved in a wide arc shortly after signal change, often dis- 

 appearing from view in the direction from which they came. Although signal 

 change was initiated when one shark had reached a distance of 5 to 10 m 

 from the source, sharks farther away also reacted accordingly. 



These sequences were especially clear for silky sharks approaching the 

 transducer during withdrawal tests in the Straits of Florida. Silky sharks 

 tested at specific locations in the Tongue of the Ocean exhibited greater 

 variation in response. In the Straits, variations from the relatively rapid drift 

 of our vessel during a given day of testing, the different drift tracks used 

 each day, and the relatively few sharks seen on a given day substantially re- 

 duced any possible effects of habituation. In the Tongue of the Ocean, the 

 same small population of silky sharks congregating around an offshore, deep- 

 moored buoy was tested over a period of some hours. This provided the 

 opportunity to determine possible changes in levels of responsiveness during 

 repetitive testing. These data indicated that the "intensity" of withdrawal 

 wanes if trials follow one another frequently. For example, if an appropriate 

 sound is transmitted once every 5 to 10 min, rapid withdrawal is seen during 

 the first two or three trials; less rapid withdrawal is elicited during the next 

 few transmissions; finally, if the same sharks are attracted again within a few 

 minutes, there will probably be little or no change in behavior when the 

 supposed aversive stimulus is transmitted. Observers who have monitored 

 both approach and withdrawal feel that withdrawal is more resistant to the 

 effects of habituation, but this must be evaluated more precisely. 



Clearly there are species differences in withdrawal response. Initial tests in 

 the Tongue of the Ocean failed to bring about withdrawal in a small number 

 of oceanic whitetip sharks (C. longimanus) under conditions that resulted in 

 excellent withdrawal by silky sharks of similar or larger size. Although a 

 more precisely monitored recent experiment showed that such responses 

 could be elicited consistently at least two or three times by whitetips under 

 nonfeeding conditions, much work remains before enough knowledge is 

 gained to explain these species differences. 



Finally, pure tones appeared quite ineffective in eliciting withdrawal, even 

 at high levels. This fits closely the fact that such sounds are also not 

 attractive to free-ranging sharks. This lack of response certainly implies an 

 inability by these animals to hear pure tones, but this is not true based on 

 laboratory findings using appropriate training techniques. This must mean 

 that in the natural environment such tones have little or no meaning. 



Various workers have tried to elicit withdrawal or avoidance responses 

 from teleosts or, at least, to redirect their movements by using sounds. While 

 a few have had varying degrees of success (e.g., Chapman 1976, Shiskova 

 1958, van Derwalker 1967), many have failed (e.g., Burner and Moore 1953, 

 Miyake 1952, Moore and Newman 1956). Most reports mention initial 

 startle responses to high-level, low-frequency sounds, but subjects apparently 

 adjust rapidly to such sounds and return within moments to prior levels of 

 activity. This again implies that such sounds have little aversive significance 



