SHARKS AND UNDERWATER SOUND 399 



that have been recently examined (Ha 1973, Myrberg et al. in press, Spanier 

 1975). Probably temporal processing by the acoustical modality of fishes is 

 an extremely sensitive and precise function. 



Another important finding by Banner (1972) was that young lemon 

 sharks do not respond to the specific nature of the source, i.e., prey versus 

 nonprey; instead, differentiation rests upon the temporal characteristics of 

 the sound, this being even more important than frequency composition. A 

 similar conclusion was reached by others dealing with adult sharks (Myrberg 

 et al. 1975, Richard 1968). Thus, this could not be attributed simply to the 

 apparent inexperience of Banner's subjects. Since such sounds are commonly 

 produced by prey (Banner, 1968, Hashimoto and Maniwa 1967, Moulton 

 1960, Nelson and Johnson 1970), the findings suggest that the rapid investi- 

 gation of certain types of sounds results in enough opportunities for prey 

 capture that appropriate responsiveness remains despite the fact that various 

 types of nonprey sources produce similar sounds. Also, the effects of habi- 

 tuation on responsiveness in the absence of positive reinforcement indicate 

 that energy would not be wasted for long in response to sounds produced by 

 sources other than prey (see page 404). 



Nelson and Johnson (1972) examined variation in pulse rate on acoustic 

 attraction in four species of Pacific reef sharks (Fig. 2, Table 4). They found 

 that low-frequency, pulsed sounds were more attractive if they had irregular 

 rather than regular pulse intervals. No significant difference was found, how- 

 ever, between trains of pulses having equal duration and those having vari- 

 able duration. 



Sound Level— To be effective, a sound must be loud enough that 

 subjects can hear it and orient to it. This factor is realistic only when 

 ambient noise level is considered at the time of stimulation. The auditory 



Sound I 



(Continuous, 10 pulses/sec) 



iiiiiiMUMiMiniiiimmnii 



iiiiiiiiiiii 



Sound 2 



(Intermittent, 10 pulses/sec) 



nr im nrnni n n 



iiiiiiiiiiii 



Sound 3 



(Intermittent, 15-75 pulses/sec) 



IIII1I1IIIIMM   



I 1 i 1 1 ' I 



5 10 15 20 25 30 



Sec 



Figure 2 Diagrammatic representation of three 25-fto 100-Hz pulsed 

 sounds used for playback to sharks. The vertical black and white bars 

 represent pulses (bursts of noise) but are not drawn to the absolute 

 time scale. The 30 -s sequences illustrated were repeated 10 times to 

 comprise single 5-min playback periods. (Nelson and Johnson 1972) 



