SHARKS AND UNDERWATER SOUND 



401 



4.5 m for the prey sounds or 10 m for various synthesized sounds. These 

 extremely short distances were understandable in this case because of the 

 nature of the prey sounds used and the extreme attenuation of signal strength 

 due to the extremely shallow water. These conditions allowed Banner to 

 demonstrate, however, that the sharks responded by well-oriented approaches 

 toward the sound source only when they were in an area in which the sound 

 level exceeded that of previously established hearing thresholds of similar- 

 sized animals under controlled laboratory conditions (see Fig. 3— measured 

 in displacement values based on demonstrated sensitivity (Banner 1967)). 

 Thus, at least in this case, directional responses appeared at the greatest 

 detection distances. This differs somewhat from results of Chapman and 

 Johnstone (1974) for the cod, in which the level required for directional 

 responses in members of that species was higher than that necessary for 

 detection. One reason for this difference may be the presence of a swim- 

 bladder in the cod and its absence in sharks. 



Qualities of a Sound that Appear Unimportant for Attraction 



Within reasonable limits, certain features of underwater sounds are apparently 

 not critical for attracting sharks. These include the duration of individual 

 pulses within a given train of pulses (Nelson and Johnson 1972) and the 

 summation, in time, of acoustic energy present in part or all of the pulse 

 train (Myrberg et al. 1972). The latter study showed that silky sharks 

 apparently find a signal with a train of 20 pulses/s, each pulse lasting 10 ms 

 (i.e., 200 ms of energy), far more attractive than a signal of only 10 pulses/s, 

 each pulse being 50 ms (i.e., 500 ms of energy). The latter, however, was far 



100 



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FRACTION OF THE DISTANCE BETWEEN SOURCE 

 AND THRESHOLD LOCATION PREDICTED FOR 320 Hz 



2.2 



Figure 3 Percentages of positive responses by young lemon sharks at locations 

 within and beyond the predicted threshold distance, the latter being calculated 

 from displacement thresholds as measured in the laboratory for conspecifics of 

 the same size. Results of attractive signals having optimum level at 320 Hz are 

 combined. (Banner 1972) 



