392 MECHANICAL AND ACOUSTICAL SENSES 



INTRODUCTION 



Divers who frequent areas where sharks prevail have long known that spear- 

 fishing greatly increases the chance of a shark encounter. Stories of such 

 encounters often are surprisingly similar, regardless of geographical location. 

 Usually sharks are not seen when the fish is speared but appear suddenly a 

 few moments later. In the few cases in which their approach is noted, the 

 rapidly moving predators often come from directions where chemical infor- 

 mation could not possibly have reached them, due to the prevailing current 

 or the extremely brief time between the spearing and their arrival. The ap- 

 proaching sharks appear well oriented; they locate the struggling fish within 

 seconds, whether in the open or hidden in a recess of the reef. Such orienta- 

 tion appears clearly to depend, however, on the struggling movements of the 

 speared fish. If the fish rests quietly, sharks only a few meters upstream 

 show no interest. 



These accounts, as well as those from the literature (e.g., Eibl-Eibesfeldt 

 and Hass 1959, Hass 1959, Hobson 1963, Limbaugh 1963, Wright 1948), 

 clearly suggest that sharks can, indeed, be attracted solely by the sounds of 

 struggling fishes. This suggestion was first tested by Nelson and Gruber 

 (1963, see also Wisby et al. 1964), who showed that the sounds of struggling 

 produced by a speared grouper consist of rapid pulses of broadband noise 

 with peak energy below 100 Hz and that similar sounds can attract several 

 species of free-ranging sharks. Playback of rapid pulses of noise with fre- 

 quencies below 60 Hz were especially effective in that regard, while sounds 

 at these same frequencies but without pulsing or with pulsed noise bands 

 between 400 and 600 had no effect. 



These findings were discussed during the Second Symposium on Marine 

 Bioacoustics at New York City in April 1966, when several speakers com- 

 mented that attempts to confirm such attraction by a variety of underwater 

 sounds had failed. These remarks immediately followed another discussion 

 that ended with most of those present accepting the idea that fishes are in- 

 capable of orienting to a sound source located beyond the region of the near- 

 field effect. 1 Since the published report on shark attraction included the 

 important point that the rapidly approaching animals appeared well oriented 

 to the sound source, such responses, if real, could have occurred only within 

 the near-field of the sound. Yet the distances mentioned in the original 

 reports (e.g., Wisby and Nelson 1964) strongly suggested that oriented move- 

 ments had begun beyond that region. This difficulty, as well as the lack of 

 confirmation, resulted in the opinion that confirmation of such results was 



lr This effect consists of the relatively large amplitude excursion of the medium close to a 

 sound source that is associated either with movement of the source in excess of the com- 

 pressibility of the medium or with the curvature of the wave front. Attenuation of this 

 effect occurs faster than does attenuation of particle motion associated with pressure 

 fluctuations; thus the near-field effect predominates only within a region generally ex- 

 tending less than one wavelength from the source; beyond that point, one enters the far- 

 field (see Banner 1972, van Bergeijk 1964). 



