SHARKS AND UNDERWATER SOUND 395 



to a variety of biological sounds. The group includes species commonly 

 found over shallow flats and reefs as well as over deep oceanic waters. Most 

 are piscivorous, but the group also includes a number of small species that 

 feed mainly on certain invertebrates (e.g., the bonnethead, Sphyrna tiburo). 

 The attractiveness of such sounds to sharks of such diverse habits suggests 

 strongly that most, if not all, other carnivorous sharks will react similarly to 

 appropriate underwater sound sources. It is noteworthy that other carniv- 

 orous fishes, such as groupers and snappers, find the same types of 

 underwater sounds attractive, although their approach to the sources is much 

 slower than that of sharks (Myrberg et al. 1969, Nelson and Johnson 1970, 

 Nelson et al. 1969, Richard 1968, Steinberg et al. 1965). 



Qualities of Attractive Sound 



Early experiments showed that not all sounds elicit approach in sharks. 

 This suggested that such animals are attending to specific qualities of trans- 

 missions. To determine these qualities, we synthesized sounds to control 

 systematically those features of obvious interest. By this means alone or with 

 natural and synthesized sound used together in the same experimental 

 design, findings from the field related closely to the meager but significant 

 results previously obtained on the hearing physiology of some sharks. 



Spectral Content— One important feature of an attractive sound was 

 its spectral content. All results indicated that a sound, to be attractive to 

 sharks, must contain frequencies below 800 or 1000 Hz; if not, approach 

 was not seen (e.g., Myrberg et al. 1969); see Figure 1. The initial findings by 

 Nelson and Gruber (1963) suggested that only very low frequencies were 

 attractive, i.e. below 60 Hz. This figure was revised upwards, however, after 

 subsequent studies showed that signals possessing higher frequencies were 

 also attractive (Myberg et al. 1972, Nelson and Johnson 1972). Yet, for 

 those species most intensively studied (e.g., the silky shark, Carcharhinus 

 falciformis), levels of attraction increased as the included wavelengths of a 

 signal increased (Myrberg et al. 1972); see Table 2. This cline of responsive- 

 ness eventually ended in similar effectiveness when octave bands of very low 

 frequencies were finally reached, i.e., 10 to 20 Hz and 20 to 40 Hz (Myrberg 

 et al. 1975a, 1976). All synthesized sounds used during controlled testing, 

 however, were bands of limited frequencies, and experiments have not yet 

 determined the band limits of an attractive sound. Nevertheless, doubtless 

 any signal whose spectrum includes an octave or more and contains fre- 

 quencies below 800 Hz will be attractive— so long as it possesses a few ad- 

 ditional qualities. 



The upper frequency limit of attractive sound, around 800 to 1000 Hz, 

 agrees well with data on the hearing abilities of those few species of sharks 

 tested under reasonably controlled conditions (the lemon shark, Negaprion 

 breuirostris- Banner 1967, Nelson 1967, Wisby et al. 1964; the bull shark, C. 

 leucas— Kritzler and Wood 1961; the scalloped hammerhead, S. lewini~Ol\a 

 1962). It is noteworthy that this limit is extremely close to the upper 

 limiting frequencies that elicit vestibular microphonics in another elasmo- 



