SHARKS AND UNDERWATER SOUND 403 



Johnson 1970, 1972). The list of patterns includes those performed in 

 apparent social contexts as well as others directed at the transducers 

 themselves. The first group includes parallel swimming, circling, leaning, 

 following, chasing, giving way, and hunching; the patterns of the second 

 group include circling the transducer, veering off, hunching, biting, "startle," 

 and head shaking. Patterns observed in either of these specific contexts 

 include spinning, gill puffing, yawning, head shaking, and thrusting. 



Given the same context, the strength of response appears to vary con- 

 siderably among sharks (Limbaugh 1963). This has been shown clearly during 

 sonic attraction. Nelson and Johnson (1972) found that the gray reef shark 

 (C. menisorrah) approached an operating transducer more rapidly and more 

 closely than did the reef whitetip (Triaenodon obesus). 



The ultimate intensity of movements by sharks in a restricted area is often 

 considered to be the feeding frenzy. Undoubtedly, this phenomenon is ini- 

 tiated and maintained partly by the social facilitation of movement by active 

 sharks being close to one another. Such an effect has been noted in the 

 vicinity of an active transducer that obviously provided an adequate stimulus 

 situation. Relative speed of movement (Myrberg et al. 1969), intensity of 

 approach (Nelson and Johnson 1972), and competitive feeding (Nelson et al. 

 1969) increase as sharks concentrate around a sound source. This suggests 

 that the underlying motivation controlling the appropriate patterns of move- 

 ment changes along a continuum and thus allows the energy used in specific 

 activities to be adjusted to some level of apparent competition. Also, the 

 number of sharks attracted to a source appears related to the number of 

 sharks present within hearing range of the sound, at least during early trials 

 (Banner 1972, Myrberg et al. 1975a, Nelson and Johnson 1972). This implies 

 that attraction is effected regardless of motivational differences. Possibly a 

 single common motivation underlies such responsiveness, but this is specula- 

 tive. There is no a priori reason to believe, however, that motivation cannot 

 change once a subject is close to the source. 



Although hundreds of tests have been conducted on shark attraction there 

 is a notable absence of reports on either intra- or interspecific aggression in- 

 volving these animals, even when many are moving about in a relatively small 

 area. This lack of obvious aggression has also been reported in various labora- 

 tory contexts (Myrberg and Gruber 1974). Yet at least some species— and 

 probably all— possess a social organization apparently based on a dominant- 

 subordinate system (Allee and Dickinson 1954, Clark 1963, Myrberg and 

 Gruber 1974). Such an organization also crosses species lines (Cousteau and 

 Cousteau 1970, Limbaugh 1963, Springer 1963, 1967). Although dominant- 

 subordinate systems have been traditionally described and discussed within 

 the context of aggression in other animal groups, the system as presently 

 described in sharks might involve quite another context, i.e., predator— prey, 

 the larger individual or the group being the potential predator and the 

 smaller individual or the single animal within the group being the potential 

 prey. Although there is some evidence mitigating against this idea (e.g., sexual 

 differences— Myrberg and Gruber 1974), if that relationship is actually the 

 basis for the apparent hierarchical organization in sharks, the continued use 



