SENSORY BIOLOGY: PACIFIC SHARKS 599 



method in the western Caroline Islands, the surface was churned con- 

 tinuously for periods of 20 to 70 s, interrupted by approximately equal 

 periods of silence. 



2. The coconut shells are shaken underwater, or scraped against the side 

 of the boat, producing intermittent bursts of low-frequency sound. Sound 

 output may be augmented by pounding on the hull or gunwales of the 

 fishing boat during the same intervals. 



These methods of attracting sharks are reported effective with grey reef 

 sharks (Carcharhinus menisorrah), reef whitetips (Triaenodon obesus), 

 hammerheads (Sphyrna sp.), tiger sharks (Galeocerdo cuvieri), and makos 

 (Isurus oxyrinchuss); this list is doubtless incomplete. In Micronesia, the 

 Gilberts, and Fiji, at least, great emphasis is placed on the importance of the 

 "right rhythm," which includes not only the rhythm of shaking the rattles 

 underwater, but also the relation of periods of sound with intervals of quiet. 

 Variability of acoustic stimulation is the common denominator of all these 

 procedures, and it is quite likely that the trial- an d-error approach has hit 

 upon the significant role of habituation in eliminating a shark's responses if 

 the stimulus is continuous. 



A particularly dramatic method of varying the acoustic attraction of 

 sharks has been documented in several islands (Eua, Iki, Moungaone, and 

 Uiba) of the Tonga group, where shark fishing has become a highly ritualized 

 procedure. Use of the rattle may be alternated with intervals when one of 

 the fishermen jumps into the water himself, splashing about to "encourage" 

 the sharks (Gibbings 1948). When sharks are attracted to the general area but 

 are not close enough for noosing (the usual method of capture there), it is 

 not unusual for a swimmer to approach the sharks and then turn to lead 

 them to where they may be captured. This is locally interpreted as over- 

 coming the sharks' "shyness"— a deficiency from which the swimmers them- 

 selves evidently do not suffer! 



The success of the island fishermen in attracting sharks with these pro- 

 cedures is all the more remarkable when it is remembered that the sound 

 sources are limited to the area next to a boat at the water surface, whereas in 

 the controlled studies of Nelson and Johnson (1972), at Eniwetok Atoll, the 

 amplified test sounds were played into the water from a transducer (speaker) 

 lowered within a few meters of the bottom or of coral reefs frequented by 

 sharks. Speedier arrivals at the sound source area would be expected under 

 the latter conditions and were, in fact, obtained (Nelson and Johnson 1972). 

 Nor can it be claimed that the optimal procedures, derived from local empir- 

 ical observations, are always used in Pacific fisheries. More experienced and 

 expert shark fishermen appear to use quite deliberate care in achieving just 

 the type of acoustic stimuli they judge effective; less experienced fishermen 

 may be more casual. 



When patience is exhausted by prolonged lack of results, however, almost 

 any shark fishing expedition appears to turn to a "try anything" approach, 

 with splashing, pounding the boat hull, clattering of rattles and any other 

 noisemaking procedure. This is typically more of an emotional catharsis for 



