478 MECHANICAL AND ACOUSTICAL SENSES 



Facilitating Observation and Experimentation 



Although designed primarily as a tool which gathers the desired data, teleme- 

 try can also be used simply to indicate the whereabouts of a transmitter- 

 equipped individual so that other kinds of observations or experiments can 

 be performed. For example, studies of the cave-dwelling habits of reef 

 whitetip sharks at Rangiroa, French Polynesia, were aided by marking indi- 

 viduals with stomach -implanted transmitters at identifiable frequencies 

 (Nelson and Johnson, in press). Divers using underwater receivers could 

 pinpoint a shark's location in a particular cave, so that its relation to other 

 sharks in that cave could be observed. Likewise, the practicality of divers 

 observing packs of adult gray reef sharks at Rangiroa was enhanced by 

 ultrasonically tagging one of the pack members (Johnson and Nelson, 

 unpublished data). 



Ethological Experiments— Some accounts suggest that certain sharks 

 may possess geographical "territories" 9 (McNair 1975). If this is true, it 

 bears directly on the shark attack problem, especially for those types of 

 attacks now being attributed to agonistic motivations (Baldridge and 

 Williams 1969, Johnson and Nelson 1973). In at least one dangerous 

 species, the gray reef shark, agonistic motivations are often manifested 

 during diver— shark encounters as distinct threat displays. What is needed 

 to confirm territoriality would be proof that a given individual shark be- 

 haves more aggressively toward intruders in one part of its home range than 

 in other parts. Marking a shark with a UST not only would permit delinea- 

 tion of the shark's home range but also would facilitate experimental diver- 

 shark encounters at various places in the home range. 



Sensory Mechanisms— Field experiments to determine sensory capaci- 

 ties and orientation mechanisms can also be facilitated by telemetering tech- 

 niques. Detailed "pictures" of a shark's swimming patterns (from compass 

 sensor, transponder, etc.) when detecting a test stimulus and orienting itself 

 to it can help elucidate the mechanism involved, e.g., whether a true direc- 

 tional response or gradient seeking. 



For example, the finding of Sciarrotta (1974) that blue sharks make an 

 evening twilight migration toward the Santa Catalina Island shoreline raises 

 the question as to how the sharks accomplish this oriented movement 

 (Figure 2). Possible navigation mechanisms include sun compass, magnetic 

 compass, and acoustic, i.e., to sounds emanating from the island. It is hoped 

 that further telemetry work may elucidate the mechanism involved. One 

 planned experiment involves capturing sharks at one site north of the island, 

 then releasing half of them there, and releasing the other half at a site south 

 of the island (Figure 28). If either the sun or the earth's magnetic field is 

 providing the orientation reference, then the "south" group should move in 

 the same compass direction as the "north" group, thereby moving away 

 from the island. If the sounds of the island are used, then both groups 



"in the broad sense, including areas of elevated dominance. 



