STUDY OF FREE-RANGING SHARKS 



479 



Figure 28 Proposed telerfietry experiment at Santa 

 Catalina Island, Calif., to determine the mechanism of 

 orientation in shoreward-migrating blue sharks: (A) 

 Capture/release site north of island. (B) Release site 

 south of island. See text for details of experiment. 



should move towards the island. If it becomes necessary to separate sun- 

 compass from magnetic -compass mechanisms, additional experiments could 

 be done involving eye occluders, attached magnets, etc. The trackings for 

 these experiments need last only from about midafternoon (tagging time) 

 until midevening. Although ultrasonic trackings would provide the fullest 

 detail on paths taken by the sharks, two-point radio trackings would still 

 answer the basic question of which way the animals moved, and more such 

 trackings could be accomplished per unit time. 



Another example of the use of telemetry for sensory experiments is the 

 study proposed by A. Schuijf and the author to elucidate the mechanism 

 of far-field directional hearing in sharks. Recent experimental evidence of 

 Schuijf (1975) and Schuijf and Buwulda (1975) indicates the need for the 

 gas bladder to resolve the 180° ambiguity in the directional hearing of cer- 

 tain teleosts. Since sharks, lacking a gas bladder, are able to "home in" 

 accurately on low-frequency sounds from distances of at least several hun- 

 dred meters (Nelson 1967, Nelson and Johnson 1972, Myrberg et al. 1972), 

 the question arises of how they handle the 180° -ambiguity problem. A 

 critical aspect of the problem concerns the behavior of the shark at the 

 moment it first responds to the sound. For instance, does it always turn 

 in the correct direction or does it sometimes turn in the opposite direction 

 but eventually double back onto the correct direction? Does it respond 

 immediately, turning sharply to the correct course, or does it circle slowly 

 for a while before selecting a direction? Since the responding sharks are 

 normally too distant to be seen by underwater observers, it is planned to 



