STUDY OF FREE-RANGING SHARKS 449 



sufficient target strength or if reference buoys are fitted with radar reflectors 

 (better yet, radar transponders). Without radar, the reference buoys could 

 carry underwater acoustic transmitters on frequencies different from that of 

 the animal being tracked. Transponding units would allow fixing the buoys' 

 positions from the boat. Timefix transmitters would be quite adequate also, 

 as any needed periodic resynchronization would pose no problem with the 

 surface buoys. Addition of a small light (flashlight bulb) to a buoy greatly 

 facilitates locating it at night. 



During good weather and good signal reception, tracking operations are 

 relatively simple and straightforward. With bad weather, poor signal propaga- 

 tion, high ambient noise, etc., tracking can become difficult, and its success 

 becomes dependent on the skill and determination of the tracking personnel. 

 In certain coral-reef areas with strong currents, rugged bottom topography, 

 and high ambient noise, it can be difficult to maintain unbroken contact 

 with an actively moving shark, and trackings can include considerable time 

 spent in searching and relocating. 



Human Fatigue— Experience has shown that the main limitation to 

 the length of a continuous tracking operation is fatigue of the tracking 

 personnel. In studies of blue sharks at sea off California, 24 h has proven 

 about the limit for continuous boat tracking by a crew of one or two persons 

 (Sciarrotta 1974). At Rangiroa, French Polynesia, gray reef sharks were 

 tracked nearly continuously for periods up to 96 h by a crew of four per- 

 sons, each putting in one solo 6-h shift per day (Johnson and Nelson, 

 unpublished data). 



Considering this human problem, any hardware improvements that make 

 tracking easier or permit more noncontinuous trackings are well worth de- 

 veloping. Manpower requirements should also be kept in mind during the 

 planning of telemetry operations. It may be that long continuous trackings 

 are not necessary to answer the biological question being asked. An inter- 

 mittent schedule with adequate rest periods is certainly easier on the crew 

 and may, in the final analysis, yield more useful information. 



Fixed Receiver Arrays— An alternative to tracking from a single boat 

 is to use an array of fixed hydrophone /receiver units distributed throughout 

 the animal's home range. Such units would either record data for later ex- 

 amination or would be linked by cable or radio to a central receiving station 

 for immediate availability. By comparison of pulse-arrival times at three or 

 more omnidirectional units, relatively exact location fixes can be made, even 

 from simple pingers. A system of this type was used by Hawkins et al. 

 (1974) for tracking cod in a Scottish sea loch. Another example is the 

 "pinpoint" system described by Holand et al. (1974) in which timefix USTs 

 are automatically tracked and plotted on an xy -recorder using a multiple- 

 hydrophone array and special microprocessor. 



Complete area coverage by such a hydrophone array is practical only in 

 relatively open environments with good acoustics. Hawkins et al. estimated 

 that four hydrophones could cover an area of about 1 km 2 . In the case of a 

 shark ranging throughout a complex environment of coral reefs, passes, and 



