STUDY OF FREE-RANGING SHARKS 



423 



at relatively shallow depths. One advantage of radio over ultrasound is that 

 signal propagation is not adversely affected in areas of high bubble entrain- 

 ment such as below waterfalls. Radio telemetry from animals in seawater is 

 possible but only at very low frequencies and at very short ranges. Mackay 

 (1970) discusses successful transmission from a dolphin in a pool 15.2 m (50 

 ft) in diameter and 2.1 m (7 ft) deep. In this case, of course, acoustic 

 telemetry could not be used because of the hearing sensitivity of the animal 

 in the ultrasonic region— a problem that is nonexistent with fish. For the 

 continuous tracking of free-ranging sharks in the marine environment, the 

 distances and depths are such that only acoustic transmission is possible. 



Furthermore, for the relatively long ranges normally required in field 

 studies, only pulsed transmission at relatively low duty cycles is practical for 

 reasons of power conservation. The power-consuming transmitter output 

 stage is thus on only for the duration of the pulse, a relatively small per- 

 centage of the time. With continuous transmission, the power-output stage is 

 on 100% of the time, and the carrier frequency produced is usually modu- 

 lated in amplitude or frequency by the information to be telemetered. 

 Continuous-transmission telemetry is normally used at very short ranges, 

 usually on captive animals, to monitor rapidly changing physiological vari- 

 ables such as the electrical activity of the heart (ECG waveform), respiratory 



Figure 2a Positions of seven blue sharks tracked by T. Sciarrotta during the spring season 

 of 1972. Note that day positions are well away from the island, while the majority of 

 night positions are near shore. (From Sciarrotta and Nelson 1977.) 



