FISHERY BULLETIN: VOL. 75, NO. 3 



under laboratory and field conditions. In sub- 

 sequent work with the horn shark, Finstad and 

 Nelson (1975) measured the effect of light inten- 

 sity on releasing activity onset, both in the natural 

 environment and in the laboratory under artificial 

 twilight transitions. For a colony of captive bon- 

 nethead shark, Sphyrna tiburo, under semi- 

 natural conditions, Myrberg and Gruber (1974) 

 reported a late-afternoon peak in patrolling speed, 

 suggesting a diurnal activity rhythm. 



Using ultrasonic telemetry, Standora (1972) es- 

 tablished a basically nocturnal pattern of activity 

 and a limited home range for the Pacific angel 

 shark, Squatina californica. His multichannel 

 transmitters were a similar, but earlier version of 

 those used in the present study. Carey and Lawson 

 (1973) tracked a free-ranging dusky shark, Car- 

 charhinus obscurus, in order to study body tem- 

 perature regulation. They used a two-channel, 

 frequency-shifting transmitter that measured 

 both surface and deep body temperatures. Thorson 

 ( 1971) monitored long-term movements of the bull 

 shark, C. leucas, with relatively long-life, sensor- 

 less pingers and automatic-recording receivers at 

 several locations. Using this technique in conjunc- 

 tion with conventional tagging, he showed that 

 bull sharks move via the San Juan River from the 

 Caribbean Sea to Lake Nicaragua. 



The present paucity of behavioral information 

 on active, wide-ranging sharks, especially pelagic 

 species, is undoubtedly due in part to the difficulty 

 of studying them by direct observation. Ultrasonic 

 telemetry now offers one promising avenue of ap- 

 proach to this problem. This paper reports on an 

 initial study using this technique to investigate 

 diel patterns of behavior in a wide-ranging pelagic 

 shark. 



METHODS 



The present study is based on 14 individual 

 telemetry trackings conducted between 3 March 

 and 7 October 1972 (Table 1). Each tracking was 

 initiated in the pelagic environment of the San 

 Pedro Channel approximately 6 to 7 km north of 

 the Isthmus, Santa Catalina Island, Calif. The 

 blue shark was well suited for this telemetry study 

 because of its moderately large size, high abun- 

 dance for most of the year, and attractability to 

 bait. The abundance and/or attractability of blue 

 sharks in the offshore baiting area was low only 

 during the months of January and February, the 

 sharks being easily obtainable the rest of the year. 



TABLE 1. — Summary of tracking data for 14 telemetered blue 



sharks. 



The estimated range in total lengths of blue 

 sharks telemetered was 1.8 to 2.6 m; for those 

 otherwise observed, 1.2 to 3.0 m. 



Telemetry System 



The ultrasonic telemetry system used in the 

 present study has been described in detail by 

 Standora (1972), Ferrel et al. (1974), and Nelson 

 ( 1974). The transmitters were of the oil-filled type, 

 about 15 to 18 cm long, 3.5 cm in diameter, and 

 emitted 10-ms pulses (tone bursts) at 40 kHz. The 

 units were set for a life of several days, and a 

 maximum range of 3 km (average conditions) to 5 

 km (ideal conditions). Data were encoded as pulse 

 rate (pulse interval) which varied with the value 

 of resistive sensors. The first two trackings 

 utilized single-channel transmitters incorporat- 

 ing depth sensors. The remaining 12 trackings 

 were performed with multichannel units (rapid- 

 multiplexing type) with various combinations of 

 sensors to measure depth, swimming speed, 

 swimming direction, and temperature. 



Two commercial tunable ultrasonic receivers 

 were used. For continuous monitoring of rela- 

 tively clear, nearby signals, the Smith-Root 

 Ta-25 4 receiver (25-80 kHz) was employed using 

 an omnidirectional hydrophone on a 25-m cable. 

 The more sensitive, narrow-band DuKane model 

 N15A235 receiver (30-45 kHz) with its staff- 

 mounted directional hydrophone was used for 

 directional tracking and for reception of weaker 

 signals. 



4 Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



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