STUDY OF FREE-RANGING SHARKS 421 



For several years, a development effort has been under way at California 

 State University, Long Beach (CSULB), aimed at producing a system for 

 telemetering several aspects of the behavior of free-ranging sharks at sea. In 

 contrast to the simpler "pingers" used in most previous fish-tracking studies, 

 this objective required a relatively powerful, complex transmitter incorporat- 

 ing several sensors and associated switching components. Standora (1972) 

 described the multichannel ultrasonic transmitter developed at CSULB and 

 its initial use in 1971-1972 for monitoring the day and night behavior pat- 

 terns of the Pacific angel shark at Santa Catalina Island, Calif. His trans- 

 mitters incorporated sensors for measuring depth, swimming speed, light, 

 and temperature, and the multiplexed data were tape recorded for later 

 manual decoding. Nine angel sharks were tagged underwater (via barbed 

 dart) and afterwards tracked from a boat for 13 to 25 h. The sharks were 

 nocturnal in activity, swam at depths of 27 to 100 m, and confined their 

 movements to a home area of about 150 hectares (Standora and Nelson 

 1977). 



Sciarrotta (1974) and Sciarrotta and Nelson (1977), using an improved 

 version of the CSULB multichannel transmitter, studied the blue shark 

 (Figure 1), a dangerous species abundant in offshore temperate waters. 

 Units containing various combinations of depth, temperature, swimming- 

 speed, and compass-heading sensors were externally applied to free- 

 swimming sharks baited to the boat several kilometers off Santa Catalina 

 Island. Telemetered data showed that the sharks were generally more active 

 at night than during the day, with swim speed, rate of change of direction, 

 and rate of change of depth all greater at night. The most interesting be- 

 havior discovered was a well- oriented shoreward migration to the island 

 at dusk (Figure 2). The sharks typically stayed near the island for several 

 hours, then gradually moved back out to sea. Deviation from straight-line 

 swimming was greatest upon the early evening arrival at the island, and it 

 was postulated that feeding may have occurred then. 



A number of multi-day telemetry trackings were conducted at Rangiroa, 

 French Polynesia during 1973-1975 as part of an overall study of the reef 

 sharks of the area (Nelson and Johnson, in press). Using both depth-sensing 

 and location-only transmitters, the units were concealed in bait and fed to 

 uncaptured sharks in a manner resulting in completely atraumatic applica- 

 tion. Home ranges and diel patterns of activity and movement were deter- 

 mined for gray reef sharks (Johnson and Nelson, in preparation) and reef 

 whitetip sharks (Nelson and Johnson, in preparation). 



ULTRASONIC TECHNIQUES 



Telemetry from fish in the natural environment has traditionally involved 

 acoustic (ultrasonic) transmission, as radio waves are greatly attenuated by 

 water, especially high -conductivity seawater. Although radio techniques have 

 recently been used for tracking free-ranging fish, e.g., salmon, trout, and bass 

 (Winter et al. 1973, Monan et al. 1975), they are practical only in freshwater 



