468 MECHANICAL AND ACOUSTICAL SENSES 



alone. A more efficient method would be to lower frequency and/or improve 

 the receiving system. 



RADIO TECHNIQUES 



Long-range radio tracking of terrestrial animals is now a widely used tech- 

 nique with an extensive literature (Mackay 1970, Long 1977). As mentioned 

 earlier, radio-tracking techniques have also been used successfully on certain 

 freshwater species in relatively shallow water (Winter et al. 1973, Monan 

 et al. 1975). However, radio techniques presently hold little promise for the 

 direct tracking of saltwater species. The reason is the severe attenuation 

 of radio signals as they propagate through a conducting medium. Accord- 

 ing to Mackay (1970), radio attenuation in seawater is approximately 

 55 dB /wavelength. Furthermore, for a given frequency, electromagnetic 

 wavelengths in seawater are much shorter than in air due to a reduction in 

 velocity of propagation, e.g., 1 MHz wavelength = 1.8 m, 150 MHz = 0.15 m. 

 Thus, at the distances and depths traversed by free-ranging sharks, radio 

 telemetry from the animal through the water appears out of the question, 

 and ultrasonic telemetry remains the only practical method. 



However, not all questions concerning the movements of sharks need be 

 investigated by continuous trackings. If one is interested in general patterns 

 over considerable periods of time, it may suffice to obtain several points 

 along the movement route. Such data could be provided by one or more 

 small radio transmitters, which, at predetermined times, detach from the 

 animal and float to the surface, from where they could be detected through 

 the air at long ranges. The simplest example would be single-radio trackings 

 providing two data points (application site, popup site), but more data 

 points could be obtained using multiple-radio packages with staggered re- 

 lease times, e.g., 24, 48, 72 h, etc. 



Timed-Release, Floating Radio Transmitter 



Following the previous reasoning, a pilot study using floating radio trans- 

 mitters has been started at CSULB, to determine movements of pelagic 

 sharks in situations in which ultrasonic trackings are logistically impractical. 

 One main advantage of the radio method is the great saving in manhours 

 spent tracking. While 24-h sonic tracking requires constant vigilance for the 

 whole time, 24-h radio operation requires attention for only the initial 

 application time, then a short time the next day to determine signal location 

 and (if practical) make the recovery. While one crew would be entirely occu- 

 pied with a single sonic tracking, it could handle radio trackings for a num- 

 ber of different sharks on the same day. 



The prototype transmitter package shown in Figure 23 uses a standard 

 "off-the-shelf," two-stage, 151-mHz Wildlife transmitter 7 and a 30-cm 



Manufactured by Wildlife Materials, Inc., Route 3, Carbondale, 111. 62901. 



