STUDY OF FREE-RANGING SHARKS 455 



combined data from several sensors (swim speed, compass, etc.) are used, 

 such "micropath" plots can potentially yield a great amount of behavioral 

 information. As a hypothetical example, a hunting shark might circle its prey 

 closely, then accelerate to attack. Such a brief, small-scale pattern would be 

 impossible to detect from the boat by hydrophone direction but would be 

 readily apparent on the computer map. 



System Performance— Range, Life, Size 



When selecting a telemetry system for a particular research need, the trade- 

 offs between transmitter range, life, and size must be considered. These three 

 obviously important factors are interdependent and to emphasize one means, 

 in effect, to sacrifice the others. Ideally, the compromise chosen should be 

 based on the requirements of the usage in mind. Thus, maximizing range 

 may be paramount when tracking a fast-swimming, wide-ranging pelagic 

 species; to achieve this, a rather large, short-lived transmitter may be chosen. 

 For monitoring a reef species with a limited, predictable home range, the 

 investigator may choose much longer life at the sacrifice of some range. For 

 relatively small species, range and/or life might have to be traded off to 

 achieve an acceptably small transmitter size. 



Transmitter range in the ocean depends on certain characteristics of the 

 telemetry system and on various environmental factors, some quite variable 

 and difficult to predict. Absolute range can thus only be estimated, and no 

 dependence should be placed on such calculations before verification by in 

 situ measurements under actual study conditions. It is useful, however, to 

 consider the primary factors governing signal-detection range and how 

 changes in these factors affect range, at least in a relative sense. 



Ultrasonic Propagation in Seawater— Transmission of sound through 

 water is a complex phenomenon, dependent on a multiplicity of factors, and 

 has been treated in detail by Albers (1965), Urick (1967, 1975), and others 

 (Eckart 1968). In regard to range estimates for USTs, spreading and absorp- 

 tion are the most important factors and the only ones for which simple cal- 

 culations can be made. 



Spreading (divergence)— As a sound pulse radiates outward from the 

 source, its energy is spread over an increasingly larger area. In a medium 

 without boundaries, this can be thought of as the surface of an expanding 

 sphere. In relatively deep, open water, spreading is usually considered to be 

 spherical, giving a decrease in sound level of 6 dB each time the distance 

 from the source is doubled. In relatively shallow water with strong surface 

 and bottom reflections, the diverging sound may become channeled, and a 

 "modified cylindrical" spreading of between 3 and 6 dB per distance- 

 doubling is sometimes appropriate (pure cylindrical spreading = 3 dB). 

 Sound spreading is independent of frequency, temperature, and salinity but 

 is affected by reflections, refractions, and losses at boundaries. Table 2 

 shows the relative transmission loss due to spherical spreading. 



