STUDY OF FREE-RANGING SHARKS 457 



Table 3. Absorption of 

 sound in seawater at 15°C* 



*Values based on Fig. 5.4 of 

 Urick(1975). 



sound wave, one example being the concentration of organisms known as the 

 deep scattering layer. Another example is the entrapment of air bubbles near 

 the ocean's surface under conditions of breaking waves. These air bubbles 

 can pose a serious tracking problem if both the telemetered animal and the 

 receiving hydrophone are near the surface. 



Downward ray bending can limit signal-detection range by the refractive 

 effect of the vertical temperature gradient usually found in the ocean 

 (temperature decreasing with depth). Thus, a transmitter near the surface may 

 be undetectable beyond a certain distance by a shallow hydrophone, whereas 

 a deeper hydrophone may still receive the signal. 



Sound rays in the ultrasonic frequencies are also easily blocked by struc- 

 tures that are large with respect to one wavelength; they do not go around 

 corners but instead form "sound shadows" behind such objects. Thus, if a 

 large underwater structure intervenes between the transmitter and receiver, 

 the signal can be severely reduced or blocked entirely. When tracking sharks 

 in coral-reef areas, it is common for the tracker to hear a strong signal 

 dwindle to weak or to nothing within a few seconds. This usually indicates 

 that the shark has moved behind a reef. Sharks that enter coral caves present 

 special problems for this reason, e.g., only a single narrow beam of sound 

 may escape from the cave. 



Ambient Noise— Background noise in the environment is one of the 

 variables that make transmitter detection distance difficult to predict. Since 

 under actual tracking conditions absolute signal level is seldom limiting, 

 signal recognition is usually a matter of discriminating signal from noise, thus 

 the noise level in the appropriate frequency band becomes an important 



