450 MECHANICAL AND ACOUSTICAL SENSES 



islands, many more hydrophones would be necessary for complete area 

 coverage. For some study objectives, however, less than total coverage may 

 suffice, e.g., detecting movements through passes or at specific feeding sites. 

 A good example is Thorson's (1971) use of strategically placed shore 

 monitors on the San Juan River to confirm movements of bull sharks be- 

 tween the Caribbean Sea and Lake Nicaragua. 



A suggested system for tracking home-ranging reef sharks would use ultra- 

 sonic receiver buoys with radio-relay capability. These units, each on a sep- 

 arate radio frequency (perhaps CB channels), would be anchored at sites of 

 interest in the shark's home range and would be monitored by radio at a 

 nearby home base. Using a dual-trace storage oscilloscope, the operator 

 would make the necessary pulse-arrival comparisons and plot the shark's 

 position to the precision possible— depending on the number of hydrophones 

 receiving the signal, the types of hydrophones, and the types of shark units 

 involved (Table 1). 



Omnidirectional hydrophones would be the simplest to use in radio-relay 

 units but would yield relatively poor range because they take in noise from 

 all directions. Directional hydrophones give better range but require a hori- 

 zontal scanning mechanism if all directions are to be monitored. Such a 

 scanning mechanism, e.g., a rotating reflector, could be continuously turning 

 or radio controlled from the base station. The relayed signal must also be 

 azimuth coded in some way. Although more complex, fewer such scanning 

 units would be required as compared to nonscanning omnidirectional units. 

 Nonscanning directional receivers can also be built on the principle of fre- 

 quency-phase comparison among multiple hydrophone elements, where 

 interelement distances are on the order of one wavelength. 



If the shark unit is a timefix transmitter, locational fixes can be obtained 

 from one directional relay unit or from two or three omnidirectional units 

 (Table 1). Resynchronization of the timefix system can be accomplished 

 whenever the shark comes into a position where its location can also be fixed 

 by one of the nontimefix methods. 



Transmitter Recovery— If desired, the transmitter package can in- 

 clude a timing device that releases the unit from the shark at the expected 

 end of the tracking. Recovery is then accomplished under water by divers or 

 from the boat if the package includes floatation. In the latter case, addition 

 of a radio transmitter to the package provides additional recovery insurance, 

 especially if the trackers had lost contact with the UST prior to release from 

 the shark. Simple magnesium breakaway links (predictable corrosion rates) 

 were used reliably for transmitter recoveries from angel and blue sharks 

 (Standora 1972, Sciarrotta 1974), but detachment times varied considerably 

 due to differences in water temperature and flow rate. This uncertainty of 

 timing can be discouraging to weary tracking personnel. 



A much more precise electronic release-timing mechanism is shown in 

 Figure 19. A digital oscillator and counting circuit are set for a pre- 

 determined time, then an SCR switch closes to connect a battery across the 

 seawater between an electrode plate and a thin breakaway wire. With about 



