STUDY OF FREE-RANGING SHARKS 



475 



Depth can be determined in addition to location if the transmitter is 

 equipped with a pressure sensor. Biologically, depth is a very useful parame- 

 ter to monitor, because sharks often show distinct daily rhythms of depth 

 preference or of rate of change of depth. An example of continuous data 

 covering several days from a gray reef shark equipped with a single-channel 

 depth transmitter is shown in Figure 27. 



Data from a single-channel, swim-speed transmitter can establish a shark's 

 basic day -night activity pattern (diurnal, crepuscular, nocturnal) in terms of 

 swimming speed and whether it ever stops swimming to rest on the bottom. 

 The most complete picture of general activity, of course, comes from multi- 

 channel transmitters having several different sensors. 



Long-Term Movement, Migration 



Since manpower requirements limit the durations of continuous ultrasonic 

 trackings, it would appear that radio methods are most applicable for study- 

 ing the longer term movements of non-home-ranging/pelagic species such as 

 the blue shark. For instance, it would be interesting to monitor the progress 

 of blue sharks off the California coast as they migrate northward in the 

 spring and southward in the fall. Only the general existence of this migration 

 is established; very few of the details are known. For example, do individuals 

 move along the coast at relatively uniform rates or do they move more errati- 

 cally, lingering at certain "temporary home ranges," such as when feeding on 

 spawning squid at Santa Catalina Island? 



Much of what is known about long-term migrations of sharks comes from 

 standard tagging and recovery programs, but such efforts are hampered by 



25 



x 50- 



K 

 Q- 

 UJ 

 Q 



75 — 



100 — 



24 



24 



12 24 



TIME OF DAY 



24 



12 



Figure 27 Example of data from a single-channel depth transmitter from one gray 

 reef shark tracked continuously for 72 h at Rangiroa, French Polynesia. Shaded areas 

 indicate times from sunset till sunrise. Note the distinct correlation between depth 

 and time of day. First point is at the site of transmitter application (self-ingested in 

 bait) in shallow water to which the shark was bait attracted. (From Johnson and 

 Nelson, unpublished data.) 



