Figure 2. — Scalloped hammerhead sharks photographed in the Gulf of California: A. Hammerhead school as viewed obliquely from 

 below. Note leading edge of school, orientation of sharks in same direction, and similar spacing between sharks. B. Side view of school. 

 C. Shark in upper right-hand corner performing headshaking behavior. Note shark with color-coded streamer tag beneath 

 headshaking shark. D. Female hammerhead possessing ellipsoid scar (above last gill slit), possibly inflicted during courtship. 



Lincoln Index capture-recapture analysis, modi- 

 fied for observational data. On the morning of 3 

 August, 21 sharks were tagged with color-coded, 

 plastic-streamer dart tags. Upon spear applica- 

 tion of the tags, the sharks often accelerated 

 momentarily, but usually remained within their 

 groups. That afternoon, 9 tagged sharks were 

 observed again within a group of 225, yielding 

 an estimate of 525 sharks in the seamount vicinity 

 (2-4 ha). We feel that this estimate was unbiased 

 because all 21 of the tagged sharks dispersed 

 among various groups after tagging. 



We observed groups of 3 to 225 scalloped ham- 

 merhead sharks per dive with a mean of 19 

 (Table 1). Groups of sharks were encountered on 

 83% of the dives, two sharks on 7%, and single 

 shark on 20% of the dives. Group size varied 

 greatly; this in part reflected the dynamic compo- 

 sition of the groups as sharks left and returned. 

 This was tested by noting the number of sharks 

 associated with a tagged shark when it was 



reobserved. Tagged sharks (39 were tagged at El 

 Bajo over the study period) were reobserved with 

 varying numbers of accompanying sharks. For 

 example, shark no. 19 was reobserved on four 

 occasions during 135 min on the morning of 6 

 August with successive groups of 50, 15, 100, and 

 10 sharks. Such dynamics also were evident to an 

 observer who remained for 120 min above one 

 large group, which varied over that time from 50 

 to 225 sharks. Subgroups of up to 50 sharks 

 departed and returned to this main group. Groups 

 either appeared to remain at one certain spot (e.g., 

 just upcurrent of the seamount at El Bajo) or 

 repeatedly to follow a path within a relatively 

 small area (ca. 2 ha). At Las Arenitas Rocks a 

 group of ca. 50 scalloped hammerheads repeatedly 

 traveled a particular oblong circuit once each 

 15 min. 



There appeared to be some departures of sharks 

 from El Bajo. Although 16 of the 21 sharks tagged 

 the morning of 4 August were reobserved later 



357 



