Scripps Institution of Oceanography

The social behavior of the scalloped hammerhead shark is described with an emphasis on determining the function of its polarized schooling. . This shark possesses a refuging social system in which individuals remain in groups in a small core of their home range during the inactive phase of their diel cycle and disperse into the surrounding environment during the active phase. Scalloped hammerhead sharks, followed by ultrasonic telemetry, swam slowly back and forth during the day along the ridge of seamount, El Bajo Espiritu Santo. These sharks moved rapidly away from the seamount late during the day or at dusk. The rhythmical dispersal and return of the sharks to the seamount was indicated by the return of telemetered sharks followed away from the seamount and the repeated observation of marked sharks at the seamount over periods up to seven weeks. At the seamount during the day the sharks can interact socially while remaining centrally positioned within their feeding arena. They are then ensured of foraging success at night. The diet of hammerheads consisted of cephalopods, fishes, and crustaceans.

The large schools were formed of Sharks spanning a size range of 88 to 371 cm. This permitted size segregation within the schools. Both the total lengths and distances to the nearest neighbors increased significantly toward the bottoms of groups at the insular site, Las Arenitas. Total lengths increased in a direction into the groups at the seamount sites, El Bajo Espiritu Santo and Gorda. The sizes and spatial relations of school members were measured from stereophotographs taken by free-diving to the edges of the schools. The presence of larger female sharks at the bottoms of groups at Las Arenitas is believed to be caused by aggressive interactions among the females of which the schools were primarily composed. The sharks within the schools were mostly immature. The onset of maturity in male hammerheads occurred at 163 cm, that of females at 217 cm.