KlYOSHI WAKABAYASHI 



Fisheries Agency of Japan 



Far Seas Fisheries Research Laboratory 



1,000 Orido, Shimizu 424, Japan 



California Academy of Sciences 



Golden Gate Park 



San Francisco. CA 94118 



TOMIO IWAMOTO 



SCHOOLING OF THE SCALLOPED HAMMERHEAD 



SHARK, SPHYRNA LEWir^I, IN 



THE GULF OF CALIFORNIA 



Groups of sharks have been witnessed occasion- 

 ally by airborne or shipboard observers ( Bass et al. 

 1975; Clark 1963; Springer 1967; Kenney 1968). 

 Such remote observations, augmented with infer- 

 ences from fishery records (Ford 1921; Olsen 1954; 

 Jensen 1965), have provided fragmentary descrip- 

 tions of these groups and speculations on their 

 function. We have studied polarized schools of the 

 scalloped hammerhead shark, Sphyrna lewini, by 

 free diving among them at several offshore sites in 

 the Gulf of California. Here we report preliminary 

 observations on the sizes, depths, movements, and 

 compositional dynamics of these schools, and the 

 sizes, sexes, and behavior of the school members. 

 Based upon these observations, we discuss the 

 possible function of such schools. 



Previous sightings by others indicated that 

 schools of S. lewini might be encountered during 

 the summer at three locations near La Paz, Baja 

 California Sur, Mexico: Las Arenitas Rocks (Isla 

 Cerralvo), El Bajo Espiritu Santo, and Isla Las 

 Animas (Figure 1). These locations were visited 

 between 26 July and 6 August 1979 aboard either 

 our 7 m fiber glass skiff or the 23 m ferrocement 

 research vessel, the Juan de Dios Batiz. At these 

 locations and others, four search techniques were 

 employed: 1) Free diving and surface swimming; 

 2) scuba; 3) baiting with ground Pacific mack- 

 erel. Scomber japonicus; and 4) playback of 

 pulsed, low-frequency sounds (for description, 

 see Myrberg 1978). Scalloped hammerheads were 

 most easily discovered and approached by free 

 diving, and data were recorded on small plastic 

 slates. Relatively few scalloped hammerheads 

 were observed by divers with scuba, even in areas 

 where this species was abundant. This was prob- 

 ably due to their avoidance of the divers' sounds 



NAUTICAL MILES 



Figure l. — The La Paz study area in the Gulf of California. 

 Grouped scalloped hammerhead sharks were encountered at 

 three sites: 1) Las Arenitas, a cluster of rocks 100 m from Isla 

 Cerralvo; 2) El Bajo Espiritu Santo, a seamount rising to within 

 14 m of the sea surface; and 3i Isla Las Animas. 



and visually conspicuous bubbles. Only three 

 scalloped hammerheads were attracted in 11.5 h of 

 baiting and none during 40 min of sound playback, 

 indicating a lack of interest in these feeding 

 stimuli. 



Both individuals and schools of Sphyrna lewini 

 were seen. Generally the sharks within schools 

 swam in a polarized manner, remaining relatively 

 equidistant from each other and swimming for- 

 ward and changing direction together (Figure 

 2A, B). For the purposes of data sampling, sharks 

 were counted during discrete "observation dives," 

 which usually consisted of free dives, but included 

 some observations from the surface. Scalloped 

 hammerheads were encountered on 91'7f of the free 

 dives at El Bajo, 58% at Isla Las Animas, and 34% 

 at Las Arenitas Rocks. These percentages are a 

 rough index of relative abundance because they 

 are probably affected by differences in search 

 success and water clarity. School sizes at El Bajo 

 were larger than those at Las Arenitas Rocks and 

 Isla Las Animas, but no large difference existed 

 between group sizes at Las Arenitas Rocks and 

 Isla Las Animas. 



The number of scalloped hammerhead sharks in 

 the vicinity of El Bajo was estimated by using the 



356 



FISHERY BULLETIN; VOL. 79. NO. 2. 1981. 



