GRUBER and COMPAGNO: TAXONOMIC STATUS AND BIOLOGY OF BIGEYE THRESHER 



patterns. The first, exemplified by methods of 

 the Japanese and Soviet high-seas pelagic fleet, 

 involves highly mobile longline fishing vessels 

 which actively seek out concentrations of preda- 

 tory fish associated with small-scale oceano- 

 graphic processes, such as plankton concentra- 

 tions, and local circulation patterns (Osipov 

 1968; Gubanov 1972). While tunas are the major 

 objective of these fisheries, sharks and billfishes 

 are an important bycatch. 



Osipov (1968) noted that, in the northwestern 

 Indian Ocean, local circulation patterns produce 

 distinct areas of plankton and fish concentrations 

 in which one or two predatory species predom- 

 inate. These associations are dynamic both in 

 species composition and time. Thus the concentra- 

 tion of any species in such an area is both spatially 

 and temporally discontinuous and falls off rapidly 

 outside the enrichment cells. As a consequence, 

 fishing vessels must move continuously in the 

 wake of fish schools as concentrations form and 

 disperse. 



Osipov (1968) identified three such areas in the 

 Indian Ocean off the Republic of Somalia. In one 

 of these plankton-enriched areas carcharhinid 

 sharks predominated, while the bigeye thresher 

 was the most plentiful shark in another. Taken 

 overall, however, the bigeye thresher amounted to 

 only 12% of the total shark catch. Thus, while the 

 distribution of A . superciliosus on the high seas is 

 patchy, they make up a reasonable proportion 

 (over 10% ) of the shark catch, at least seasonally. 



The bigeye thresher is also commercially impor- 

 tant in the short-range pelagic fishery operating 

 off the northwestern coast of Cuba (Guitart 

 Manday 1975, footnote 10). However, the pattern 

 of distribution is quite different from that in the 

 Indian Ocean. Longlines are set year round in the 

 Cuban fishery and 11 shark species are caught in 

 commercially exploitable numbers. Of 11 species, 

 the bigeye thresher is the third most abundant 

 and amounted to some 20% by weight of the total 

 1973 shark catch. The Cubans have been fishing 

 this species more effectively in recent years and 

 have doubled their catch between 1971 and 1975. 



Seasonal distribution is also evident in the 

 Cuban catch records (Guitart Manday 1975). The 

 poorest catches are in March-June. The catch of 

 bigeye threshers gradually increases over the 

 summer and peaks in the fall around September- 

 October, to decline again in the winter. 



Bigeye threshers occasionally enter the market 

 when they are caught by sport and commercial 



anglers fishing for swordfish off southeastern 

 Florida. Since both species are caught at night 

 near the surface in the Florida Current it is not 

 surprising to see several bigeye threshers each 

 year captured by commercial longliners or during 

 the swordfish tourneys. Incidentally, many of 

 these animals are foul-hooked as described above, 

 perhaps reflecting a preference to attack bait with 

 their caudal fins. However, in this fishery the 

 hook is usually attached to a nylon monofilament 

 leader specifically to avoid catching sharks. 

 Thus the low incidence of mouth hooked bigeye 

 threshers could reflect losses due to biting through 

 the leader. 



Finally, this species has been captured a few 

 times in gill nets set at moderate depth, to 160 m 

 (Fitch and Craig 1964; Telles 1970; Bass et al. 

 1975). 



ACKNOWLEDGMENTS 



This investigation was supported by grants 

 from the Biological Oceanography Section of 

 NSF (OCE-78-26819) and ONR-Oceanic Biology 

 (N00014-75-C-0173) to S. H. Gruber and by a 

 writer's grant from FAO to L. J. V. Compagno. We 

 wish to thank Gloria Lerma for her assistance 

 in the literature surveys; Bruce Welton, Pearl 

 Sonada, Stewart Springer, and Gordon Hubbell 

 for providing specimens and dried jaws from their 

 extensive collections; Joan Brownell and Eugene 

 Flipse for the X-ray analyses; Fred Karrenburg 

 for preparing the photographs in Figures 1, 2, and 

 3; Marie Gruber for rendering the line drawings; 

 and Denise Hurley and Helena Detorres for typing 

 the final copy. 



We are also grateful to Pflueger Taxidermy, 

 Inc., especially Susie Hass, Ralph Grady, and 

 Tim Master, for informing us whenever a bigeye 

 thresher was received for taxidermy. We 

 thank Bill Harrison and Dan Kipnis for collecting 

 several fresh bigeye threshers for us. 



We thank John Fitch for pointing out the 

 Zane Grey article and Izumi Nakamura for 

 the Mediterranean record. We especially thank 

 Susumu Kato for providing us with his complete 

 bigeye thresher records and Dario Guitart 

 Manday for providing unpublished data from the 

 Cuban longline fishery. 



Finally we are grateful to C. Richard Robins 

 and Donald P. de Sylva for critically reading 

 this manuscript and providing many thoughtful 

 improvements. 



637 



