SHARKS OF THE GENUS Carcharhinus Associated with the Tuna Fishery in the Eastern Tropical Pacific Ocean Circular 172 UNITED STATES DEPARTMENT OF THE INTERIOR FISH AND WILDLIFE SERVICE BUREAU OF COMMERCIAL FISHERIES ABSTRACT The nature of the shark problem in the American purse seine fishery for tuna is discussed. Outlined are aspects of the problems that are under study by the Bureau of Commer­ cial Fisheries Biological Laboratory, San Diego, California. A pictorial key, and photographic and verbal descriptions are presented of seven species of sharks of the genus Carcharhinus associated with tuna in the eastern tropical Pacific Ocean. UNITED STATES DEPARTMENT OF THE INTERIOR Stewart L. Udall, Secretary James K. Carr, Under Secretary Frank P. Briggs, Assistant Secretary for Fish and Wildlife FISH AND WILDLIFE SERVICE, Clarence F. Pautzke, Commissioner BUREAU OF COMMERCIAL FISHERIES, Donald L. McKernan, Direetor SHARKS OF THE GENUS Carcharhinus ASSOCIATED WITH THE TUNA FISHERY IN THE EASTERN TROPICAL PACIFIC OCEAN by Susum.u Kato Circular 172 Washington, D.C. June 1964 CONTENTS age Intr oduc tion ......•........ 1 Some aspects of the shark study. 2 Biology of the sharks 2 Shark behavior ....• 3 Aid of fishermen needed .. Economic importance of sharks A guide to sharks of the genus ('arcltarl'nu as ociated with the purse seine fishery in the eastern tropical aciiic Ocean ... 5 Introduction to the use of the key .................. Key to sharks of the genus ('arcl!arlt'nIL associated with the tuna fishery in the eastern tropical acific Ocean . 7 Descriptions and notes ......... 10 Blacktip shark, rarcltarhinu limbatu 10 Pigeye shark, ('arcJ.arl.inu azur u .••• 10 Silvertip shark, C'archarlinu platyrlyndu 1 1 Galapagos shark,earcharltwu gaZapag n 1 1 Bay shark, {;ardarhinus lamlt lla .•.•. •• 12 Net-eater shark,('arc!wrltinu malp lo n i 12 Bignose shark, CarcltarMnu alt'mu 13 Acknowledgments •. 13 References ................... 22 11 SHARKS OF THE GENUS Carcharhinus ASSOCIATED W ITH THE TUNA FISHERY IN THE EA STERN TROPICAL PACIFIC OCEAN by Susumu Kato Fishery Biologist (Research) Bureau of Commercial Fisheries Biological Laboratory U.S. Fish and Wildlife Service San Diego, California INTRODUCTION The U.S. tuna fishery in the eastern The study of the operations of the tuna tropical Pacific Ocean is principally a fishery and the application of ocean­ purse seine operation. Typical vessels ographic and biological findings in im­ have a capacity of 120 to 500 tons of tuna, proving fishing efficiency are the main and the fishery ranges from southern Cali­ functions of the Bureau of Commercial fornia to northern Chile and offshore Fisheries Biological Laboratory, San several hundred miles. The waters between 0 Diego. Because of the widespread problem latitudes 25 N. and 50 S., which are rich of sharks, a study of the sharks associated in yellowfin and skipjack tunas, are also with the tuna purse seine fishery has well supplied with sharks, some species of been undertaken. The objectives of the which appear to be associated with tuna. study are (1) to determine whether shark Although tuna is probably not a major food damage to nets and catch can be reduced, item for sharks under natural conditions, (2) determine the population structure of the pur se seining operation makes it simple the species involved, (3) increase our for sharks to obtain a free meal. When the knowledge of the life histories of the net is pursed and the tuna are confined pelagic sharks, and (4) compIle a check in a small bag of net, many become gilled list of the pelagic sharks found in the area or die of suffocation. It is easy for sharks of study. to feed on these tuna, in the meantime ripping the net so that the entire catch Most of the sharks that are associated is sometimes lost. Then the boat has to with the tuna fishery belong to the genus remain idle for several hours or even Carcharhinus. These sharks are quite similar days while the crew mends the net. Hun­ in appearance, and it is difficult to dif­ dreds of tons of fish and much fishing ferentiate the various species. The main time are lost because of shark attacks. purpose of this paper is to point out, Most of the damage is done by sharks on by means of photographs and verbal de­ the outside of the net; those within the scriptions, the major differences that net do some damage to the tuna and net, characterize the various species. A general but they suffocate quickly and are trouble­ account of the shark study being done by some mainly because they slow the net­ the Bureau of Commercial Fisheries at stacking and brailing operations. San Diego is also presented. 1 Fl re l.--Repalrl shark dama e. SOME ASPECTS OF THE SHARK STUDY BIOLOGY OF 1H E SHARKS date, 180 sharks of several specles have been tagged at three locations: Islas A study of the life histories of sharks Revillagigedo, off Guatemala, and off south­ includes such aspects as reproduction, ern Columbia. At the present time we are growth, food habits, mortality, distribu­ using strap tags attached to the dor sal fin, tion, and migration. In addition, taxonomy with or without a colored vinyl disc (flg. 2) and population dynamics must be investi­ but in the future we may also use dart tags gated. The information collected thus far (similar to the tuna tag) or disc tags at­ is presented in the discussion of individual tached to the dorsal fin. species. Much more data are needed to fill in the many gaps in our knowledge. We have asked the masters of purse selne vessels to record incidents of shark dam­ It has become clear during the course of age to their nets and of sightings of large our investigation that there is a great need aggregations of sharks in their logbooks. for a thorough study of the classification From these records, obtained through the of the sharks of the eastern Pacific. cooperation of the Inter-American Tropical J. A. F. Garrick of the U.S. National Mu­ Tuna Commission, we hope to estimate seum, Smithsonian Institution, is currently more accurately the extent of damage to working on the taxonomy of carcharhinid nets and catch and to obtain information on sharks on a worldwide ba sis. The Bureau of the distribution of sharks. The response Commercial Fisheries is cooperating with has been good, and we have been able to Garrick by collecting specimens for him. derive some idea of shark distribution A modest tagging program has been initi­ (fig. 3) from logbooks covering the first ated to study the migration of sharks. To three quarters of 1962. Shark distribution 2 derived from logbook accounts is ine'­ lO-lO tricably tied in with distribution and abund­ ance of tuna, and fishing effort. Blank areas in the figure do not necessarily indicate the absence of sharks, but may be due to absence of fishIng effort in those areas. For the same reason, seasonal distribution cannot be determined except for those areas that are fished throughout the year. were: SHARK BEHAVIOR commercIal of black dye In recent year s there has been an in­ creased interest in shark behavior. Scien­ tists in many parts of the world are currently studying the basic behavior pat­ terns of sharks and are carrying on a search for an effective shark repellent. Although their results will be of help in our investigations, we need to experiment with our own species, because shark be­ havior differs from species to species. In several preliminary experiments, we tested the effects of various shark re­ pellents on sharks associated with the tuna certain of fishery. These tests involved groups of pellents. th nel metal str • 3 BAJA ~\ MEXICO CALIFORNIA ""~• REVILLAGIGEDOS IS ••• : • CLIPPERTON I •0 COLO~BIA COCOS I • • SINGLE ATTACK • • IIALPELO I 0 • 2-4 ATTACKS •• • 5-7 ATTACKS GALAPAGOS IS •2f~: • • Figure 3.--Shark attacks on nets and catch, as logp,ed by purse seiners from January to October 1362. Examination of the stomachs of sharks 3 . Recovery of shark tags. has shown that they subsist primarily on the same food organism s a s tuna found in 4 . The number of tons of fish lost due to the same area: small fishes, squid, and shark attacks . crabs. 5. The amount of damage to nets (for Other aspects of shark behavior being example, how many man-hours are spent studied are: (l) The nature of the shark­ m ending net s). tuna association, (2) influence of other factors in their distribution, e.g., water Such information, if kept by fishermen temperature, species of tunas and food in their logbooks, will be of invaluable organisms present, other oceanographic assistance in our investigation. and biological variables, and (3) patterns in their behavior. ECONOMIC IMPORTANCE OF SHARKS AID OF FISHERMEN NEEDED The economic potential of sharks lies in To obtain adequate data on the popula­ the diversity of products that can be mar­ tions of sharks that are associated with keted: (l) flesh, for fresh fillet, dried meat, the purse seine fishery, we need the aid and meal, (2) teeth, for curios, (3) fins, for of the fishermen. The following are ex­ Chinese soups, (4) hide, for leather, anc' amples of data that fishermen can provide: (5) liver, for vitamin A and oils. 1. The number (or tons) of sharks caught These shark products are marketed at in individual sets. present in the United States, though in limited quantities. Other countries, such as 2. The species of sharks caught or In­ Australia, Japan, and Mexico are utilizing volved in attacks. sharks to a much greater extent. As the 4 world's population increases rapidly, there will be continuing pressure for new sources of protein.