CONSIDERATION OF THREE PROPOSED MODELS OF THE 

 MIGRATION OF YOUNG SKIPJACK TUNA (KATSUWONUS PEL AMIS) 



INTO THE EASTERN PACIFIC OCEAN' 



F. Williams^ 



ABSTRACT 



Previous evidence suggested that most exploited skipjack tuna (Katsuwonus pelamis) in 

 in the eastern Pacific Ocean have a central Pacific spawning origin. Three models are 

 now proposed of the migration of young skipjack into eastern Pacific fishery areas; 

 these are (i) the active migration model, (ii) the passive migration model, and (iii) the 

 gyral migration model. Data utilized and theories advanced in the detailed development 

 of the models are discussed. Mechanisms and timing in all three migration models are 

 dependent on oceanographic conditions and events in the central-east Pacific, which thus 

 have a controlling eff'ect on migration success of incoming young fish. Current skipjack 

 research cruises, in part designed to test the validity of the models, are outlined. 



Skipjack tuna {Katsuwonus pelamis) are widely 

 distributed in tropical and subtropical surface 

 waters of world oceans. In the Pacific Ocean 

 there are three principal fisheries: the Japanese 

 home islands fishery, including the Ryukyu- 

 Tokara-Izu-Bonin Islands (1956-1969, range 

 87,000-252,000 short tons) ; the Hawaiian Is- 

 lands fishery (1956-1969, range 3,000-8,000 short 

 tons) ; and the eastern Pacific fishery from Cal- 

 ifornia to northern Chile (1956-1970, range 

 52,000-132,000 short tons). In addition, there 

 are skipjack fisheries off Taiwan and the Phil- 

 ippines, developing ones of various sizes in Mi- 

 cronesia, Melanesia, and Indonesia, and sub- 

 sistence fisheries in many other island groups, 

 such as the Society-Tuamotu Islands. With the 

 regulation of yellowfin catches in the eastern Pa- 

 cific through an annual catch quota and the gen- 

 eral decline in Japanese longline catch rates of 

 tunas, the fishing industry has been showing in- 

 creased interest in skipjack for a greater share 

 of total tuna catches. 



' Contribution from the Scripps Institution of Ocean- 

 ography. 



* Institute of Marine Resources, Scripps Institution 

 of Oceanography, University of California at San Di- 

 ego, P.O. Box 109, La Jolla, CA 92037. 



Fujino (1967, 1970, and in press) has shown 

 that genetic studies indicate a subpopulation of 

 skipjack in the western Pacific distinct from that 

 present in the central-east Pacific. Seasonal 

 mixing of the two subpopulations, or replace- 

 ment one by the other, is considered to take 

 place in the area immediately to the east of an 

 arc through the New Hebrides-Solomon-Carol- 

 ine-Mariana-Bonin Islands chains to the waters 

 off the northeast coast of Japan. 



The hypothesis on North Pacific skipjack ad- 

 vanced by Kawasaki (1965a, b) proposed a 

 transpacific population with a common element 

 and radiation outwards of juveniles from a cen- 

 tral Pacific spawning area with eventual return 

 to that area of sexually mature fish. This aspect 

 of Kawasaki's hypothesis does not seem tenable, 

 in view of Fujino's work. It is possible though 

 that Pacific-wide changes in environmental con- 

 ditions may cause apparently similar fluctuations 

 in skipjack abundance in different areas through 

 effects on recruitment and distribution. 



Schaefer (1963) and Rothschild (1965) re- 

 ported on the structure of skipjack populations 

 in the central-east Pacific and stated that skip- 

 jack in the eastern Pacific fishery have a central 

 Pacific origin. This inference was based pri- 

 marily on the indicated general lack of spawning 



Manuscript accepted April 1972. 



FISHERY BULLETIN: VOL. 70, NO. 3, 1972. 



741 



