returns. Unfortunately tagging of skipjack tuna in the 

 past has not been as widespread nor as consistent and 

 intensive as desirable. In the few areas where tagging 

 has been done on a moderate scale, i.e., in the eastern 

 and northwestern Pacific, the few tag recoveries have 

 aided in developing parts of this model. Much more 

 tagging effort is needed in these and other areas, 

 however, if we are to gain a better knowledge of 

 skipjack tuna migration in the Pacific Ocean. A model 

 such as this could be the basis upon which a Pacific- 

 wide tagging experiment could be developed. 



SUMMARY 



1. Skipjack tuna are taken mainly in surface fisheries 

 that are restricted to coastal waters. Consequently, our 

 knowledge about the distribution of these fish is equally 

 limited. To get a better idea of the distribution, abun- 

 dance, and movement of skipjack tuna in noncoastal 

 waters, data of the Japanese tuna longline fishery from 

 1964 through 1967 were analyzed by determining the 

 catch-per-unit effort (CPUE) for each 5° area of latitude 

 and longitude by quarters for each of the years. Con- 

 tours were drawn of the CPUE values to determine 

 high-CPUE areas or cells and their movements from 

 quarter to quarter. 



2. The concentrations of zero catches at the northern 

 and southern boundaries of the fishery indicated that 

 fishing had occurred throughout the distributional range 

 of the skipjack tuna. 



3. Large skipjack tuna were found to be concentrat- 

 ed a) east central equatorial Pacific; b) eastern Pacific 

 south of lat. 10°S; c) central North Pacific, north and 

 northwest of Hawaii, and in Hawaiian waters; d) west- 

 ern North Pacific, south and east of Japan; and e) 

 western South Pacific, northeast and east of Australia. 

 The distribution of CPUE was characterized by a) the 

 absence of uniformity in the northward and southward 

 shifts of the northern and southern boundaries and along 

 the boundaries across the Pacific; b) the persistence of a 

 very large area of better-than-average CPUE in the east 

 central equatorial Pacific (between long. 130°W and 

 180°, and between lat. 10°N and 15°S); and c) the 

 disparity between high catch areas of the longline and 

 the surface fisheries. 



4. The relative abundance of adult skipjack tuna was 

 suggested in the relative abundance of larvae in tropical 

 and subtropical waters. The abundance was low in the 

 eastern Pacific east of long. 120°W, high in the central 

 Pacific between long. 120° W and 160°E, and low in the 

 western Pacific between long. 120° and 160°E. A similar 

 trend also was noted in the longline catches of skipjack 

 tuna. 



5. The apparent movement of groups of skipjack 

 tuna in the Pacific appeared to coincide with the circula- 

 tion of the major ocean currents. The movement was 

 counterclockwise in the southern hemisphere and clock- 

 wise in the northern hemisphere, except in the eastern 

 Pacific where the movement appeared counterclockwise, 

 corresponding with the flow in the north equatorial 

 water mass. 



The movement patterns of high CPUE suggested the 

 occurrence of a number of groups of skipjack tuna, 

 possibly 14 or more, in the Pacific. The movement of 

 the four westernmost groups, two each in the northern 



and southern hemispheres, generally agreed with those 

 of the two western Pacific subpopulations of skipjack 

 tuna proposed by Fujino (1972). Each group of the 

 central-eastern Pacific subpopulation of (Fujino 1970a) 

 seemed to follow its own seasonal migratory pattern 

 that permitted movement of adults or their progeny 

 from one area to the next, e.g., 1) skipjack tuna in 

 the eastern Pacific northern fishery apparently origi- 

 nated in and returned to central equatorial waters, 

 with part of the group moving into Hawaiian waters in 

 subsequent seasons and part to the Line Islands area 

 south of the equatorial countercurrent; 2) skipjack tuna 

 in the Midway-Hawaii area were apparently composed of 

 groups from the eastern and equatorial Pacific, from 

 waters farther west between Midway and Wake Island, 

 and possibly from the Marshall Islands and Japanese 

 offshore surface fishery; and 3) part of the skipjack tuna 

 in the eastern Pacific southern fishery apparently origi- 

 nated in the South Pacific to the east and south of the 

 Marquesas Islands. 



6. The migration of skipjack tuna from the eastern 

 Pacific to Hawaii had been established by recoveries 

 there of five fish tagged in the eastern North Pacific 

 and one fish tagged in the eastern Pacific near the 

 southern edge of the Equatorial Countercurrent (lat. 

 4°11'N, long. 119°W). The migratory routes of these 

 fish as well as a seventh fish tagged in the eastern 

 Pacific northern fishery and recovered near Christmas 

 Island were postulated. The five fish recovered in Ha- 

 waii presumably departed the eastern Pacific fishery 

 area in late fall and arrived in Hawaiian waters via the 

 California Current Extension in the following spring or 

 summer. The fish tagged near the southern edge of the 

 Equatorial Countercurrent moved through the eastern 

 Pacific fishery and then to Hawaiian waters via the 

 California Current Extension where it was recaptured 

 soon upon arrival. The fish tagged off Baja California 

 and recaptured east of Christmas Island left the tagging 

 area in late fall, arrived in equatorial waters via the 

 California Current Extension and remained there a full 

 year before being recaptured. The latter two recoveries 

 were especially noteworthy since they demonstrated that 

 skipjack tuna can move across the Equatorial Counter- 

 current. 



ACKNOWLEDGMENTS 



I thank William H. Bayliff, Inter-American Tropical 

 Tuna Commission, La Jolla, Calif.; and Frank Williams, 

 Rosenstiel School of Marine and Atmospheric Science, 

 University of Miami, for reviewing the manuscript and 

 for their many helpful suggestions. I also thank my 

 colleagues at the National Marine Fisheries Service, 

 Honolulu, Hawaii, for their helpful criticism and sugges- 

 tions during the preparation of the draft manuscript. 



LITERATURE CITED 



BROADHEAD, G. C. and I. BARRETT. 



1964. Some factors affecting the distribution and apparent 

 abundance of yellowfin and skipjack tuna in the Eastern 

 Pacific Ocean. Inter-Am. Trop. Tuna Comm. Bull. 8:417-473. 

 BLACKBURN. M., and F. WILLIAMS. 



1975. Distribution and ecology of skipjack tuna, Katsuwonus 

 pelamis, in an offshore area of the eastern tropical Pacific Ocean. 

 Fish. Bull., U.S. 73:382-411. 



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