Distribution, Relative Abundance, and Movement of 



Skipjack Tuna, Katsuvxmus peUmds, in the Pacific 



Ocean Based on Japanese Tiina Longline Catches, 1964-67 



WALTER M. MATSUMOTO' 



ABSTRACT 



Catch data of the Japanese tuna longline fishery from 1964 to 1%7 were analyzed to determine 

 the distribution, abundance, and movement of skipjack tuna, Kattuwonui pelamii, in offshore waters 

 of the Pacific Ocean. 



Large skipjack tuna, as well as larvae, were found to be concentrated mainly in the east central 

 equatorial Pacific. Movement of skipjack tuna stocks was determined by following the shifting of 

 high-CPUE (catch per unit effort) cells from one quarter to the next. The apparent movement of 

 skipjack tuna stocks in the Pacific appeared to coincide with the circulation of the major ocean 

 currents; counterclockwise in the southern hemisphere and clockwise in the northern hemisphere, 

 except in the eastern Pacific where the current flow is counterclockwise. The movement patterns of 

 high CPUE suggested that skipjack tuna adults or their progeny could move from one area to the 

 next. The movement pattern was used also to determine the probable migratory routes followed by 

 skipjack tuna tagged in the eastern Pacific and recovered near the Hawaiian and Christmas islands. 



INTRODUCTION 



Skipjack tuna, Katsuwonus pelamis, are taken near 

 the surface and at moderate depths in the Pacific Ocean, 

 but the surface fisheries, utilizing pole and line and 

 purse seine, account for about 99% of the catch. For 

 various reasons, most of the catches throughout the 

 major fisheries are made within 320 km (ca. 200 miles) 

 of shore (Fig. 1). Consequently, although much has been 

 learned about the skipjack tuna within the surface fish- 

 eries, knowledge of their distribution and movements in 

 mid-oceanic areas is limited. 



In an attempt to determine the population structure 

 and migration of the skipjack tuna in mid-oceanic areas, 

 Miyake (1968) examined the longline catch data of Ja- 

 panese commercial (1956-64) and research (1949-65) ves- 

 sels. He did not draw any conclusions from his study 

 because he believed the data were inadequate. 



Kasahara (1968) also examined the Japanese longline 

 catch data to determine the distribution and centers of 

 abundance of skipjack tuna in the Pacific Ocean. His 

 report included hypothetical mig^ratory routes of skipjack 

 tuna as postulated by Naganuma (unpublished manu- 

 script^) based on the catches and monthly variations in 

 length frequencies of skipjack tuna taken by the Japan- 

 ese longline fishery (year not gfiven). Certain aspects 

 about the routes postulated by Naganuma require clari- 

 fication; for example, no return routes toward the equa- 

 tor are given for fish in the central and western North 

 Pacific, and the direction of movement of skipjack tuna 

 in the central South Pacific differs from the results of 

 my study. 



' Southwest Fisheries Center, National Marine Fisheries Service, 

 NOAA, Honolulu, HI 96812. 



^Naganuma, A. (Unpubl. manuscr.) [Tohoku Regional Fisheries 

 Research Laboratory, Shiogama City, Miyagi Prefecture. Japan.) 



My study also is based upon the Japanese tuna 

 longline catch data to determine centers of skipjack tuna 

 abundance and, by tracing the seasonal shifting of high 

 catch-rate areas, to derive the migration paths of skip- 

 jack tuna in the Pacific Ocean. As might be expected in 

 studies of this kind, some deg^ree of subjectivity was 

 involved in the analyses. Nevertheless, models such as 

 this are necessary first step in advancing our knowl- 

 edge about the oceanwide movement of the skipjack 

 tuna. 



LONGLINE CATCH DATA 



Source of Data 



Several sources of tuna longline data are available, 

 but the data are not all equally useful. Miyake (1968) 

 g^ves a detailed account of the nature of these data. The 

 best data are those published in the annual report of 

 effort and catch statistics by area in the Japanese tuna 

 longline fishery beginning with the year 1962 (Fisheries 

 Agency of Japan, Research Division, 1965, 1966, 1967a, 

 1967b, 1968, 1969). However, because the Japanese 

 Government's requirement that all fishing vessels submit 

 copies of their fishing logbooks at the termination of 

 each trip did not go into effect until 1963 and because 

 the extent of longline fishing coverage in the eastern 

 and southern Pacific was poor through the first quarter 

 of 1963 (Miyake 1968), data for 1962 and 1963 are not 

 included in this study. 



Evaluation of LongUne Data 



The tuna longline is a passive gear designed to catch 

 large fish about 70 to 145 m below the surface (Yoshi- 

 hara 1954). Its design and fishing characteristics are not 

 ideal for catching skipjack tuna, but in the absence of 



