Hypotheses on the Origin of Exploited 



Skipjaclc Tuna (Kotsuwonus pelamis ) 



in the Eastern and Central Pacific Ocean 



By 



BRIAN J. ROTHSCHILD 



Fishery Biologist (Research) 



Bureau of Commercial Fisheries 



Biological Laboratory, Honolulu, Hawaii 



ABSTRACT 



A set of hypotheses has been formulated to account for the origin and move- 

 ment of exploited groups of skipjack tuna ( Katsuwonus pelamis ) in the eastern 

 and central Pacific Ocean. The hypotheses take into account the available evi- 

 dence on larval distributions, gonad indices, size distributions, tag recoveries, 

 catch predictions, and immunogenetic studies. The evidence suggests that most 

 skipjack taken by the eastern Pacific skipjack fisheries originate in the central 

 Pacific. It is likely that the equatorial region of the central Pacific contributes a 

 major portion of the recruitment stockfor the eastern Pacific. Large proportions 

 of the Hawaiian catch also may originate in the Equatorial Zone. Skipjack catch 

 predictions are discussed in the context of evidence which indicates that year- 

 class-strength phenomena affect the Hawaiian landings. The need for more evi- 

 dence on the origin and movements of harvested skipjack is emphasized. 



INTRODUCTION 



Skipjack tuna ( Katsuwonus pelamis ) are distri- 

 buted throughout the tropical and subtropical Pa- 

 cific Ocean. In recent years yields from Pacific 

 skipjack fisheries have been of the order of 550 

 million pounds annually. There are now three 

 major fisheries: the eastern Pacific fishery, in 

 the coastal waters of North, Central, and South 

 America from Baja California to Peru (161.4 

 million pounds in 1962); the Hawaiian fishery 

 (9.4 million pounds in 1962); and the fishery off 

 the Japanese archipelago (375 million pounds in 

 1962). Formerly there was a fishery in Micro- 

 nesia, which produced 72.8 million pounds in 1937 

 (Shapiro, 1948: p. 59). Although this area has 

 been virtually unfished for skipjack over the past 

 2 decades a fishery is now recommencing. 



The apparent decline in eastern Pacific yellow- 

 fin tuna stocks beingfished (e.g., Schaefer, 1961, 



1962,1963) and the increasing world demand for 



1/ 

 tunas (Chapman)- suggest that fishing for skip- 

 jack may become greatly intensified, both in 

 existingfishing areas and in areas not now fished. 

 Management may eventually become necessary to 

 ensure continued high yields of skipjack. 



To place future investigations and possible 

 management measures on a rational and eco- 

 nomically sound basis, we need knowledge of the 

 origins and relationships of the groups of skip- 

 jack that are being harvested or may be harvested 

 in the future. A first step in organizing and en- 

 larging this knowledge is the construction of a 

 set of hypotheses incorporating available data 

 into a testable, integrated picture of the popula- 

 tion biology of the Pacific skipjack. The presen- 

 tation of such hypotheses is the purpose of this 

 paper. Since informationfrom Japanese sources 

 (e.g., Kawasaki, 1955) indicates that the skipjack 

 caught off Japan originate from spawnings in the 

 Ryuku-Izu-Bonin Islands area, far from the areas 

 of direct concern to American fishermen, I will 

 discuss only the origin and relationships of the 

 skipjack of the eastern and central Pacific. I do 

 not attempt to review all the literature on skip- 

 jack populations because adequate reviews have 



— ' Chapman, W. M. Recent trends In world tuna 

 production and some problems arising therefrom. 

 Paper presented to the Symposium on Scombroid 

 Fishes. Marine Biological Association of India, 

 Mandapam Camp, South India, 12-15 January 1962. 

 (In press.) 



