Distribution of Forage of Skipjack Tuna (Euihynnus pelamis) 

 in the Eastern Tropical Pacific^ 



By 

 MAURICE BLACKBURN- and R. MICHAEL LAURS' 



ABSTRACT 



The fishery for skipjack tuna (Euthijnnus pelamis) in the eastern Pacific Ocean 

 might be extended if offshore areas of high skipjack abundance were known. One would 

 expect the numbers of skipjack in these offshore areas to be related to the distribution of 

 known skipjack forage organisms in the micronekton. The EASTROPAC oceanographic 

 cruises yielded net-caught micronekton samples over large parts of the eastern tropical 

 Pacific during seven successive 2-month periods. From these samples, the occurrence of 

 organisms known to be prey of skipjack was expressed as ml/ 1000 m''. Charts of night 

 and day concentrations in the upper 200 m were produced for each of the seven periods. 



In the region from long 92 to 1 19 W the major areas of maximum concentration 

 of potential skipjack forage remained essentially constant during inost periods. Two of 

 these areas lie parallel to the equatorial upwelling zone — one just to the north, the other 

 just to the south. Another zonal area of abundant forage generally occurs between lat 

 6 and 14 N. Forage is also frequently abundant between lat 14 and 20 N and long 

 107 and 119 W. 



The concentrations of skipjack forage in these areas are compai-able with those in 

 nearshore parts of the eastern tropical Pacific, where the present skipjack fishery occurs. 

 The abundance of skipjack in the forage-rich offshore areas might therefore be sufficient 

 to support commercial fishing operations. Sea-surface temperatures are generally suit- 

 able for skipjack in those areas. On three recent crossings of the equatorial region at 

 about long 119 W, skipjack appeared to be abundant in the first three areas of high 

 forage concentration mentioned above. 



INTRODUCTION tropical Pacific, and is not being fully exploited 



(Rothschild, 1965; Joseph and Calkins, 1969). 

 The United States tuna fishery in the eastern 

 Most of the fishery for skipjack tuna (Eu- tropical Pacific may become increasingly de- 



thynnus pelamis) in the eastern tropical Pa- pendent upon skipjack. The tropical Pacific 



cific lies within a few hundred miles of the tuna fleet continues to grow in size in spite of 



American coast. Skipjack taken in these waters the fact that the fishery for yellowfin tuna 



probably represent adolescent stages of a mi- (Tlninmts albacares) is regulated, and other 



gratory population which breeds in the central than skipjack there is no equally available and 



'This work was supported by the National Marine ^Research Biologist, Institute of Marine Resources, 



Fisheries Service (former Bureau of Commercial Scripps Institution of Oceanography, University of 



Fisheries), both directly and through Contracts 14-17- California at San Diego, P.O. Box 109, La Jolla, Calif. 



0007-458, 14-17-0007-742, 14-17-0007-963, and 14-17- 92037. 

 0007-2311 with the Institute of Marine Resources, 



University of California. It was part of the research of ''Oceanographer, National Marine Fisheries Service, 



the Scripps Tuna Oceanography Research Program of Fishery-Oceanography Center, P.O. Box 271, La Jolla, 



the Institute of Marine Resources. Calif. 92037. 



