HAWAIIAN-CAUGHT SKIPJACK TUNA AND THEIR PHYSICAL 



ENVIRONMENT 



GUNTER R. SeCKEL^ 



ABSTRACT 



Empirical associations between the occurrence of skipjack tuna (Katsuwonus pelamis) 

 in Hawaiian waters and environmental conditions point to the current system as an im- 

 portant cause for the variations in the Hawaiian fishery. Large interyear differences 

 of sea-air interactions in the skipjack spawning areas may affect larval survival and 

 year-class strength. A numerical drift model was used to investigate the contribution 

 of currents to the travel of skipjack from the eastern North Pacific to Hawaii. Floating 

 objects introduced in the model ocean at long 120 °W and lat 10° to 20° N converge toward 

 the northern edge of the North Equatorial Current near Hawaii in 21 to 23 months. The 

 time of freedom of skipjack tagged in the eastern North Pacific and recovered in Hawaiian 

 waters is of the same magnitude. It is concluded that for skipjack a possible mode of 

 travel from the eastern North Pacific to Hawaii is drifting in the North Equatorial 

 Current. The variability in abundance and size-frequency distributions observed in the 

 Hawaiian fishery can be caused by changes in the current system. Numerical models 

 of the type presented can be verified and so permit progress from the exploratory to the 

 experimental phase in fisheries investigations. 



Over a decade ago empirical associations were 

 established between environmental parameters 

 and the availability of skipjack tuna {Katsuwon- 

 us pelamis) to the Hawaiian fishery. Since that 

 time oceanographic studies have provided addi- 

 tional information to augment these associations 

 that can now be used to postulate causal relation- 

 ships. Such hypotheses are an essential step in 

 bringing ecological studies from the exploratory 

 to the experimental phase, numerical analysis, 

 and, eventually, prediction. 



In this paper I will briefly review the empirical 

 associations that were established and introduce 

 new information that has resulted from the 

 Trade Wind Zone Oceanography investigation. 

 The empirical associations lead to two environ- 

 mental processes that must be included in nu- 

 merical models describing the distribution of 

 skipjack. One concerns the sea-air interaction 

 processes that may affect productivity and larval 

 survival, and the other concerns the current field 

 that affects the distribution of skipjack schools. 



' National Marine Fisheries Service, Pacific Environ- 

 mental Group, Monterey, CA 93940. 



The latter is illustrated by a numerical "drift 

 model" for a portion of the Pacific North Equa- 

 torial Current. 



The purely physical explanations that are 

 given for the varying availability of skipjack 

 in the Hawaiian fishery and the implications of 

 the drift model results may conflict with beliefs 

 based on biological considerations. Such con- 

 flicts can be resolved if the proposed hypotheses 

 are tested experimentally as suggested in this 

 paper. 



It is fitting that this work should be reported 

 in Dr. Sette's Festschrift. He provided great 

 impetus and leadership to the integration of en- 

 vironmental studies with fisheries research. The 

 work on Hawaiian-caught skipjack and on their 

 environment was initiated at the time Dr. Sette 

 directed the Pacific Oceanic Fishery Investi- 

 gations (POFI) in Hawaii. 



REVIEW OF EMPIRICAL ASSOCIATIONS 



THE HAWAIIAN SKIPJACK FISHERY 



The Hawaiian skipjack fishery has been de- 

 scribed by Yamashita (1958) and Uchida 



Manuscript accepted March 1972. 



FISHERY BULLETIN: VOL. 72, NO. 3, 1972. 



763 



