IX 



Skipjack Ecology 



Approximately 550 million pounds of skipjack tuna were 

 taken from the Pacific Ocean in 1962, and there are indica- 

 tions that the intensity of fishing for this fish on a Pacific- 

 wide basis is increasing. The skipjack is probably the most 

 promising species which might be exploited to supplement the 

 apparently limited yellowfin resources of the U.S. fisheries 

 in the eastern Pacific. The possible increase in skipjack ex- 

 ploitation emphasizes the need to estimate the abundance of the 

 resource and the effects of fishery activities on its long-term 

 yield potentials. 



Despite the present and potential importance of the skip- 

 jack fishery, little is known of the number of genetically sep- 

 arate skipjackpopulations, their spawning habits, movements, 

 growth, and other dynamic parameters essential to the calcu- 

 lation of resource abundance and exploitation effects, as well 

 as determination of the need for management and of appropri- 

 ate management activities. 



Significant progress in our understanding of skipjack ecol- 

 ogy has been made recently. After a detailed review of pre- 

 vious studies, a model was designed that collates all pertinent 

 information now known about the origin and distribution of ex- 

 ploited stocks of skipjack in the central and eastern Pacific 

 Ocean. This model, now nearly completed, will enable us to 

 assess our present knowledge of the skipjack tuna and to plan 

 the future course of our research. 



The model suggests that skipjack catches both in the east- 

 ern Pacific and the Hawaiian fishery contain large components 

 that originated in the equatorial region of the central Pacific. 

 When the model is completed, the testing of its hypotheses 

 will begin. 



Another important step was the definition of a unit of effort 

 tor the Hawaiian fishery. This definition provides, for the 

 first time, a basis for the study of tuna population dynamics 

 in the Hawaiiantishery. The unit of effort was defined so that 

 the varied fishing abilities of the vessels in the fishing fleet 



(fig. 12) could be converted into standard comparable units of 

 effort. 



TUNA LIVE-BAIT FISHING 



Figure 12. 



-Live-bait; fishing In Hawaiian sampan 

 skipjack fishery. 



In the study of catch per unit of effort (fig. 13), we dis- 

 covered that the catch per standard effective trip for the 

 Hawaiian skipjack tuna fishery was not associated with fishing 

 intensity. This observation suggests that annual fluctuations 

 in catch for the Hawaiian fishery are largely associated with 

 abundance and availability of skipjack and not with activities 

 of the fishery, at least as they presently exist. 



Noting that there was a steady decline in the number of men 

 fishing per trip (fig. 14), we expected that boats with reduced 

 crews would make smaller catches. On the contrary, catches 

 did not show a decline. The fishermen increased their indi- 

 vidual effectiveness by flipping the fish directly on the deck 

 by allowing the barbless hooks to disengage themselves. 

 Before crew reductions, the fishermen had grasped and un- 

 hooked each fish as it was lifted out of the water and swung 

 toward the boat. 



