FISHERY BULLETIN: VOL. 70, NO. 3 



relatively high bigeye tuna catches occurred in 

 the equatorial Countercurrent, 



7. Deep-swimming albacore were most abun- 

 dant in the southwestern area, e.g., around lat 

 5°S, long 170°W. 



8. Skipjack tuna were found over the entire 

 area. 



9. Deep-swimming yellowfin and bigeye tunas 

 were larger in the east (e.g., 120°W) than in 

 the west (e.g., 130°E) . The reason for this dif- 

 ference is obscure. 



10. Analyses of tuna catches and properties of 

 the environment in a meridional plane revealed 

 the following: There is upwelling and enrich- 

 ment at the equator; the upwelled water is dis- 

 placed northward; zooplankton peaks are at the 

 center of upwelling, apparently because of a time 

 lag in the development of tuna forage. 



11. The water flow near the equator differs 

 from theclassical concept in that empirical mea- 

 surements suggest the northward motion is of 

 the same magnitude as the westward motion. 

 Thus the South Equatorial Current just north 

 of the equator should be regarded as a series of 

 parallel, homologous, northwesterly flow sys- 

 tems. 



12. Variation in surface temperatures at the 

 equator is a function of surface winds, through 

 alteration of the rate of upwelling. 



13. Interaction of geographical variation in the 

 vertical distribution of nutrient salts and the 

 geographical distribution of wind stress cause 

 the greatest enrichment through equatorial up- 

 welling to occur in the central area (near long 

 140°W). 



14. In the area surveyed, between long 120°W 

 and 180°, both zooplankton and yellowfin tuna 

 were most abundant near the center of the area 

 (near long 150°W). 



15. When northeast trades prevail, yellowfin 

 tuna seem most abundant south of the equator; 

 during southeast trades, north of the equator; 

 and during variable winds, the peak abundance 

 straddles the equator. 



16. Between the equator and the Countercur- 

 rent the presence of colder water is associated 

 with more phosphate, more zooplankton, and 

 fewer yellowfin tuna. Warmer water is associ- 

 ated with less phosphate, less zooplankton, and 



more yellowfin tuna. We believe that these dif- 

 ferences are a reflection of the time lapse since 

 enrichment and that variations at a geographical 

 locality are in response to variations in wind 

 flow that aflFect the rate of upwelling. 



17. There are suggestions of annual seasonal 

 variations in the abundance of yellowfin tuna 

 near the equator, but these may be masked by 

 longer term secular trends. 



18. Changes in abundance at a point on the 

 ocean may result from east-west shifts in the 

 yellowfin tuna population as well as north-south 

 shifts, or simply general dispersion from or con- 

 centration at that point. All of these variations 

 may be logically attributed to variations in the 

 enrichment at the equator, brought about by var- 

 iations in the winds. 



19. Surface tunas are seen more often close to 

 land, possibly because more birds are present 

 there. 



20. In the equatorial central Pacific, yellowfin 

 tuna tend to predominate in surface schools close 

 to land and skipjack tuna far from land. 



21. Near the equator, the abundance of surface 

 schools does not seem to coincide with the peaks 

 in abundance of the deep-swimming tunas. Food 

 concentrating mechanisms appear to be more 

 important than basic food supply in aff'ecting the 

 local abundance of surface-swimming schools. 



ACKNOWLEDGMENTS 



We take great pleasure in acknowledging 

 Oscar E. Sette for the leadership and guidance 

 provided the authors during the investigation of 

 the tuna resources of the equatorial central Pa- 

 cific; the investigation upon which this report 

 is based. As Area Director of the Hawaii Area 

 from 1949 to 1955, 0. E. Sette initiated this tuna- 

 oceanography study of the vast unknown equa- 

 torial waters in 1949. 



Practically the entire scientific staflf of the 

 former Bureau of Commercial Fisheries Biolo- 

 gical Laboratory, Honolulu, helped collect the 

 data at sea during the 1950-53 period. In the 

 Laboratory, Jean S. Hailing and Richard N. 

 Uchida were responsible for much of the pro- 

 cessing of biological data. In addition to 0. E. 



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