NOURISHMENT OF CENTRAL PACIFIC STOCKS OF TUNA 135 



are feeble and do not appear regularly as should be true if upwelling 

 at the northern boundary of the countercurrent were a fundamental 

 part of the circulation. 



The surface upwelled water immediately south of the equator may 

 move northward across the equator or it may move southward away 

 from the equator depending on wind direction. At the time of SMITH 

 Cruise 11, the phosphate values, the temperatures, and the plankton 

 catches suggest that it was moving southward away from the equator. 

 There is no evidence of convergence and indeed Cromwell's model calls 

 for none south of the equator unless winds blow from the north as well 

 as from the east. Without convergence, the biota produced in the water 

 drifting south from the center of divergence, upwelling, and enrich- 

 ment may simply disperse without producing concentrations of tuna 

 such as appear in the convergent zone north of the equator. 



However, good catches of albacore taken by our research vessels 

 on several occasions between 5° and 17°S. latitude, suggest that the 

 southward drifting biota may not always completely disperse before 

 high trophic levels are reached. 



We may conclude from our examination of the distribution of 

 physical, chemical, and biological properties of sea water in the meri- 

 dional plane across the equatorial current system at 155°W. longitude: 



(1) divergence at the equator is induced by easterly winds; (2) up- 

 welling associated with the divergence brings inorganic nutrient salts 

 to the surface layers; (3) plankton develops in the enriched water; 



(4) the enriched water bearing the plankton moves away from the 

 equator; (5) convergence may occur either north or south of the equator 

 depending on the meridional component of the wind direction; (6) with 

 a northward meridional component, as in southeast trade winds, the 

 convergence is north of the equator; (7) enhanced productivity resulting 

 from equatorial divergence, coupled with the concentrating effect of 

 convergence on the biota as it passes through the successive trophic 

 levels, provides a feeding ground for tuna in the convergent zone; 



(8) yellowfin tuna concentrate abundantly on this feeding ground. 



Turning away from the meridional aspect and remembering that 

 the divergence-convergence system described above will be most effec- 

 tive when there is a strong, steady wind of long reach from an easterly 

 direction, we may examine the east-west distribution of elements perti- 

 nent to equatorial productivity. In August, when southeast trade winds 

 are best developed over the equator, the wind conditions for diver- 

 gence and upwelling are at their best in the eastern half of the Pacific 

 as may be seen from Fig. 8A. Arrows representing wind vectors are 

 shown for the tier of five-degree squares lying on each side of the equa- 



