FISHERY BULLETIN: VOL. 70, NO. 3 



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J I I I L. 



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.8 10 1.2 1.4 1.6 

 PHOSPHATE /JG AT/ L 



1.8 2.0 2.2 



Figure 22. — Temperature-phosphate curves from the 

 equator at several longitudes. The thermocline approx- 

 imately corresponds with the 21 °C isotherm. The ap- 

 proximate depth in meters for each group of curves is 

 shown. Where one longitude is represented by several 

 curves, the stations are separated in time from several 

 weeks to several months. 



in the distribution of phosphate. Accepting the 

 differences among longitudes (Figure 22) as 

 vahd, it is clear that, though inequality in wind 

 stress might be directly related to the amount 

 of upwelling and consequently of cooling of the 

 euphotic zone (as appears to be the case), these 

 inequalities in wind stress will not necessarily 

 be directly related to variations in the amount 

 of enrichment. A given wind stress will prob- 

 ably produce the most enrichment along long 

 140°W, whether the energy is chiefly dissipated 

 in the region of the thermocline or is dissipated 

 uniformly with depth. The difference, however, 

 will be most striking if the principal effect of the 

 wind is expended in disturbing the thermocline. 

 For instance, water diffused up from the ther- 

 mocline along long 140°W will contain nearly 

 twice the amount of nutrient salts as water dif- 

 fused up from the thermocline at long 117°W. 

 Considering the variations in the vertical dis- 

 tribution of nutrient salts (Figure 22) and the 



geographical distribution of wind stress (Fig- 

 ure 20) , it appears that on the average the great- 

 est enrichment might be in the center of the 

 system (near long 140°W), for in this region 

 wind stress is high and the thermocline is 

 abundantly supplied with nutrients. To the 

 west, enrichment could very well fall off, since 

 the wind stress declines, and to the east, enrich- 

 ment might decrease because the thermocline is 

 impoverished. 



The discussion above is admittedly based on 

 scanty and perhaps unreliable data. The gen- 

 eral conclusion — that we should expect the most 

 enrichment near the center of the area under 

 consideration — is, however, compatible with the 

 biological data to be considered next. Even more 

 important, the phosphate-temperature curves in- 

 dicate an east-west heterogeneity in the distri- 

 bution of physical and chemical properties. This 

 heterogeneity is consistent with the model of flow 

 pattern (Figure 19) evolved from longline drift 

 data. 



Biological properties. — The biological proper- 

 ties along a latitudinal axis (Figure 23B, C) do 

 not follow the temperature gradient (Figure 23 A 

 and Table 2) but rather tend to be distributed 

 in conformance with the general enrichment pat- 

 tern described in the previous section on nutri- 

 ents. Both plankton and yellowfin tuna tend to 

 be highest in the center of the region, where the 

 measurements suggest enrichment is greatest. 



The plankton distribution (Figure 23B) is 

 somewhat unreliable to the east of long 150°W 

 because the samples are few and unequally dis- 

 tributed in time (King, 1954; King and Hida, 

 1957). Nevertheless, all evidence points to a 

 peak of plankton coinciding with or slightly west 

 of the peak in enrichment. 



The distribution of yellowfin tuna (Figure 

 23C) is more orderly than that of zooplankton, 

 possibly because of a more orderly temporal and 

 spatial stratification of the samples. The peak 

 abundance of tuna is close to the longitude of 

 maximum enrichment and the longitude of max- 

 imum standing crop of zooplankton. 



Though there is general agreement between 

 the standing crop of zooplankton and tuna, there 

 are several discrepancies. For instance, the 



894 



