MURPHY and SHOMURA: PRE-EXPLOITATION ABUNDANCE OF TUNAS 



Table 2. — Mean surface temperatures by longitudes and months for lat 0° 

 to 5°N (basic data from U.S. Weather Bureau, 1938). 



350 



UJ 



a: 



UJ 



a. 



s 



UJ 



I 



< 



APR. MAY JUNE JULY AUG. SEPT OCT NOV DEC. JAN. FEB. MAR. 



Figure 21. — Gradients of mean surface water temper- 

 ature, equator to lat 5°N, and stress of southeast winds 

 across the equator between long 120° and 170°W. We 

 prepared the index of southeast wind stress by multiply- 

 ing the percentage occurrence by the average force. The 

 basic data are from the four wind roses that focus on 

 the intersection of the equator and the appropriate long- 

 itude in the U.S. pilot charts for 1953. Half of the due 

 east winds were arbitrarily assigned to the southeast 

 quadrant. 



Because of the lack of measurements of other 

 salts, we will consider only inorganic phosphate. 

 Even a discussion limited to this property must 

 be considered poorly documented, however, be- 

 cause of the few measurements and the inherent- 

 ly erratic nature of phosphate determinations. 

 In considering the distribution of phosphate, 



we have constructed several temperature-phos- 

 phate curves from along the equator (Figure 

 22). We elected to use the temperature-phos- 

 phate curve because it seemed to yield the best 

 available index of enrichment. Although low 

 temperatures appear to be a good index of up- 

 welling and relative enrichment when a single 

 meridional section is being considered, it does 

 not necessarily follow that relative coolness a- 

 mong meridians along the equator is a good index 

 of enrichment, for there are marked east-west 

 gradients in such factors as the depth of the 

 thermocline (Austin, 1958). Temperature- 

 phosphate curves should permit evaluation of 

 the differences associated with these gradients 

 and should indicate the "enrichment potential" 

 along the equator. 



The surface temperature relations among the 

 temperature-phosphate curves in Figure 22 are 

 consistent with the general east-west distribu- 

 tion of temperature, i.e., cooler water to the east 

 suggests that they represent typical rather than 

 atypical conditions. Phosphate, however, does 

 not follow temperature when the several longi- 

 tudes are considered together. Inorganic phos- 

 phate is lowest along long 117°W, where the 

 surface water is coldest, and highest along long 

 140 °W, where water temperature is interme- 

 diate. These differences in the basic tempera- 

 ture-phosphate relation appear to persist to rath- 

 er low temperatures and therefore to consider- 

 able depths. 



We will not attempt to explain the differences 



893 



