FISHERY BULLETIN: VOL. 70, NO. .1 



is more readily reconcilable with this concept 

 than with the classical flow pattern depicted in 

 Figure 5. 



Geographical Variation 



We have advanced the concept that the flow 

 pattern in the northern latitudes of the equator- 

 ial waters consists of a series of homologous 

 northwesterly flowing cells. A study of the dis- 

 tribution of physical and biological properties, 

 e.g., winds, water temperature, nutrients, plank- 

 ton, and fish, indicates that they are more readily 

 reconcilable with the homologous cell concept 

 than with the classical flow pattern depicted in 

 Figure 5. 



Winds. — The wind system is the source of 

 energy for the equatorial circulation, and it is 

 logical to approach the problem of geographical 

 variation of tuna by first examining the vari- 

 ations in the winds. The average distribution 

 of winds (Figure 20) clearly shows a peak of 

 wind stress near long 130°W, and in this area 

 the equator is dominated by the southeast trades. 

 Farther west the two trade systems are about 

 equally distributed and the average force de- 

 clines. In the far west the northeast trades 

 dominate and the average force is low. 



Water temperature. — If the wind is the causal 

 force behind the upwelling and poleward dis- 

 placement of enriched (cooled) water, there 

 should be a demonstrable relation between var- 

 iation in the mean wind and variation in the 

 mean water temperature. Table 2 depicts the 

 mean monthly temperature of the surface waters 

 north of the equator. It clearly shows the gen- 

 eral trend to warmer water in the west in the 

 zone between the equator and lat 5°N, which is 

 the expected result of the wind gradient (Fig- 

 ure 21). 



More significant is the variation in the size 

 of the water temperature gradient from east to 

 west. For instance, during March there is prac- 

 tically no slope but during October there is a 

 steep slope (Table 2). The size of these grad- 



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150° 140° 



LONGITUDE 



II0°W 



Figure 20. — Average direction and force of winds over 

 the equator. The values were derived from U.S. pilot 

 charts for 1953. In arriving at a value for a given long- 

 itude we averaged the winds in the four 5° squares focus- 

 ing on the intersection of the meridian and the equator. 

 East winds were apportioned equally between northeast 

 and southeast. 



ients, when plotted against a measure of the 

 gradient of southeasterly winds (Figure 21), 

 suggests a causal relationship. When the wind 

 gradient is large the gradient in water temper- 

 ature is steep, and vice versa. We can now pro- 

 ceed with confidence on the assumption that var- 

 iations in the winds are responsible for varia- 

 tions in water temperature through altering the 

 rates of upwelling and the northward displace- 

 ment of cool water. 



Nutrients. — It is now in order to examine the 

 distribution of nutrient salts in the euphotic 

 zone along the equator to see whether the con- 

 centration varies with the amount of upwelling 

 as evidenced by winds and water temperature. 



892 



