MURPHY and SHOMURA: PRE-EXPLOITATION ABUNDANCE OF TUNAS 



100 



^ 200 



a 



UJ 



Q 300 



400 



500 



' I * dp.ei)ii t If I I ( }\i : i: ; : ^ |ij ] r I ) c 



J I c 



J I I 1_ 



18" N 



Figure 16. — Distribution of inorganic phosphate along long 150°W, September 26 through 

 October 4, 1951, Hugh M. Smith cruise 11. PO4 in micrograms per liter, isopleth intervals 

 0.2 /tg atom per liter (Austin, 1954b). 



ship, we have elected to use long 150°W, because 

 more information is available from that merid- 

 ian than from any other and it lies in the center 

 of the region in which yellowfin tuna are most 

 abundant (Figure 6). 



Oceanography 



A meridional temperature section along long 

 150°W (Figure 15) clearly shows the several 

 features described in plane view (Figure 5). 

 North of lat 10°N the isotherms sloping to the 

 right indicate the westerly flowing North Equa- 

 torial Current. Between lat 10° and 5°N the 

 isotherms slope to the left, indicating the east- 

 erly Countercurrent. South of the Countercur- 

 rent lies the westerly flowing South Equatorial 

 Current. 



The cool water at the equator is the result of 

 divergence which disturbs the stability of the 

 thermocline and thereby increases the rate of 

 upward diffusion (up welling). Under these con- 



ditions the surface waters are always cooler than 

 adjacent waters. Nutrient salts, such as phos- 

 phate (Figure 16), are also aff'ected, and their 

 distribution suggests that nutrients are diffusing 

 into the euphotic zone near the equator faster 

 than they are being utilized. According to Crom- 

 well (1953), these enriched waters moving 

 northward and southward within the framework 

 of the westerly flow are warmed and the nutri- 

 ents are utilized by organisms. 



Between the upwelling at and near the equator 

 and the southern boundary of the Countercur- 

 rent at about lat 5°N (Figure 15), there is con- 

 vergence and sinking of the northward flowing 

 waters (Cromwell, 1953). The downward 

 sloping arrows in Figure 17 show schematically 

 the convergence with its attendant sinking. 

 Among other features shown in the figure are the 

 easterly flowing Countercurrent (flow is toward 

 the observer), the westerly North Equatorial 

 Current to the north, and the westerly South 

 Equatorial Current to the south. The figure also 



887 



