MURPHV and SHOMURA : PRE-EXPLOITATION ABUNDANCE OF TUNAS 



ated with the lowest catch rates shown in Fig- 

 ures 29 and 30. 



A second group of three profiles (panel B) has 

 in common cool water at the equator and a steep 

 gradient rising from south to north. This steep- 

 ness of the gradients also suggests active upwell- 

 ing and northward movement, and the average 

 low water temperature implies the presence of 

 immature water. Like the two profiles shown in 

 panel A, these three are also associated with poor 

 catch rates. 



The profiles in the last group (panel C) show 

 relatively little gradient, at least from lat 1.5° to 

 5°N. Such "flat" profiles at high temperatures 

 should arise if upwelling and northward move- 

 ment of water has slowed down, possibly in re- 

 sponse to a shift from southeast to northeast 

 winds or simply a marked decline in wind force. 

 The four high catch rates shown in Figures 29 

 and 30 were obtained during these four periods. 



The nature of the sequence of events leading 

 to a relatively high abundance of yellowfin tuna 

 can be inferred from the temperature profiles. 

 First a body of enriched water should be created, 

 and this body of water should be permitted to 

 mature relatively undisturbed; that is, the north- 

 south system should be slowed down. The dis- 

 tribution of the winds (Figure 20) suggests that 

 such a sequence of events is most apt to occur in 

 the central portion of the area (long 140°- 

 160°W). In the east, southeast trades of high 

 velocity persist during most of the year, possibly 

 creating a south-to-north circulation with at- 

 tendant convergence of such vigor that the water 

 rarely matures in terms of tuna forage produc- 

 tion. In the center, the southeast trades still 

 have the substantial velocity needed to induce 

 upwelling, but their frequency is only about 50%, 

 suggesting there is frequently an opportunity for 

 the water to mature. In the far west, neither 



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JRM 12 (AUG 1952) 



JRM 13 (OCT 1952) -f-25 



CHG 15 (APR 1954) 



JL 



Figure 31. — Surface temperature profiles 

 on longline fishing cruises along long 148° 

 to 155°W. A. Low temperatures between 

 lat 1° an(i 5°N. B. Low temperatures and 

 a steep north-south gradient between lat 

 1° and 5°N. High temperatures between 

 lat 1.5° and 5°N. 



LATITUDE 



HMS 1 1 (SEPT 1951 ) - 

 JRM II (FEB 1952) 

 JRM 15 (MAY 1953) 

 JRM 16 (AUG 1953) 



24 

 29 

 28 

 27 

 26 

 25 

 24 



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