yellowfin were most abundant near the Equator, where enrichment 

 through upwelling results in an augmented supply of plankton (King and 

 Demond 1953) presumably accompanied by an increase in tuna forage. 

 This higher yellowfin abundance near the E H uator, as compared with 

 regions to the north and south, is clearly shown by the POFI surveys 

 and by Japanese tuna fishing. Bigeye were more abundant than yellow- 

 fin north of the Equator, in the Countercurrent (roughly 5°N. -10°N. 

 latitude), although the population of bigeye there is not nearly as dense 

 as that of the yellowfin to the south. At the present time the higher 

 catches of albacore south of the Equator and west of 150°W. longitude 

 cannot be associated with any particular feature of the environment. 



The temperature sections drawn from bathythermograms taken 



on each fishing line are shown in figures 13 to 19 (appendix). Each 



section, except those of Manning cruise 15 (figs. 15 and 16), shows 



the characteristic features that appear to be associated with the tuna 



o o 



populations. The isotherms sloping downward from about 10 to 5 N. 



latitude roughly coincide with the easterly flowing Countercurrent, 



where bigeye are more abundant than yellowfin. South of 5 N. latitude 



is the westerly flowing South Equatorial Current, where yellowfin are 



most abundant. Centered approximately at the Equator are the doming 



isotherms and lower surface temperatures associated with equatorial 



upwelling. Yellowfin are generally most abundant between the doming 



and the Countercurrent. 



However, during May and June of 1953 the characteristic 

 features of the current system were virtually absent (figs. 15 and 16) 

 along 150 W. and 170 W. longitude. Instead of the well defined slope 

 usually associated with the Countercurrent, the slope was almost 

 imperceptible. The doming at the Equator was also very weak. For 

 instance, the 80 F. isotherm did not reach the surface on 150 W. , 

 whereas in all sections before and after this crossing the surface 

 water at the Equator in this area has been cooler than 80 F. These 

 observations suggest a slowing down or stopping of the entire equatorial 

 current system, which was further indicated by the unusually slight 

 drift of the longline on the stations of Manning cruise 15. However, 

 the fishing results (fig. 2) show that this peculiar condition of the cir- 

 culation did not affect the catches adversely; on the contrary, fishing 

 along 170 W. was better than in our previous experience (Murphy and 

 Shomura 1953b) although it is entirely possible that these events in the 

 environment did affect the distribution of yellowfin at some later time. 



11 



