Farther east, in the central Pacific, a zone in the vicinity of the Equator, extending 

 to a few degrees north, is enriched by wind-induced upwelling (Cromwell 1954). The nnost 

 important winds in this area are those from the southeast quadrant, which create upwelling and 

 displace the upwelled water to the northward. This upwelling results in an increased supply of 

 zooplankton (King and Demond 1953), and this basic food appears to be related to the relatively 

 dense population of yellowfin found near the Equator (Murphy and Shomura 1953a). The location 

 of tuna in the western Pacific is very similar and it seems possible that similar ecological 

 factors, influencing the distribution of tuna in the central Pacific, are operating farther to the 

 west. 



However, in the western Pacific the relative absence of vigorous winds, particularly 

 those from the southeast quadrant (table 11), would seem to preclude the same intensity of both 

 the upwelling and the northern displacement of the richer water described by Cromwell (1954); 

 at best a system activated by these winds would be weak or sporadic. The general weakness of 

 the winds in the western Pacific is indicated in table 1 1 by the size of the index numbers, which 

 are measures of the relative constancy and force of winds from the southeast quadrant. Pilot 

 chart indices derived for the central Pacific (140 W. long.) were in general 2 to 10 times those 

 from the western Pacific during the 2 years of mothership records, indicating that the forces 

 activating the circulation are of a lesser order of nnagnitude in the western Pacific, 



An examination of the seawater temperature sections in figures 12-15 reveals a 

 lack of evidence of upwelling in any of the sections; that is, the cooler deeper isotherms do not 

 reach the surface near the Equator as is typical of the temperature sections from the central 

 equatorial Pacific (Cromwell 1951, 1954; Yoshida et al, 1953), While it is true that the 3 sections 

 shown in figure 12 are pre-war and those in figure 14 adapted from Robinson were from 1946 

 data, the feature of the lack of doming of the isotherms at the Equator is common to all the 

 sections including those taken by the Japanese research vessel Tenyo Maru in August and 

 September 1951 in the region of the mothership operations (fig, 13), These two sections are the 

 most pertinent as they were taken concurrently with the mothership fishing at a time when yellow- 

 fin showed a definite peak of abundance just north of the Equator (fig, 7), 



Accompanying the general lack of evidence for upwelling in the western Pacific, as 

 contrasted with the central Pacific, are differences in the catch rates of yellowfin tuna even 

 though the pattern of latitudinal distribution is similar. In the central Pacific, with good evidence 

 of strong upwelling, catches averaging around 6 per 100 hooks have been experienced to the north 

 of the Equator (Murphy and Shomura 1953a, b, and unpublished data). In the apparent absence 

 of upwelling in the western Pacific, catches were in the order of 2 to 3 fish per 100 hooks. How- 

 ever, the fact that the relative latitudinal distribution of yellowfin in the western Pacific, where 

 evidences of upwelling appear to be absent at least part of the time, is similar to that in the 

 central Pacific, where upwelling is definitely evident most of the time, suggests a possible con- 

 flict with the upwelling-plankton-forage-tuna cycle proposed for the central Pacific (Murphy and 

 Shomura 1953), This conflict may not be real, however, because there may be upwelling in the 

 western Pacific during some months and during sonne years as indicated by Mao and Yoshida 

 (1953), and the distribution pattern of the yellowfin found there may reflect an integration of 

 average conditions rather than the monnentary ecological situation measured by hydrographical 

 or biological means. It may also be that the peculiar ecological conditions in the immediate 

 vicinity of the Equator favor yellowfin at the expense of other fishes occupying the same position 

 in the food chain, with or without upwelling. Even slight and occasional upwelling superimposed 

 on this background might have permitted the expansion of the yellowfin population. 



Superimposed on the rather regular occurrence of a peak in yellowfin abundance near 

 the Equator are the variations in abundance with time. Analysis of the mothership catch records, 

 while only for two years, suggests that there may be regularly occurring cycles of abundance 

 within a year. 



