time. The Japanese catches are of the same general magnitude as ours but are less variable. 

 The variability of our catches can probably be attributed to the small amount of fishing effort 

 represented. From these data it can be concluded that the catch rates obtained by small experi- 

 nrjental sets of gear are representative of what might be expected from a large commercial set. 



SIZE OF TUNAS 



Length frequencies of the yellowfin tuna taken on the nine sections (table 7) suggest 

 a tendency for the catches to be composed of larger fish toward the eastern end of the survey 

 area, although the difference is not as pronounced as indicated in earlier surveys (Murphy and 

 Shomura 1953b). A recapitulation of data for yellowfin is given in table 7 and in figure 9, based 

 on data in Murphy and Otsu (1954), Murphy and Shomura (1953a, b), and table 7. This clearly 

 shows that longline yellowfin increase in size from west to east. The simplest explanation of this 

 size differential is that the growth rate of the yellowfin ch2inge8 across the Pacific. This is most 

 likely a reflection of the relative availability of food in the several areas. 



Table 7. --Length frequencies of yellowfin tuna taken by longline fishing gear 



y Stations 1-19 

 Z_l Stations 20-22 

 2/ Stations 23-31 



This proposal is compatible with our estimates of the annount of upwelling in the 

 three areas figured. In the westernmost area (140 E. to 170 E. longitude) there is no evidence 

 that upwelling takes place along the Equator (Murphy and Otsu 1954). In the central area (180° 

 to 150 W. longitude) nnoderate to strong upwelling is a persistent feature of the hydrography 

 along the Equator (Austin 1954, Cromwell 1953, Murphy and Shomura 1953a, b, and figs. 5-7 of 

 this report). In the easternmost area (145 W. to 120 W. longitude) upwelling appears to be very 

 intense, at least at times (figs. 2-3). 



16 



