DEEP-SWIMMING TUNAS 



This section presents all the data on the dis- 

 tribution of deep-swimming tunas but empha- 

 sizes the yellowfin tuna and how the environ- 

 ment influences their distribution. Scientists 

 have advanced certain hypotheses to explain the 

 variations in the abundance of yellowfin tuna 

 in the central Pacific (see, for example, Crom- 

 well, 1953) . These hypotheses, however, involve 

 only portions of the total mass of data, and usu- 

 ally only a limited aspect of the variations in 

 abundance, e.g., longitudinal variation. 



The most elegant way to examine hypotheses 

 about fish and the environment is to show step 

 by step how precise interactions of energy flow 

 and behavior (in a broad sense) result in a par- 

 ticular distribution of fish. This approach is not 

 possible with the present data. Instead we have 

 adopted the more generalized approach of ad- 

 vancing a hypothesis about one aspect of the 

 data, i.e., the average distribution of yellowfin 

 tuna with respect to the average distribution of 

 environmental properties. Then we examine 

 this hypothesis with respect to details of var- 

 iation in abundance such as the north-south dis- 

 tribution and east-west distribution at particular 

 times and places, and time variation at particular 



FISHERY BULLETIN: VOL. 70, NO. 3 



places. Our assumption is that if the same basic 

 hypothesis stands up under this kind of cro'ss- 

 examination, its merit is essentially proved, even 

 though a great deal remains to be learned about 

 the exact way the ecosystem functions. This 

 approach results in some repetition of argu- 

 ments, especially as it has been possible to ex- 

 plain nearly all aspects of variation in abundance 

 with the same hypothesis — namely, that varia- 

 tions in the dominant species, the yellowfin tuna, 

 are influenced by variations in the wind-driven 

 ocean circulation that affect the food supply of 

 that species. 



AVERAGE AREAL DISTRIBUTION 



In this section wp provide a brief description 

 of the circulation of the major surface currents 

 and the general distribution and relative abund- 

 ance of the deep-swimming tunas and skipjack 

 tuna in the central equatorial Pacific Ocean. 



Oceanography 



The dominant features of the eastern tropical 

 Pacific are the three great currents (Figure 5) : 

 the westerly flowing North Equatorial Current 



70" w 



Figure 5. — Schematic representation of the major surface currents and surface isotherms in the central 

 and eastern tropical Pacific. The area most intensively surveyed is shaded. Arrows denote approximate 

 current direction. (Adapted from Schott, 1935.) 



880 



