FISHERY BULLETIN: VOL. 70, NO. 3 



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Figure 17. — Schematic representation of a meridional section of the equatorial central Pacific 



(Sette and staff of P.O.F.I., 1954, p. 3). 



shows a feature new to this discussion, the Equa- 

 torial Undercurrent described by Cromwell, 

 Montgomery, and Stroup (1954). This latter 

 current is usually found in the upper portion of 

 the thermocline and rarely flows as a surface 

 current. 



Tunas 



Our aim is to compare the meridional distribu- 

 tion of fish with certain properties in the envi- 

 ronment. Ideally, we should compare the weight 

 of fish life at a given trophic level with such ele- 

 ments as plankton abundance and nutrient level. 

 This comparison is difficult with the material at 

 hand, but, as shown in Appendix A, the catches 

 of yellowfin tuna, the dominant species, appear 

 to reflect the abundance of the larger predators. 



Tuna-Oceanography 



In the discussion of the areal distribution of 

 tunas we pointed out that the peak abundance 

 of yellowfin tuna was near the equator. We now 

 propose to examine the hypothesis that this 

 striking concentration of yellowfin tuna is the 

 result of the following sequence of events: (1) 

 upwelling at the equator, (2) northward dis- 

 placement of the water, (3) relatively rapid de- 



velopment of zooplankton, (4) a slower devel- 

 opment of tuna forage, and (5) congregation of 

 tuna in the region best supplied with forage. 



It can be inferred from the distribution of 

 yellowfin tuna and water temperature in Figures 

 6 and 15 that there is a general coincidence be- 

 tween the location of upwelling and the zone in 

 which yellowfin tuna are most abundant. Fig- 

 ure 18 clearly shows this relation as well as the 

 distribution of other variables such as plankton 

 and current direction. 



In the top panel (A), the temperature struc- 

 ture shows the cool surface waters at the equator 

 that can be attributed to upwelling. That this 

 is accompanied by enrichment was shown in 

 Figure 16 in the distribution of inorganic phos- 

 phate. The sharp downward slope of the iso- 

 therms between lat 5° and 9°N is the region of 

 the Countercurrent. 



The second panel (B) shows the drift of the 

 longline while in the water. This drift, which is 

 essentially the movement of the gear between 

 0600 and 1600 hr, shows the direction of the 

 several major water movements discussed ear- 

 lier. North of lat 9°N, the drift is to the west, 

 indicating the North Equatorial Current. Be- 

 tween lat 5° and 9°N, the drift is to the east, 

 suggesting the Countercurrent. Except for the 

 station nearest the equator, the drift is westerly 

 south of lat 5°N, indicating the South Equatorial 

 Current. The easterly drift at the equator is 



888 



