134 EIGHTH PACIFIC SCIENCE CONGRESS 



Therefore it is not surprising to find the tuna feeding ground 

 displaced from the upwelHng area in the direction of movement of the 

 water in the meridional plane. However, it would be expected that 

 the plankton, occupying an intermediate trophic level, would also oc- 

 cupy an intermediate geographic position. This it does according to 

 some of our other sets of data for trans-equatorial sections. But pre- 

 ponderantly the peak of zooplankton abundance lies closer to the up- 

 welling area than to the fish concentration and sometimes, as in the 

 SMITH Cruise 1 1 example, it almost coincides with or even lies slightly 

 south of center of upwelling, while the tuna are located well north of it. 



This may not be as anomalous as it appears at first sight. Our 

 plankton catches are proportional to the standing crop. The standing 

 crop, in turn, is the product of inter-action between replenishment and 

 losses from mortality, including particularly consumption by other 

 forms in a given water mass. As a consequence the plankton concen- 

 tration should everywhere be lower than at its center of production, 

 which in the present instance appears also to be the center of upwelling. 

 Simultaneously with the drift of plankton away from the production 

 center and its decline in abundance, it is passing through successional 

 forms, some of them being successive stages in development of indivi- 

 duals, others being successive stages in a food chain. If we may postu- 

 late that particular successional forms of zooplankton are required by 

 the particular nektonic forms which constitute forage for the tuna then 

 we have an explanation for the displacement of tuna away from the 

 center of plankton production. Unfortunately we have not yet learned 

 how to sample quantitatively the nekton of tuna-forage size or to learn 

 its feeding habits, and this explanation remains conjectural. 



North of the concentration of tuna in the convergence zone lies 

 the equatorial countercurrent marked by elevated surface temperature, 

 upward tilting from south to north of the thermocline (Figs, 6A and 

 7B) and the eastward drift of longlines (Fig. 6B). Here the surface 

 layers are poor in phosphate (Fig. 7C) and the zooplankton catches are 

 also poor (Figs. 6C and 7B). However, near the northern boundary 

 of the countercurrent, the upper portion of the thermocline, roughly 

 located by the 26.7 °C (80 °F) isotherm, lies only 50 meters below the 

 surface and water from the thermocline layer might occasionally be 

 stirred by the wind into the overlying warm layer. Possibly such stirring 

 accounts for the distribution of properties found by the Carnegie as 

 depicted by Sverdrup, Johnson, and Fleming ("The Oceans", Fig. 198, 

 p. 710) and interpreted by them to indicate upwelling at the northern 

 boundary of the countercurrent. Some of our hydrogiaphic sections 

 contain indications of enrichment at this location, but the indications 



