These are longitudinal sections prepared from bathythermographs 

 taken concurrently vn.th the fishing stations o They indicate such 

 recognizable features as the shallow thermocline at the northern 

 boundary of the Countercurrento Proceeding southward the thermo- 

 cline and isotherms are found progressively deeper to a point north 

 of the Equator„ As the Equator is approached the isotherms come 

 closer to the surface and the surface temperature decreaseso This 

 is an indication of upwelling in the vicinity of the Equator, a 

 process that brings cooler, nutrient-rich water into the photo- 

 synthetic zoneo This upwelling is wind-induced and during periods 

 of winds from the southeast quadrant ■•"he upwelled water is displaced 

 northward, (Cromwell 1951^ Cromwell^)o An increased supply of 

 zooplankton develops in this upwelled water (King and Demond 1953) „ 

 and so provides greater opportunity for the maintenance of a popu- 

 lation of animals such as tuna than is provided by the areas to 

 the north and south not affected by the equatorial enrichment. In 

 the upper panel of figure 14 are shown the catches at fishing 

 stations along 150° \V<, longitude. The lower panel shows the plankton 

 catches that were made at each fishing stationo The general corres- 

 pondence between fish and plankton is rather striking and provides 

 an excellent indication that yellowfin tuna were abundant in the 

 area more favorably supplied with basic foodso 



Though the correspondence between the abundance of yellow- 

 fin tuna and zooplankton is striking, the two distributions diverge 

 in several details o These do not appear serious, and since we can- 

 not offer well-founded explanations for the divergences, they are 

 mentioned only very briefly together with some of the possible reasons 

 for noncorrespondenceo First, the peak of tuna catch does not coin- 

 cide with the peak of zooplankton abundance. Reintjes and King (1953) 

 have shown that yellowfin tuna do not forage extensively on plankton 

 but rather consume the small fishes, crustaceaj and molluscs that 

 feed on zooplankton. Because of this unsampled link and the probable 

 northerly drift during the time lags in passing through the unsampled 

 stage of the food cycle, the location of the tuna stocks should not 

 be expected to coincide exactly with the location of the planktorio 

 Second, the poorer catches relative to plankton abundance north of 

 6° N, latitude may be in part due to some of the hooks' fishing in 

 waters too cold for yellowfin tuna. For instance, at 8° N„ latitude 

 on 150° W, longitude, where the thermocline was very shallow (figo ll), 

 a measurement in the field indicated that the deep hooks were fishing 

 in water of 56° F, or coldero Third, and finally, the question may 

 be raised as to why the bigeye tuna catches, for instance, were not 

 greater where the yellowfin were abundant. In fact, as shown in 

 tables 2 and 3, the bigeye appear to be more abundant north of 6° 

 No latitude, where few yellowfin are caughto Pending study of com- 

 parative feeding habits of the two species » no explanation is 

 ventured. The small numbers of albacore and skipjack in the catches 



Y 



Cromwell, Townsend, MSo Circulation in a meridional plane in 

 the central equatorial Pacifico 



19 



