NOURISHMENT OF CENTRAL PACIFIC STOCKS OF TUNA 133 



bound transit was interrupted for 15 days at 2°N. latitude to occupy 

 an east-west line of fishing stations. These will be omitted from our 

 discussion. 3 The observations on temperature, phosphate, plankton, and 

 tuna are diagramed in Figures 6 and 7. 



The upwelling at or very near the equator is evident at the sur- 

 face from the drop in temperature, and in cross section by the extension 

 to the surface of the water cooler than 26.7 °C (80 °F) and richer in 

 phosphate than 0.4 microgram atoms per liter. The transport near the 

 surface of the upwelled water northward and southward from the equa- 

 tor is evident from the extension of the higher phosphate concentration 

 in both directions from the equator. It is also evidenced by the north- 

 ward component in the direction of longline drift ^ north of the equator 

 and the southward component when they were fished south of the equa- 

 tor (Fig. 6B). 



The movement of the water in surface layers toward the north 

 away from the equator decelerates and may terminate. In either event 

 there is convergence and probably sinking. The northern limit of the 

 convergent zone was at 4°50'N. latitude at the time of the northbound 

 passage of the SMITH and is clearly marked by the abrupt change in 

 surface temperature at that latitude (Fig. 7A). This also approximates 

 the southern boundary of the countercurrent. A month earlier, during 

 the southbound passage, this feature was about one degree farther 

 south judging from the change in slope of the 15.6° and 26.7°C iso- 

 therms and from the drift of the longline gear (Figs. 6 A and B). 



Yellowfin tuna were markedly concentrated between the equator 

 and the northern limit of the convergence, the catch averaging four 

 times as many fish per unit of fishing effort between 1° and 6°N. as 

 elsewhere (Fig. 6D). 



In the sequence of events between the enrichment of the water 

 at the equator and the production of food for the tuna, several trophic 

 levels intervene. According to stomach examinations tuna eat prac- 

 tically all but the largest members of the bathypelagic nekton, small 

 fish and squid forming their main diet. The fish and squid feed on 

 other nekton or on the zooplankton, which in turn feeds on phyto- 

 plankton. Time must intervene and transport of the biota takes place 

 between enrichment of the surface waters and the production of feed 

 for the tuna. 



'They are reported by Murphy and Shomura: "Longline fishing for deep-swimming tunas 

 in the central Pacific, 1950-51." Department of the Interior, P'ish and Wjldlite Service, Special 

 Scientific Report: Fisheries No. 98, May 1953. 



* The longline fishing gear is a system of horizontal and vertical lines suspended from the 

 surface by buoys. It is allowed to drift freely in the water during the fishing operation, 

 which occupies most of the daylight hours. The drift of this gear is mainly determined by 

 the movement of the upper 100- to 150-meter stratum of water, though it is also influenced 

 somewhat by surface wind and waves. The gear usually drifts far enough for its course and 

 direction to be determined by celestial navigation. 



