FISHERY BULLETIN: VOL. 70, NO. 3 



than the plankton. This asymmetry seems anom- 

 alous until it is considered in the light of the 

 general northward transport of the enriched 

 water and the probable time required for tuna 

 forage to appear after the enrichment at the 

 equator. 



Tunas do not consume the smaller zooplank- 

 ters that make up the bulk of the catches of the 

 1-m nets of 30 X X X gage mesh used to sample 

 zooplankton. Rather, tunas forage on the higher 

 trophic levels, from euphausiids to all but the 

 largest nekton (Reintjes and King, 1953). If 

 the food supply is the chief determinant of tuna 

 abundance, the tunas, which are capable of rapid 

 and extensive migration, will concentrate where 

 they can most readily obtain food. In the ab- 

 sence of poleward motion we would presume that 

 tuna forage as well as tunas would concentrate 

 or develop at the equator, where the enrichment 

 and the zooplankton are centered. There is pole- 

 ward motion, however, and the successive troph- 

 ic levels above the zooplankton would be expected 

 to have their greatest population density at some 

 distance from the equator. 



SPACE AND TIME VARIATION 



In this section we focus attention on the var- 

 iation in the distribution of yellowfin tuna and 

 the variation in distribution of properties in the 

 environment and attempt to show their relation. 

 The zone between the equator and the Counter- 

 current receives primary attention simply be- 

 cause more information is on hand from this 

 arra than from others. 



Space and time variation in the tuna distri- 

 bution have been discussed in earlier publica- 

 tions. Murphy and Shomura (1953b) pointed 

 out that when and where southeast winds pre- 

 vailed the yellowfin tuna seemed to be concen- 

 trated north of the equator; when northeast 

 winds prevailed the yellowfin tuna were concen- 

 trated south of the equator; and during periods 

 of variable winds they straddled the equator. 

 These observations were amplified and further 

 discussed by Sette (1955, 1958), who suggested 

 that the average distribution of the catch rates 

 might well be a function of geographical vari- 



ation in the strength of the trades, if a suitable 

 but unknown lag period is assumed between max- 

 ima of upwelling and the occurrence of tuna 

 forage. 



The waters of the equatorial current system 

 are in a state of constant flux, with upwelling, 

 poleward motion, sinking, and westerly motion 

 integrated into a whole that represents a con- 

 tinuously changing environment. In this envi- 

 ronment we have made "instantaneous" obser- 

 vations at selected points and times, and as ex- 

 pected the data obtained are dissimilar in respect 

 to space and time. In theory, at least, a true 

 concept of the relative magnitudes of the several 

 components of the water motion should make it 

 possible to unify these results into a coherent 

 whole. It appears that vertical motions (up- 

 welling) and sinking (convergence) are counter- 

 balancing within the geographical limits (north- 

 south) of the area under consideration, but the 

 relative strengths of the westward and north- 

 ward motions, which cannot be counterbalancing, 

 have not been adequately defined, 



Sette (1958), in interpreting the then avail- 

 able information, assumed that the western com- 

 ponent of flow near the equator was of over- 

 riding importance in compcirison with the pole- 

 ward flow associated with divergence and 

 upwelling. This assumption is consistent with 

 the mean surface flows indicated in climatologi- 

 cal-type summaries of ship's drift observations 

 (e.g.. Figure 5) but is not consistent with cer- 

 tain empirical determinations derived from the 

 drifts of longlines and drogues. These deter- 

 minations suggest that the average poleward 

 component of motion to the north of the equator 

 on long 120°W would bear little relation to events 

 at lat 5°N on long 140°W and even less to events 

 at lat 5°N on long 150°W, In other words, water 

 parcels upwelled at the equator on long 120°W 

 are probably lost from the system by the time 

 they are transported westward to long 130°W, 

 because their northward motion will have car- 

 ried them to the Countercurrent (near lat 5°N). 

 Here, they must sink because the Countercurrent 

 is warmer and less saline than the waters to the 

 south. 



Most of the evidence for these arguments lies 

 in measurements of the drifts of sets of longline 



890 



