FISHERY BULLETIN: VOL. 70, NO. 3 



Associated with the longitudinal variations in 

 average catch rates, the distribution of yellowfin 

 tuna on a meridional plane appears to differ 

 with latitude (Figure 24). In the extreme east 

 (long 120°W), where enrichment is low, abun- 

 dance of yellowfin tuna is also low and there is 

 little evidence of increased abundance associated 

 with the equator. Farther west (long 130°W) 

 the abundance of yellowfin tuna is higher, and 

 the expected peaking under prevailing southeast 

 trades appears north of the equator. In the cen- 

 ter (about long 150°W), where enrichment is 

 thought to be highest and southeast trades pre- 

 dominate, abundance is high and centered well 

 north of the equator. In the west, where en- 

 richment is low and northeast trades are pre- 

 dominant, abundance of these tuna falls off and 

 their distribution center is at or slightly south 

 of the equator. 



Discussion. — The available physical measure- 

 ments, e.g., drogue movements and longline 

 drifts, suggest that the northward movement 

 from the equator is of considerable importance 

 relative to the westward motion, and this con- 

 clusion is at variance with the results, based on 

 ship's logs, depicted on either pilot charts or 

 other summaries, such as our Figure 5. 



As discussed in earlier sections, northward 

 displacement of tuna with respect to plankton 

 (Figure 25) is attributed to the time lag in the 

 development of tuna forage, for during this pe- 

 riod the waters are moving in a northerly direc- 

 tion. This northward displacement appears 

 perfectly logical, but if so, and if the westerly 

 flow is greater than the northerly flow, as we 

 have generally assumed, the peak of tuna abun- 

 dance should be displaced well to the west of the 

 zooplankton peak. 



The available data (Figure 23) give little or 

 no evidence of westerly displacement of the tuna 

 peak relative to the zooplankton peak. This ob- 

 servation, coupled with empirical measurements 

 showing that the northerly component of flow 

 is vigorous and that the phosphate distribution 

 is heterogeneous, makes it difficult to accept the 

 earlier concept of an overriding western element 

 of flow. The model, elaborated in the previous 

 sections, that is consonant with all the evidence 



regards this portion of the South Equatorial 

 Current from the equator to lat 5°N and long 

 120°W to 180° as comprising a series of homol- 

 ogous, parallel, northwesterly transport systems 

 (Figure 19). 



Temporal Variation 



Variations in enrichment and distance from 

 the source of enrichment were advanced as ex- 

 planations for north-south and east-west varia- 

 tions in the abundance of yellowfin tuna in the 

 region north of the equator. Most of the north- 

 south distributions were readily reconcilable 

 with temperature, phosphate, and plankton 

 fields. The east-west distributions can also be 

 explained in the same general terms, but the 

 arguments are somewhat weaker, because the 

 sampling program gave first priority to eluci- 

 dating the distribution along the north-south 

 axis. As a final step we propose to show that 

 even the temporal variations in yellowfin tuna 

 can be explained within the same framework as 

 the spatial variation. 



Murphy and Shomura (1953b) pointed out 

 that during seasons or in areas dominated by the 

 southeast trades yellowfin tuna tend to be con- 

 centrated north of the equator and that during 

 periods of variable or northeast winds they tend 

 to straddle the equator (Figure 26). This find- 

 ing is consistent with the hypothesis, because 

 either light, variable, or northeast winds should 

 reduce the northward motion of water from the 

 equator. 



Averages such as those in Figure 26 may be 

 misleading simply because the sampling has not 

 been well stratified. Furthermore, although 

 there are climatological seasons along the equa- 

 tor, they may frequently be altered or obscured 

 by short-term changes to the extent that during 

 a given year the "seasons" may not materialize. 

 Because of the few meteorological observations 

 in the area under consideration, it is difficult to 

 evaluate the years in which we fished in terms 

 of the normal. For these reasons, we evaluate 

 the temporal changes in fish catch along a mer- 

 idian in terms of the environment prevailing at 

 the time of the catches, thus overcoming the lack 



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