FISHERY BULLETIN: VOL. 70, NO. 3 



the velocity nor the frequency of the southeast 

 trades appear high enough to create substantial 

 bodies of enriched water north of the equator. 



We now have two processes that might ac- 

 count for poor yellowfin tuna fishing in the east 

 and west and good fishing in the center. The 

 one, involving the wind pattern just described, 

 rests on the assumption that if enriched water 

 is to result in a dense population of yellowfin 

 tuna, it must remain relatively undisturbed in 

 the euphotic zone long enough for a forage pop- 

 ulation to develop. The other, discussed in the 

 section on nutrients, is based on the vertical dis- 

 tribution of phosphate at various longitudes 

 along the equator. Phosphate distribution, when 

 viewed against the wind distribution, suggests 

 that the absolute amount of enrichment might 

 be greatest in the center of the area, and it is 

 in this area that the enriched water appears to 

 have a reasonable chance to mature before con- 

 vergence. The two processes are independent 

 but interact in a way consistent with the occur- 

 rence of the best yellowfin tuna fishing in time 

 and space. 



In addition to variation in the catch rates at 

 a given longitude, there have been changes in 

 the meridian of the best catches. For the year 

 1953 there are enough records to illustrate this 

 phenomenon (Figure 32). Such shifts are in 

 harmony with the present theories of the factors 

 controlling yellowfin tuna abundance. The yel- 

 lowfin tuna are highly motile, and we expect them 

 to be most abundant where conditions best suit 

 them. For instance, if upwelling is too vigorous 

 over the area from long 120° to 150°W and less 

 vigorous farther west, conditions should be bet- 

 ter in the west. This situation may well explain 

 the pattern in May-June of 1953 (Figure 32). 

 On the other hand, in August 1953 yellowfin tuna 

 seemed to be most abundant near long 155°W; 

 this distribution would be expected if strong up- 

 welling and northward displacement at some 

 time in the past was followed by a period of rel- 

 ative quiet. 



These deductions are difficult to test critically, 

 i.e., it is difficult to make a critical comparison 

 between fish catch and some factor in the envi- 

 ronment such as temperature because of the het- 

 erogeneity of the water when considering the 



equator as a whole. However, there is a sug- 

 gestion of an association in our data: During 

 February-April 1953 (Figure 32) the catch was 

 better on long 140°W than on long 150°W, and 

 at the same time the mixed layer was about 0.5°C 

 lower on long 150°W than on long 140°W. This 

 is a reversal of the average trend (Figure 23) 

 and could only have resulted from the water at 

 long 150°W being more recently aff'ected by up- 

 welling and northward displacement than that at 

 long 140°W. In perfect agreement with our hy- 

 pothesis, this relatively newer water at long 

 150°W had a smaller population of tuna. 



In summary, it has been possible to develop a 

 functional hypothesis consonant with the me- 

 chanics of the equatorial system that can logic- 

 ally account for all, or nearly all, of the major 

 observed variations in the abundance of the 

 large deep-swimming yellowfin tuna. The the- 

 ory successfully embraces variations in catch 

 rate in respect to time and in respect to space. 

 Since the root of the system lies in the winds, 

 it should be possible eventually to anticipate the 

 variations in the yellowfin tuna population by 

 examining variations in the wind regime. 



SURFACE TUNAS 



The principal methods we have used to sample 

 surface tunas are surface scouting and trolling. 

 Live-bait fishing has also been used occasionally. 

 These are the same methods used to locate tunas 

 by the several live-bait and purse-seine fisheries 

 in the Pacific. 



Each of these survey methods has inherent 

 weaknesses that cannot be estimated quantita- 

 tively. Perhaps the greatest difficulty in surface 

 sighting lies in the state of the sea. The central 

 Pacific is nearly always choppy, practically pre- 

 cluding detection of schools at the surface unless 

 they are breaking water or are accompanied by 

 feeding birds, e.g., in our experience 85% of all 

 schools were first sighted through the accompa- 

 nying "working birds." In eff'ect, we do not see 

 tuna unless their presence is marked by birds; 

 this relation casts doubt on the census method. 

 The problem, however, may not be as serious 

 as it seems, for the association between birds 



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