FISHERY BULLETIN: VOL. 70, NO. 3 



E 180* 



170* 



N 

 20* 



10' 



0* 



10' 



ISO* 



150* 



140* 



20* 



130* 



120* W 



Figure 9. — The distribution of skipjack tuna in the equatorial central Pacific Ocean as 

 shown by longline catches. A dot represents capture of one or more skipjack tuna. 



Yellowfin tuna were found over the entire 

 survey area (Figure 6) , but the peak abundance 

 occurred near the equator in the zone enriched 

 by the equatorial upwelling. Betvv^een this zone 

 and lat 10°N, the numbers decreased rapidly. 

 Abundance south of the equator also declined 

 but not as precipitously as to the north. Even 

 near the equator this species was scarce at the 

 eastern end of the survey area; it was most 

 abundant in the center (between long 140° and 

 160°W) and somewhat less abundant to the west 

 of long 160°W. 



Bigeye tuna were not as abundant as yellowfin 

 tuna, but their distribution, like that of the yel- 

 lowfin tuna, was far more uniform over the area 

 covered (Figure 7). On the basis of the small 

 catches, two tongues of relatively high abun- 

 dance were at the two ends of the survey area 

 (Figure 7) . These tongues, lying between lat 5° 

 and 10°N, roughly coincide with the zone of the 

 Countercurrent. 



The distribution of albacore resembled neither 

 that of the yellowfin nor the bigeye tunas. They 

 were most numerous in the southwestern portion 



of the survey area (Figure 8) and their abun- 

 dance declined to the northeast. Additionally, a 

 broad zone to the north appeared to be com- 

 pletely devoid of the species. The reasons for 

 this distribution are not at all clear. 



In addition to the yellowfin tuna, bigeye tuna, 

 and albacore that fit our definition of deep-swim- 

 ming tunas, the longline captured a few skip- 

 jack tuna (Figure 9). The skipjack tuna is a 

 smaller species (rarely over 18 kg) than the 

 longline is designed to capture. Nevertheless, 

 the gear is not absolutely selective because it has 

 taken skipjack tuna as small as 2 kg. 



The sporadic catches of skipjack tuna by long- 

 line are too scanty to estimate relative abundance 

 and are therefore indicated in Figure 9 only as 

 locality records. The skipjack tuna were present 

 over a vast area of the central Pacific. Studies 

 of the distribution of tuna larvae (Matsumoto, 

 1958) suggest that skipjack tuna are more 

 abundant than yellowfin tuna in the central Pa- 

 cific, the implication being that the equatorial 

 central Pacific has a large underutilized stock of 

 skipjack tuna. 



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