FOOD OF BIGEYE AXD YELLOWFIX TUNA 



75 



BIGEYE 



I YELLOWFIN 



70 



60 



o 

 o 



I 



o 

 g 



n: 

 ill 

 a. 



I 

 o 



50 - 



40 



30 - 



20 





 220 



200 



180 



160 



140 



120 



100 



80 



60 



40 



20 





 I 4 



X 



O I 2 



T 



T" 



(84) 



(52) 



1 



(44) 



I 



(24) 



(58) 



(47) 





(164) 



I 



I 



(86) 



m 

 t 



m. 



(3) 



(58) 



(88) 



(23) 



(4 7) (164) 



(98) 



VA 



(14) ,^ 



J 



1 ;>^ 



 ^^ 



i 



^ 



5 



SEC 



4 



DIV 



CONV 

 AREA 



2 



CC 



FinuRE 11. — Variations with the current system in (A) 

 yellowfin and bigeye catch on longline gear, (B) average 

 volunie of food per stomach, and (C) average volume of 

 food per pound of body weight. Boundaries for each di- 

 vision of the current .system are defined in the text. Part 

 A i.s derived from cruises 7, 11, and 18 of the Iltiiih M. 

 Smith, cruises 11, 12, 13, 14, and 15 of the John If. Mann- 

 ing, cruise 1 of the Charle.<; II. Gilbert, and cruise 1 of 

 the Cavalieri. Xumberof ob.servations, as stations fished 

 (part A) or .stomachs examined (parts B and ('), is 

 shown in parentheses. 



volume of stomach contents.' The divergence 

 zone at tlie Equator produced good catches of 

 yellowfin, but these fish contained the lowest food 

 volumes. On the basis of both the average volume 

 of food per stomach and the average volume of 

 food per pound of l)ody weiglit — disregarding the 

 three stomachs collected from the NEC — we 

 judge that the yellowfin captured in areas 2, 3, 

 and 5 were equally well fed. In the bigeye, there 

 is a suggestion of parallel variation in catch rate 

 and volume of stomach contents. This species 

 was tlie best fed in areas 1 and 2, which were also 

 the areas of best catch. The bigeye from near the 

 Equator (area 4), where catches were poorest, con- 

 tained the lowest food volumes. 



Table 7 illustrates variations in certain food 

 components as related to the system of currents. 

 The consumption of Crustacea by yellowfin is 

 rougldy in accordance with the varying abundance 

 of zooplankton as determined from oiu' plankton 

 surveys (King and Demond 1953, King 1954). 

 Tiieir utilization l)y l)igeye is quite different, 

 iiowever, and may be related to differences in the 

 kinds of organisms involved. In tlie food of 

 yellowfin, for example, the crustacean fraction 

 was principally amphi|)ods, with isopods and crab 

 larvae of some importance; the bigeye had fed 

 chiefly on shrimp and euphausids. The complex 

 variations in the consumption of squid, Bramidae 

 (chiefly Collijbu-y drachme), Gempylidae (chiefly 

 Gempylus serpens), and total fish are difficult to 

 understand, since we lack information on the lati- 

 tudinal variations in abundance of these forage 

 organisms. 



We should like ne.xt to exan)ine in greater detail 

 the differences between the CC (urea 2) and the 

 convergent zone (area 3) with respect to volume 

 and composition of food utilized as related to deptli 

 of capture of the tinias. As previously stated, 

 the CC is a region of relatively good catch for 

 l)igeye and of poor catch for yellowfin. Bigeye 

 from this region contained about 50 percent more 

 food in their stomachs than did the yellowfin, 

 but they averaged somewhat larger in body size. 



• It was previously reported (Reiiiljes :iiicl Kiiiix I9.i:t) thai on one eniisc 

 (eruise n, Hugh \f. Smith) there was some iiuliealion that for yellowfin the 

 average volume of stomaeh contents varied directly with the ("ateli rate. 



