66 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



RESULTS 



The food of both the yellowfin and the bigeye 

 was primarily fish, of great variety, and squid 

 (table 11, Appendix). Other mollusks, such as 

 the argonauts and octopods, and crustaceans 

 were of minor importance.^ Figure 5 ilhistrates 

 the percentage of occurrence of the major food 

 items. Figure 6 shows the percentage of aggre- 

 gate total volume of each major food element, 

 which indicates its relative importance by bulk. 



Representatives of 48 fisli families and 1 1 

 invertebrate orders were found among the stomach 

 contents of the yellowfin, as compared with 36 

 fish families and 9 invertebrate orders for the big- 

 eye* Despite this great variety in the food, only 



OTHER 

 FOOD 



Figure 5. — Percentage of occurrence of the major food 

 elements. 



s Among the results of this study, not referred to elsewhere in the report 

 but perhaps worthy of mention, were observations on the number of stomach 

 parasites. Among the bigeye, 26 percent of the stomachs examined were 

 Infested with nematodes and 32 percent with trematodes. The infestation 

 was somewhat less among the yellowfin, being 16 percent for nematodes and 

 26 percent for trematodes. 



* The greater variety in the food of the yellowfin as compared with the big- 

 eye is due, we believe, simply to the fact that more than twice as many yellow- 

 fin stomachs were examined. 



a few items were of primary importance to either 

 species. For both the yellowfin and the bigeye, 

 those food elements ranking high in number, 

 volume, and frequency of occurrence were squid, of 

 the families Ommastrephidae and Loliginidae, and 

 among the fish the pomfret {CoUyhus drachme) and 

 snake mackerel {Gempylus serpens) were important. 

 Certain fishes, such as the tunas (Thunnidae) and 

 the sun fishes (Molidae), were relatively important 

 in volume but ranked low in number and frequency 

 of occurrence, indicating that they are only occa- 

 sionally utilized. Crustacea of the order Stomato- 

 poda, prominant in number in the food of yellow- 

 fin, were completely lacking from the bigeye 

 stomachs. The young of other tunas, especially 

 skipjack, formed a much more important part of 

 the yellowfin diet than that of the bigeye. In the 

 following sections of this report we shall try to 

 describe the major differences and similarities in 

 the foods of these two species of tuna as related to 

 such factors as size of the tuna, area and depth of 

 capture, season, and features of the equatorial- 

 current system. 



Variation in Food with Size of Tuna 



In general, for both yellowfin and bigeye, there 

 was an increase in food volume per stomach with 

 an increase in size of the tuna (fig. 4). With the 

 hope of minimizing the effects of this factor, in 

 our examination of differences in the food specif- 

 ically related to size of tuna we have split the 

 data for both species into two size groups, (1) 

 those less than 140 cm.^ and (2) those 140 cm. and 

 over, in fork length (table 2). This provided for 

 each species two groups of fish roughly equal in 

 number. In the yellowfin the larger size gfoup 

 contained 29 percent more food per stomach, and 

 in the bigeye it contained 16 percent more. The 

 ratios of stomach content to body weight are 

 almost identical for the two species (table 2). 

 Although Crustacea make up a very small per- 

 centage of the food of these large, deep-swimming 

 fish, in both species tlie smaller specimens con- 

 sumed greater amounts of such organisms as 

 crab larvae, shrimp, and ampliipods. In both 

 species, the larger specimens consumed less fish 

 and more mollusks — as percentage of total 

 volume — than did the smaller size group; this 

 was particularly true for the bigeye. The per- 



' A 140-cm. yellowfin from the equatorial Pacific weighs approximately 

 118 pounds, while a 140-cm. bigeye weighs approximately 127 pounds. 



