FOOD OF YELLOWFLN TUNA 



99 



The maximum size of organisms used for food 

 is dependent on the size of the tuna. Some 

 stomachs were distended by large carangid and 

 scombroid fishes. Certain elongate fishes such as 

 the alepisaurids and snake mackerels were folded 

 two or three times in the stomach. One stomach 

 from a yellowfin (1,358 mm. fork length) contained 

 a skipjack, Katsuwonus pelamis, exceeding 400 

 mm. in length, and vertebral remains found during 

 the study suggest that prey of this size is not 

 uncommon. The maximum size of food organisms, 

 estimated from these data, is approximately 

 one-third the length of the yellowfin. 

 Gorging 



Observers aboard the John R. Manning reported 

 an aggregation of crab megalops in the vicinity of 

 Christmas Island (Line Island Group) on May 12 

 and 13, 1950. The larvae were so numerous that 

 the screened intake of the heat exchanger for the 

 vessel's engines became clogged and had to be 

 cleaned on several occasions. Stomachs were 

 preserved from 21 yellowfin, ranging from 879 to 

 1,365 mm. fork length, captured in the vicinity at 

 this time. All contained crab larvae as the prin- 

 cipal food component, averaging 1,500 larvae 

 (180 cc.) per stomach. Many fish were gorged 

 and regurgitated large amounts of the larvae 

 when they were landed. 



Diurnal variation 



The time of capture is known for 660 yellowfin 

 caught at the surface. Of these, 398 yellowfin 

 landed between 6 a. m. and 12 m. had an average 

 stomach-content volume of 22.1 cubic centimeters; 

 74 of the 398 fish had empty stomachs. The 

 average volume of stomach contents of 258 yellow- 



fin landed between 12 m. and 6 p. m. was 49.7 

 cubic centimeters; of this number, 14 had empty 

 stomachs. These results indicate that the yellow- 

 fin sampled were not necessarily night feeders, as 

 sometimes reported (Ban 1941), but that they 

 had definitely been feeding during the day and 

 probably right up to the time of capture. These 

 data are based primarily on small and medium- 

 sized yellowfin, all surface-caught fish; conse- 

 quently, the implications may possibly not apply 

 to yellowfin in general. 



Variation in volume of stomach contents 



Table 4 shows the distribution of stomach- 

 content volumes according to an arbitrary scale 

 devised by the authors. The large percentage of 

 empty or almost empty stomachs is surprising, 

 but may be related to the rate of digestion or 

 feeding habit. It is difficult to believe that the 

 food volumes of less than 25 cc. which were found 

 in 58 percent of the yellowfin examined represent 

 an average daily ration for these large, fast- 

 moving fish. We conclude that the rate of diges- 

 tion is very rapid or that the fish must depend 

 largely on occasional opportunities to gorge. 



The frequency of empty stomachs and of vol- 

 umes less than 3 cc. appears to vary considerably 

 among the catches obtained by the three fishing 

 methods. The high percentage of empty stomachs 

 among the pole-and-line-caught yellowfin may be 

 the result of these fish coming from surface schools, 

 whereas there are indications that the catch for 

 the other two fishing methods was not from well- 

 defined schools but rather was composed of indi- 

 vidual fish or of fish from small congregations. 

 Also, the different fishing methods, by taking fish 



Table 4. — Distribution of the volume of stomach contents from 1,097 yellowfin tuna, for each fishing method and for 



all methods combined 



' Less than 0.1 cc. 



