108 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



fish taken near land (table 7). Also, fish from the 

 subsurface contained as much food as those from 

 the surface. 



Although we believe this report provides the 

 most complete information available on the food 

 of the yellowfin, the more important observations 

 in other areas (Kishinouye (1917, 1923) for the 

 Bonin Islands and general western Pacific; 

 Nakamura (1936) for the Celebes Sea; Marukawa 

 (1939) and Ban (1941) for the equatorial western 

 Pacific; Kanamura and Yazaki (1940a, 1940b) and 

 Ronquillo (1950) for the Philippines; Herald 

 (1949) and Welsh (1949) for the eastern Pacific 

 and Hawaiian Islands) and those reported here are 

 in general agreement on the chief constituents of 

 the food of the yellowfin. The number and kind 

 of individual species listed may vary with the 

 locality and the extensiveness of the work done. 

 A summary of the species taken shows that yel- 

 lowfin have an extremely varied diet and feed to 

 a large extent not only on reef fauna but also on 

 organisms of the open ocean, both plankton and 

 nekton. Since most pelagic crustaceans, molluscs, 

 and fish, within rather broad size limits, appear to 

 be acceptable as food, the distribution and abun- 

 dance of maturing and adult yellowfin are most 

 probably not determined by the occurrence of 

 specific food items, but rather are influenced by 

 the total amount of food present in an area. 



The food of juvenile yellowfin is yet to be in- 

 vestigated. Whereas the adults accept a wide 

 variety of food, the juveniles may be more specific 

 in their feeding habits. In such a case, the abun- 

 dance of a particular food or foods could have an 

 important influence on the survival and growth 

 of the young. Few yellowfin less than 400 

 mm. (about 2.8 lb.) are taken by present fishing 

 methods. 



Also there remains the problem of evaluating 

 the abundance of tuna food organisms in different 

 areas and at different depths to determine which 

 regions of the Pacific may potentially support the 

 greatest populations of tuna. This phase of the 

 problem, however, awaits the development of 

 effective gear for quantitative sampling. 



SUMMARY 



1. This study is based on the quantitative anal- 

 ysis of the stomach contents of 1,097 

 yellowfin tuna (Neothunnus macropterus) 

 taken in the central Pacific. 



2. The fish were captured by three fishing 



methods, trolling, pole-and-line, and long- 

 line, came from varying habitats, onshore 

 and offshore, surface and subsurface, and 

 were of different size groups. 



3. The 1,097 stomachs contained a total volume 



of 52,336.1 cc. of food, of which 47 percent 

 by volume was fish, 26 percent squid, and 25 

 percent crustaceans. 



4. Yellowfin appear to accept a great variety of 



animal food, and take advantage of what- 

 ever is most plentiful in the area at the time. 



5. They feed on very small plankton organisms 



as well as large squid and fish one-third 

 the length of the tuna. 



6. Yellowfin captured in the afternoon by troll- 



ing and live-bait fishing had more food in 

 their stomachs and fewer empty stomachs 

 than those captured in the morning; 

 therefore, feeding must take place during 

 daylight hours. 



7. The yellowfin taken at the surface had been 



feeding primarily on crustacean larvae 

 and fish, the fish taken beneath the surface 

 on fish and squid. 



8. The yellowfin taken near land had fed on 



fish, crustacean larvae, and squid; those 

 caught away from land on fish and squid. 



9. Small yellowfin fed preponderantly on crusta- 



cean larvae; medium-sized yellowfin fed 

 on fish, crustacean larvae, and squid ; while 

 the large yellowfin fed mainly on fish and 

 squid. 



10. Yellowfin from offshore areas contained as 



much food in their stomachs as those 

 captured just off the reef, and they fed 

 at subsurface levels as well as at the surface. 



1 1 . The average volume of stomach contents was 



(cruise 11, Hugh M. Smith) roughly pro- 

 portional to the concentration of zoo- 

 plankton in the area in which the fish were 

 captured, i. e., the yellowfin tuna captured 

 in the rich zone near the Equator con- 

 tained greater amounts of food — zooplank- 

 ton, forage fish, and squid — than those 

 captured at more northerly or southerly 

 latitudes. 



12. Further research is needed on the food of 



juvenile yellowfin; on the abundance of 

 food organisms in different areas of the 

 Pacific and at different seasons to determine 



