80 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



6. Stomatopod crustaceans, common in the food 

 of yellowfin, were completely lacking from the 

 bigeye stomachs. The young of other tunas, 

 mostly skipjack, formed a much more important 

 part of the yellowfin diet than of the bigeye diet. 



7. In both species, the larger tuna had more 

 food in their stomachs than did the smaller fish, 

 but the larger fish contained less food per pound of 

 body weight than did the smaller fish. There were 

 few completely empty stomachs. 



8. In both tunas, the smaller individuals con- 

 sumed a greater proportion by volume of crusta- 

 ceans and fish and a lesser proportion of mollusks 

 than the larger size group. The same fish families 

 were prominent in the diet of botli size groups. 



9. There was an increase in volume of stomach 

 contents with depth of capture for the yellowfin; 

 in the bigeye, the largest volumes were found in 

 specimens from intermediate depths. There was 

 no marked variation in composition of stomach 

 contents over the range of depth sampled (esti- 

 mated at 150 to 500 feet), which may be evidence 

 that both the forage organisms and the tuna range 

 rather freely tlnougliout this water layer. 



10. In both yellowfin and bigeye, fish were con- 

 sumed in greatest amount during the period April- 

 July, and in least amount during August and Sep- 

 tember. There was little correspondence between 

 the two species in the seasonal variation in the 

 other major food items. 



11. In respect to longitudinal variations in the 

 food, the two species were similar in the lower 

 volume of total fish in the stomach contents of 

 those tunas captured in the central region (140°- 

 150° W. longitude) of the sampled area. The 

 utilization of specific foods did not vary with 

 longitude in any regular pattern for the two species. 



12. When classified according to natural subdi- 

 visions of the equatorial current system, the volume 

 of stomach contents in the bigeye varied directly 

 witli the longline catch rate, while in tlie yellowfin 

 there was little change in volume of stomach con- 

 tents with even a marked change in catch rate. 



13. Tuna ttiat were dead when landed con- 

 tained, on the average, more food in their stom- 

 achs than those landed alive. 



14. Despite the difl'erences that we have pointed 

 out, tlie foods of the yellowfin and bigeye are re- 

 markably similar. We conchuh', tlierefore, that 



when occupying the same general area the two 

 species have essentially the same feeding habits. 

 If there is any marked food selection, it must be 

 exercised b\' seeking different areas for feeding. 



LITERATURE CITED 



BERfi, L. S. 



1947. Classification of fishes both recent and fossil. 

 J. VV. Edwards, Ann Arbor, Mich. 517 pp. 

 Brock, V. E. 



1949. A preliminary report on Parathunnus sibi in 

 Hawaiian waters and a key to the tunas and tuna- 

 like fishes of Hawaii. Pacific Science, vol. 3, No. 3, 

 pp. 271-277. 

 Cromwell, Townsenu 



1951. Mid-Pacific oceanography, January through 

 March 1950. U. S. Fish and Wildlife Service, Spec. 

 Sci. Rept. — Fisheries No. 54. 77 pp. 



1953. Circulation in a meridional plane in the central 

 equatorial Pacific. Journal of Marine Research, vol. 

 12, No. 2, pp. 196-213. 



Kanagawa Prefectire Fisheries Experiment Station 



1951. Report of South Seas tuna fishery investigations. 



162 pp. (Partial tran.slation from the Japanese by 



W. G. Van Campen in the files of Pacific Oceanic 



Fishery Investigations.) 



King, J. E. 



1954. Variations in zooplankton abundance in the cen- 

 tral equatorial Pacific, 1950-52. Fifth Meeting, Indo- 

 Pacific Fisheries Council, Symposium on marine and 

 fresh-water plankton in the Indo- Pacific, pp. 10-17. 



KiNfj, J. E., AND Joan Demond 



1953. Zooplankton abundance in the central Pacific. 

 U. S. Fish and Wildlife Service, Fishery Bull. 54, 

 vol. 82, pp. 111-144. 



MlRPHY, G. I., AND R. S. ShOMURA 



1953a. Longline fishing for deep-swimming tunas in 

 the central Pacific, 1950-51. U. S. Fish and Wildlife 

 Service, Spec. Sci. Rept. — Fisheries No. 98. 47 pp. 



1953b. Longline fishing for deep-swimming tunas in 

 the central Pacific, January-June. 1952. U. S. Fish 

 and VV'ildlife Service, Spec. Sci. Rept. — Fisheries No. 

 108, 32 pp. 



1955. Longline fishing for deep-swimming tunas in the 

 central Pacific, August-November 1952. U. S. Fish 

 and Wildlife Service, Spec. Sci. Rept. — Fisheries No. 

 137. 42 pp. 



Nakamira, Hiroshi 



1949. The tunas and their fisheries. Tokyo: Takeuchi 

 Shobo. Tran.slated from the Japanese by W. G. Van 

 Campen, U. S. Fish and Wildlife Service, Spec. Sci. 

 Rept.— Fisheries No. 82, 1952. 115 pp. 



NisKA, E. L. 



1953. Construction details of tuna longline gear used 

 by Pacific Oceanic Fishery Investigations. U. S. Fish 

 and Wildlife Service, Commercial Fisheries Review, 

 vol. 15, No. 6, pp. 1-6. 



