but does not occur regularly in the possibly 

 more epipelagic range of the tuna. Another 

 possibility, of course, is that it is fed on selec- 

 tively by the porpoise and not by the tuna. 



Onychoteuthidae 



Onykia sp. was not found in the stomachs of 

 Thunnus and has not been taken by dip net at 

 the surface in the area. However, it was found 

 in the stomachs of the two species of porpoise 

 and may be a mesopelagic species. Other mem- 

 bers of the family, OnycJioteuthis sp. for ex- 

 ample, are regularly taken at or near the sur- 

 face at night. 



SUMMARY 



While there is great overlap among the diets 

 of the two porpoises and the tuna in mixed ag- 

 gregations, there is evidence of specialization 

 in prey items, time of day at which feeding 

 occurs, and possibly in maximum feeding depth. 

 Although the three species are intimately as- 

 sociated in tight mixed groups, they do not 

 necessarily feed on the same items at the same 

 time in the same place. If the association is 

 food-based, however, the epipelagic ommastre- 

 phid squids seem to be the most probable can- 

 didates for a binding common factor in this 

 region. The apparent trophic relationships 

 among the three species can be crudely sum- 

 marized in terms of taxa and depth distribution 

 of the major prey items (Figure 2). 



YELLOWFIN TUNfl 



SPOTTED PORPOISE 



SPINNER PORPOISE 



Figure 2. — Summary of apparent relative feeding habits 

 of yellowfin tuna, spotted porpoise, and spinner porpoise 

 in mixed aggregations in the eastern tropical Pacific, based 

 on stomach-content data presented here and in Fitch and 

 Brownell (1968) and Alverson (1963). Widths of lines 

 crudely approximate relative importances of the major 

 food categories. 



FISHERY BULLETIN: VOL. 71, NO. 4 



ACKNOWLEDGMENTS 



The senior author's wife, Gerlind, spent count- 

 less hours rough-sorting thousands of otoliths. 

 John E. Fitch identified the otoliths, using his 

 extensive reference collection to make direct 

 comparisons in most cases. Clyde F. E. Roper 

 provided much help in identifying squids. 

 Richard H. Rosenblatt furnished information 

 on fish-depth distribution. The captains and 

 crews of the tuna seiners MV Carol Virguiia 

 (now Carol S.) and MV Pacific Queen made 

 possible collections of the stomachs. Craig J. 

 Orange collected the Pacific Queen specimens. 

 The Inter-American Tropical Tuna Commis- 

 sion furnished measurements of a sample of 

 the Carol Virginia tuna catch. Carl L. Hubbs 

 and Richard H. Rosenblatt read the manuscript. 

 We thank these persons and others who assisted 

 us in the project. 



LITERATURE CITED 



Alverson, F. G. 



1963. The food of yellowfin and skipjack tunas in 

 the eastern tropical Pacific Ocean. [In Engl, and 

 Span.] Inter-Am. Trop. Tuna Comm., Bull. 7;293- 

 396. 

 Clarke, M. R. 



1962. The identification of cephalopod "beaks" and 



the relationship between beak size and total body 



weight. Bull. Br. Mus. (Nat. Hist.) Zool. 8:419-480. 



1966. A review of the systematics and ecology of 



oceanic squids. Adv. Mar. Biol. 4:91-300. 



Ebeling, a. W. 



1962. Melamphaidae. I. Systematics and zooge- 

 ography of the species in the bathypelagic fish genus 

 Melamphaes Giinther. Dana Rep. Carlsberg 

 Found. 58, 164 p. 



Ebeling, A. W., and W. H. Weed III. 



1963. Melamphaidae. III. Systematics and dis- 

 tribution of the species in the bathypelagic fish 

 genus Scopelogadits Vaillant. Dana Rep. Carls- 

 berg Found. 60, 58 p. 



Fitch, J. E., and R. L. Brownell, Jr. 



1968. Fish otoliths in cetacean stomachs and their 

 importance in interpreting feeding habits. J. Fish. 

 Res. Board Can. 25:2561-2574. 



Gibes, R. H., Jr., and C. F. E. Roper. 



1972. Ocean acre. Prehminary report on vertical 

 distribution of fishes and cephalopods. In C. B. 

 Farquhar (editor). Proceedings of an international 

 symposium on biological sound scattering in the 

 ocean, p. 1 19-133. U.S. Navy, Wash., D.C. 



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