STOMACH CONTENTS OF PORPOISE, STENELLA 



SPP., AND YELLOWFIN TUNA, THUNNUS ALBACARES, 



IN MIXED-SPECIES AGGREGATIONS 



W. F. Perrin,' R. R. Warner,- C. H. Fiscus,^ and D. B. Holts' 



ABSTRACT 



Haul-by-haul analysis of stomach contents of spotted porpoise iSienella aiieniiuta), spinner 

 porjxjise (S. lonsiirosiris), and yellowfin tuna (Thunnus albacares) from six net hauls made 

 on mixed-species aggregations in 1968 indicates that an ommastrephid squid (probably 

 Dosidicus fiifius) was the most important food item in terms of co-occurrences in the three 

 species and in terms of the volume and number. The ommastrephid occurred in significant 

 numbers and/or volume in each haul in all the species examined, albeit to a lesser degree 

 in the spinner porpoise than in the other two species. Overlap of elements in the stomach 

 contents was greatest between the tuna and the spotted porpoise. In addition to the om- 

 mastrephid, the small scombrid Auxis sp. and epipelagic exocoetids such as Oxypoilutmplms 

 micropterits were important to both. The portunid crab Eiiphylux duvii was very important 

 in the tuna but absent in both species of porpoise. Onychoteuthid and enoploteuthid squids 

 were important in some hauls in both species of porpoise but were all but absent from the 

 tuna. Small mesopelagic fishes, mainly myctophids and gonostomatids, were important in 

 the spinner porpoise, but not in the spotted porpoise or the tuna. Relative frequencies of 

 empty stomachs and state of digestion of stomach contents indicated that the spinner por- 

 poise does not feed at the same time as the spotted porpoise and tuna. In spite of overlap 

 among the three species in nearly all of the food components, these results suggest that the 

 tuna and spotted porpoise feed together largely on epipelagic prey, whereas the spinner 

 porpoise for the most part feeds deeper and at different times of day; furthermore, only the 

 tuna eats crustaceans. 



Small pelagic delphinids {Ste)ieUa attenuata, 

 S. longirosins, and Delphiniis delphis) in the 

 eastern tropical Pacific are commonly en- 

 countered with yellowfin tuna {Thunnus alba- 

 cares) in large mixed-species aggregations. The 

 association between the fish and the cetaceans 

 is very tight (Perrin, 1969, 1970). Although the 

 reason for the association is unknown, the possi- 

 bility that it is food-based immediately suggests 

 itself. Alverson (1963) examined the stomach 

 contents of 2,846 yellowfin tuna from the eastern 

 tropical Pacific and found the major components 

 to be fish (46.9% of total volume) and crusta- 

 ceans (45.4% ). Cephalopods accounted for only 



' Southwest Fisheries Center, National Marine Fisheries 

 Service, NOAA, La Jolla, CA 92037. 



- Scripps Institution of Oceanography, La Jolla, CA 

 98037. 



3 Marine Mammal Division, Northwest Fisheries Center, 

 National Marine Fisheries Service, NOAA, Seattle, WA 

 98115. 



7.6% of the volume. He encountered a wide 

 variety of food items and changes in species 

 composition from area to area. He therefore con- 

 cluded that yellowfin tuna are nonselective 

 feeders, foraging on whatever pelagic or benthic 

 organisms of suitable size are locally available. 

 Fitch and Brownell (1968) reported on fish 

 identified from otoliths taken from spotted por- 

 poise, Stenella graffmani (= attenuataY and 

 spinner porpoise (S. longirostris) caught in 



^ The taxonomy of the spotted porpoises is confused. 

 Recent morphological studies (Perrin, 1972) have shown 

 that the spotted porpoise in the eastern Pacific is con- 

 specific with the spotted porpoise that occurs around 

 Hawaii. Accordingly, the name 5. citteniiatci (Gray, 1846), 

 which was applied by True (1906) to the Hawaiian form, 

 is used here for the eastern Pacific form. This name has 

 priority by date over 5. graffmani (Lonnberg, 1934). This 

 usage is provisional, pending the outcome of current 

 studies comparing forms in the Atlantic and Pacific. S. 

 aiteiuiata may be a junior synonym of 5. frontalis (Cuvier, 

 1829), which was described from the tropical North 

 Atlantic. 



Manuscript accepted May 1973 



FISHERY BULLETIN: VOL. 71, NO. 4, 1973. 



1077 



