weight/d for dolphin and 10-127r for porpoise. The 

 species fed to these captive animals were not iden- 

 tified. 



Size of Prey 



Higgins (1919) mentioned a stomach containing 

 six large Pacific sardine, Sardinops sagax, each 

 about 30 cm long. Houck (1961) reported a dolphin 

 with a stomach full of Pacific saury and with a 33 

 cm jack mackerel, Trachurus symmetricus, 

 wedged in its throat. Unfortunately, no count of 

 the Pacific saury was given. Fitch and Brownell 

 (1968) reported that otoliths representing 29 

 Pacific whiting 40-50 cm long and 14 Pacific whit- 

 ing about 20 cm long were recovered from a dol- 

 phin stomach. The sizes offish in stomach contents 

 were measured only from whole or nearly whole 

 specimens or those with complete vertebral col- 

 umns (Table 2). All salmon consumed by dolphin 

 were generally immature, showing 0-age ocean 

 growth, although a few showed 1, 2, and 3 ocean 

 annuli. 



Scheffer (1953) reported on stomach contents of 

 two Ball's porpoise taken off Oregon, each of which 

 contained four Pacific whiting about 45 cm long. 

 These records represent the largest fish recovered 

 from porpoise stomachs. Mizue et al. (1966) re- 

 ported only one occurrence of sockeye salmon, O. 

 nerka, from stomachs of 148 porpoise taken in con- 

 junction with the high-seas salmon gill net fishery. 

 No mention was made of the size of this fish, al- 

 though they did state that adult salmon are prob- 

 ably not taken by animals of this species. 



In our collections, there was no evidence of large 

 fish being broken up prior to ingestion by either 

 dolphin or porpoise. Captive bottlenose dolphin, 

 Tursiops truncatus, have been observed to break 

 up food species."* The teeth, jaw structure, and 

 relative neck mobility of the white-sided dolphin 

 are similar to those of the bottlenose dolphin and 

 would allow such behavior in this species more so 

 than in the Ball's porpoise. The maximum size of 

 prey eaten is apparently limited by the predator's 

 ability to capture and swallow whole fish. 



The size of prey listed in Table 2 does not neces- 

 sarily indicate that these fish are the largest con- 

 sumed by the seal. Seal generally swallow smaller 

 fish and squid whole below the surface whereas 

 larger fish are brought to the surface and broken 



••William Gilmartin, Naval Ocean Systems, San Diego, Calif., 

 pers. commun. 1978. 



into smaller pieces by grasping them with their 

 teeth and shaking them violently from side to side. 

 The largest fish we have seen taken by a seal was a 

 king-of-the-salmon (length 170 cm) which we took 

 away from the animal as it attempted to break the 

 fish into smaller pieces at the surface. 



Conclusions 



The Pacific white-sided dolphin and the Ball's 

 porpoise feed primarily on small schooling fishes 

 and cephalopods. They, like the northern fur seal, 

 are opportunistic feeders, preying on available 

 species, including some that are commercially im- 

 portant such as salmon, anchovy, jack mackerel, 

 and Loligo opalescens. Meaningful estimates of 

 the dolphin and porpoise populations are unavail- 

 able, and too few stomachs have been examined to 

 make any estimate of the percentage of commer- 

 cially important fishes included in the diet. 



Regardless of the time of day collected, stomachs 

 may contain undigested fish indicative of recent 

 feeding. Based on stomach content volume and 

 time of collection, large stomach volumes were 

 most often observed in animals collected before 

 1000 h in the morning, indicating that most feed- 

 ing is done at night or in the morning. 



Northern fur seal tend to congregate in areas of 

 abundant food supply and usually feed at night, 

 probably because most prey species rise toward 

 the surface after dark and are more readily avail- 

 able (Fiscus et al. 1964). Food species consumed by 

 the seal vary by area, but the important food in the 

 diet of this mammal in a given area, based on 

 percentage of stomach content volume, generally 

 does not change — only ranking by volume 

 changes. The animals collected on the continental 

 shelf appear to feed on fishes, whereas those taken 

 beyond the shelf feed primarily on squids. 



Acknowledgments 



The authors wish to thank John Fitch, Califor- 

 nia Bepartment of Fish and Game, for identifica- 

 tion of fish otoliths from the stomachs of two dol- 

 phins; Eric Hochberg, Curator of Invertebrate 

 Zoology, Santa Barbara Museum of Natural His- 

 tory, for identification of the pelagic octopus, 

 Ocythoe tuberculata, beaks; and Bale Rice, North- 

 west and Alaska Fisheries Center, NMFS, NOAA, 

 for his review of the manuscript. 



957 



