Unidentified fishes and six species of squids were 

 also found in trace amounts. The stomachs of three 

 porpoise taken 56 km southwest of Pt. Reyes on 21 

 February 1966 contained no northern anchovy but 

 did contain typical open ocean dwelling fishes and 

 squids. Off Washington trace amounts of 

 eulachon, Thaleichthys pacificus., rockfish, 

 Sebastes spp.; sablefish, Anoplopoma fimbria; 

 flatfish, Pleuronectidae; American shad, Alosa 

 sapidissima; capelin, Mallotus villosus; and 

 squids, L. opalescens , Gonatidae, Gonatus sp., and 

 0. borealijaponicus , were found in stomachs of 

 three porpoise. 



Discussion 



Distribution of Prev 



Based on identified prey species, it appears that 

 all three of the mammals feed in the epipelagic 

 (0-200 m) and mesopelagic (200-1,000 m) zones of 

 the ocean and that over the continental shelf, they 

 may descend to the bottom (200 m or less) on occa- 

 sion as demonstrated by the presence of demersal 

 species in their stomachs. Many of the fishes and 

 squids rise to or toward the surface at night, 

 thereby becoming more readily available and 

 perhaps ruling out the necessity for long, deep 

 dives. Kooyman et al. (1976), reporting on fiar seal 

 diving behavior, indicated that dives between the 

 surface and 20 m lasting <1 min may be for shal- 

 low feeding or travel and that dives between 20 

 and 140 m of 3.3-3.4 min duration may be hunting 

 and feeding dives. They reported dives deeper than 

 140 m; the deepest reported dive lasted 5.4 min and 

 reached 190 m. A study of blood oxygen levels of 

 three genera of porpoise by Ridgway and Johnston 

 (1966) reported that the Pacific white-sided dol- 

 phin cannot swim as fast and probably cannot dive 

 as deep as the Ball's porpoise. 



Prey Species 



Off the California coast all three of the mam- 

 mals feed primarily on small schooling fishes and 

 cephalopods, including the northern anchovy, 

 Pacific saury, and Pacific whiting. Other species of 

 fish were probably taken as the opportunity arose. 

 The primary fish species in dolphin collected off 

 the Washington coast were salmonids (genus On- 

 corhynchus). Because the latter collection was 

 made in a small area over a 3-d period, the taking 

 of salmonids may have been opportunistic and 



short term rather than typical of more routine 

 feeding. This sample is too small to conclude that a 

 major predator-prey relationship exists between 

 the Pacific white-sided dolphin and the salmonids. 



Cephalopods are probably more important as 

 prey species than indicated by the relative volume 

 of stomach contents in the collection. Except for 

 the chitinous beaks, cephalopods are probably 

 more rapidly digested than fish. Marine mammals 

 are more likely to feed on squids during the night 

 because the vertical migration of many species 

 brings them closer to the surface waters (Roper 

 and Young 1975; Pearcy et al. 1977). Collection of 

 dolphin during the day would give adequate time 

 for digestion of fleshy parts, thus leaving the large 

 numbers of indigestible chitinous beaks often 

 found in stomachs. 



The fishes and squids identified in the stomachs 

 of the dolphin, porpoise, and seal taken during this 

 study are presented in Table 2. 



Stomach Capacity of Predators 



The 44 Pacific white-sided dolphin were all 

 adult or subadult animals, presumably with 

 stomachs of maximum size. Eleven stomachs con- 

 tained only trace amounts of food, and 33 con- 

 tained food contents varying from 10 to 3,490 cm^ 

 (10-3,745 g). 



The dolphin whose stomach contained the most 

 food from California waters had eaten 68 anchovy 

 (1,770 cm^ or 1,895 g). Anchovy grow to 18-20 cm 

 and may weight 57 g (Fitch and Lavenberg 1971). 



Off Washington, the largest dolphin stomach 

 examined contained the remains of nine salmon 

 (including identifiable remains of four coho salm- 

 on and two pink salmon) which measured 26.0, 

 27.0, 31.0, 31.5, 31.5, 33.0, 33.0, 33.0, and 33.5 cm. 

 The full stomach measured 40 cm long and 24 cm 

 at the widest point (outside stomach dimensions), 

 and the stomach contents represented 4.4% (3,490 

 cm^ or 3,745 g) of the total body weight of the 

 animal. 



The largest stomach content from a Ball's por- 

 poise off California contained 58 northern anchovy 

 (1,000 cm3 or 1,090 g). Off Washington, the largest 

 stomach content contained fragments of five cape- 

 lin, four eulachon, one flatfish (family Pleuronec- 

 tidae), and trace amounts of squids (Gonatus sp. 

 and Gonatidae) (130 cm^ or 125 g). Five stomachs 

 contained food volumes varying from 5 to 1,000 

 cm^ (5-1,090 g) whereas four stomachs contained 

 only trace amounts of food. Except for occasional 



955 



