Lowry et al Food habits of Zslophus cahfornianus at San Clemente Island, California 519 



dicated seasonal and yearly variability in the diet of SCI 

 sea lions. Northern anchovy apparently was the major 

 food source because it was represented by high index 

 values and consistently occurred in the sea lions' diet 

 during the 5-year period. Pelagic red crab, jack mack- 

 erel, Pacific whiting, rockfish, market squid, black- 

 smith, and Pacific mackerel appear to supplement the 

 diet of California sea lions. 



A major uncertainty with using scat analysis to study 

 pinniped food habits is how these results reflect what 

 was initially consumed by the animals. Studies on cap- 

 tive pinnipeds indicate that recovery rates are (1) lower 

 than ingestion rates for all prey; (2) lower for prey 

 species with small otoliths than prey with large otoliths; 

 and (3) lower for young (i.e., small) prey than for older 

 (i.e., large) prey of the same species (Hawes 1983, 

 da Silva and Neilson 1985, Murie and Lavigne 1986, 

 Bellinger and Trillmich 1988, Harvey 1989). Propor- 

 tions of different prey species in scats were found to 

 correspond to proportions of prey fed (Bellinger and 

 Trillmich 1988). Pitcher (1980) found no difference 

 between occurrence of fish in stomachs and feces of 

 harbor seals Phoca vitulina, but cephalopods were 

 under-represented in their feces. 



The prey that occurred in 10% or more of scat sam- 

 ples ranged from small-sized (i.e., northern anchovy, 

 market squid, and pelagic red crab) to prey that, when 

 mature, could be potentially large-sized (i.e.. Pacific 

 whiting, jack mackerel, rockfish, and Pacific mackerel). 

 Northern anchovy, being small-sized with small oto- 

 liths, would probably represent a higher proportion of 

 the diet than indicated in scat samples. Market squid 

 would also represent a higher proportion of the diet 

 because beaks are often regurgitated and would be 

 underrepresented in scats. 



Results from SCI indicated differences in both prey 

 composition and temporal variability from those 

 described by Bailey and Ainley (1982) for sea lions at 

 the Farallon Islands off central California. We found 

 that several prey were consistently consumed (e.g., 

 northern anchovy, jack mackerel. Pacific whiting, 

 rockfish, market squid, blacksmith, and Pacific mack- 

 erel), whereas they reported only two prey (Pacific 

 whiting and rockfish) that were regularly eaten. They 

 found Pacific whiting to occur in the sea lions' diet dur- 

 ing spring and summer, and juvenile rockfish during 

 the fall and winter. At SCI we found that the presence 

 of various types of prey in the diet of sea lions fluc- 

 tuated year-to-year and sometimes seasonally within 

 the same year. These differences between two island 

 areas may be due to differences in the sampling periods 

 (1981-86 for our study; 1974-78 for the Farallon Island 

 study), to differences in prey availability in each area, 

 or to differences in the sexual composition of animals 

 inhabiting each area. 



We would expect similar diets for sea lions using 

 islands within close proximity of each other. Bata from 

 a study of food habits on San Miguel Island (SMI) dur- 

 ing spring and summer in 1978-79 by Antonelis et al. 

 (1984) were compared with our results. Four types of 

 prey taxa (Pacific whiting, rockfish, northern anchovy, 

 and market squid) were consistently found in scats of 

 SCI and SMI sea lions. Jack mackerel, consistently 

 found at SCI, was a minor prey item at SMI. The 

 presence of pelagic red crab at SCI was due to the 

 California El Nino, which did not occur during the SMI 

 study. Blacksmith were not found in the diet of sea lions 

 at San Miguel Island, but occurred frequently in the 

 SCI study. Because the distribution and availability of 

 these prey differ over time, we are unable to test our 

 expected similarities in prey consumption until concur- 

 rent studies are conducted. 



Availability and abundance of sea lion prey have been 

 demonstrated by Bailey and Ainley (1982) and An- 

 tonelis et al. (1984) to influence diet of California sea 

 lions. Evidence that abundance, if not availability, of 

 prey influences consumption is also found from our 

 study at SCI when we compare anchovy occurrence in 

 scats with anchovy biomass estimates for each year 

 (Bindman 1986, Methot and Lo 1987). The decline in 

 the occurrence of anchovy in the scats during 1982 and 

 1984 coincided with lower estimates of anchovy bio- 

 mass of 415,000 mt for 1982 and 309,000 mt for 1984 

 (Bindman 1986). Increased occurrence of anchovy in 

 scats during 1983, 1985, and 1986 coincided with 

 greater estimates of anchovy biomass: 623,000 and 

 522,000 mt (Bindman 1986), and 764,000 mt (Methot 

 and Lo 1987), respectively. Becline in anchovy biomass 

 may have caused SCI sea lions to consume other species 

 in 1982 and 1984, resulting in increased occurrence in 

 the latter. 



There was more variability in occurrence of different 

 prey taxa per scat in 1982 and 1984 when the estimated 

 population biomass of anchovy was low, and less vari- 

 ability in 1983 and 1985 when there was a greater 

 biomass. The diet of SCI sea lions, therefore, may be 

 strongly affected by the abundance of northern an- 

 chovy. As anchovy becomes scarce, we expect SCI sea 

 lions to consume other types of prey and exhibit greater 

 variability in the number of different prey consumed. 



We expected the non-El Nino periods to have similar 

 occurrences of prey taxa consumed, which would dif- 

 fer from the El Nino distribution. In fact, all three 

 periods exhibited significantly different distributions 

 of prey taxa and in their occurrence. 



The pattern in the commercial harvest of market 

 squid (which is not affected by regulations and quotas) 

 observed in California before, during, and after El Nino 

 (Worcester 1987) closely resembled the pattern ob- 

 served in the occurrence of this prey in the diet of SCI 



