Seasonal and Annual Variability 

 in the Diet of California Sea Lions 

 Zalophus californianus at San Nicolas 

 Island, California, 1981-86 



Mark S. Lowry 



Southwest Fisheries Science Center, National Marine Fisheries Service. NOAA 

 P.O. Box 271. La Jolla. California 92038 



Brent S. Stewart 



Sea World Research Institute. Hubbs Marine Research Center 

 1700 South Shores Road. San Diego. California 92109 



Carolyn B. Heath 



Fullerton College, 321 E. Chapman Avenue, Fullerton. California 92634 



Pamela K. Yochem 



Sea World Research Institute, Hubbs Marine Research Center 

 1700 South Shores Road, San Diego, California 92109 



John A/I. Francis 



Smithsonian Institution, National Zoological Park, Washington, D.C. 20008 



California sea lions haul out and 

 breed on islands along the Pacific 

 coasts of southern and Baja Califor- 

 nia and in the Gulf of California. 

 About half of all births occur in 

 southern California waters (Fig. 1), 

 principally at San Miguel and San 

 Nicolas Islands (Stewart et al. In 

 press). Some studies have shown 

 that diet varies geographically, per- 

 haps influenced by seasonal vari- 

 ability of various prey (Fiscus and 

 Baines 1966, Ainley et al. 1982, 

 Everitt et al. 1981, Hawes 1983, 

 Antonelis et al. 1984, Aurioles et al. 

 1984, Antonelis et al. 1990, Lowry 

 et al. 1990). Here we describe sea- 

 sonal and annual variability in the 

 diet of California sea lions that haul- 

 ed out at San Nicolas Island from 

 1981 through 1986. 



Methods 



We collected sea lion scats every 

 one or two months at several rook- 

 eries and hauling grounds at San 



Nicolas Island (Fig. 1). Scats were 

 washed through nested sieves 

 (mesh sizes of 2.8mm, 1.5 mm, and 

 0.71 mm) to recover hard parts such 

 as fish otoliths, cephalopod beaks 

 (i.e., mandibles), shark teeth, and in- 

 vertebrate exoskeletal fragments. 

 We identified prey species by com- 

 paring those remains with museum 

 and personal voucher collections. 

 The frequency of occurrence (FO) 

 of each prey taxon was calculated 

 as the proportion of scats in each 

 sample that contained at least one 

 hard part from a prey taxon. We 

 pooled monthly samples into four 

 groups: winter (December-Febru- 

 ary), spring (March-May), summer 

 (June-August), and autumn (Sep- 

 tember-November). We tested the 

 null hypotheses that frequencies of 

 occurrence of the most common 

 prey (those that occurred in 5% or 

 more of all scats) did not differ 

 among seasons nor among years 

 using a two-way analysis of vari- 

 ance (Program 7D of BMDP-87; 



Dixon 1987); we excluded data for 

 1981 from this analysis because we 

 lacked data for some seasons. We 

 tested the null hypothesis that diet- 

 ary diversity did not differ among 

 seasons or among years using multi- 

 way contingency table analysis 

 (Program 4F of BMDP-88; Dixon 

 1988). 



Results 



We examined 1232 scats from sum- 

 mer 1981 through autumn 1986; 

 1085 of those contained identifiable 

 prey hard parts (Table 1). We iden- 

 tified 32 prey taxa to species and 6 

 to genus, but only two genera of 

 cephalopods and five of fish occurred 

 in 5% or more of the scats. There- 

 fore, we confined our analyses of 

 seasonal and annual variability in 

 diet to the latter 7 taxa. Northern 

 anchovy Engraulis mordax was the 

 most common prey, occurring in 

 about half (50.6%) of all scats; Pa- 

 cific whiting Merluccius productus, 

 jack mackerel Trachurus symmetri- 

 cus, rockfish Sebastes spp., and mar- 

 ket squid Loligo opalescens each oc- 

 curred in about 22-30% of scats 

 (Table 2). 



We are limited in our generaliza- 

 tions about seasonal trends in the 

 frequencies of occurrence of prey 

 species because of strong seasonal- 

 annual interactions (ANOVA, P< 

 0.05). There are, however, some dif- 

 ferences apparent among some sea- 

 sons and years (Fig. 2). Northern 

 anchovy was eaten more often in 

 winter than in other seasons in 

 1982, 1983, and 1984, but not in 

 1985 or 1986; anchovy was present 

 in more than 60% of scats each 

 season in 1986. Furthermore, an- 

 chovy occurred in scats more often 

 from 1984 through 1986 than in 

 earlier years. Pacific whiting was 

 eaten frequently from summer 1981 

 through spring 1982, and in spring 



Manuscript accepted 12 December 1990. 

 Fishery Bulletin, U.S. 89:331-336 (1991). 



331 



