FISHERY BULLETIN: VOL. 82, NO. 1 



tion and preference of California sea lions which 

 necessitate additional research. 



Unfortunately, virtually no information has been 

 reported on the digestive rates or retention time of 

 the prey species' hard parts in California sea lions. 

 Therefore, it is presently impossible to ascertain how 

 many meals, or portions thereof, are represented in a 

 single scat. There is some evidence, however, from 

 feeding studies (Pitcher 1980) of harbor seals, Phoca 

 uitulina, and (Miller 1978) northern fur seals, 

 Callorhinus ursinus , which indicates that cephalopod 

 beaks are not readily passed through the intestinal 

 tract and are regurgitated. This would result in an 

 underrepresentation of cephalopod beak percent- 

 age-of-occurrence data from scats as suggested by 

 Morejohn et al. (1978). Furthermore, the possible 

 occurrence and identification of hard parts of second- 

 ary prey (from the stomach of the prey of the marine 

 mammal) could bias the results of scat or stomach 

 analysis (Perrin et al. 1973). 



Additional information on the feeding habits of 

 California sea lions can also be obtained from the 

 weight estimates of the four major prey species iden- 

 tified in this study. The 1978 and 1979 percentages 

 of total weight estimates (Fig. 4) for each major 

 species showed seasonal changes that are similar to 

 the analysis of percentage of occurrence (Fig. 2), 

 although there are a few exceptions. In 1979 the 

 market squid weight estimate showed a slight de- 

 crease, instead of an increase, from spring to sum- 

 mer, however, of more importance, is its relationship 

 to Pacific whiting. The estimated weight of market 

 squid from the scats clearly exceeded the relative 

 weight of Pacific whiting and other prey species con- 

 sumed during the spring and summer of 1979. These 

 results suggest market squid may be a more impor- 

 tant food item than was predicted from the analysis 

 of their percent of occurrence. The importance of the 

 squid in the diet of the California sea lion during the 

 summer months near the northern California Chan- 

 nel Islands was also documented by Rutter et al. 

 (1904), when they found that 84.6% (n = 13) of the 

 animals examined had squid in their stomachs. 



Bailey and Ainley (1982) estimated the spring and 

 summer percent (weight) of Pacific whiting in the 

 California sea lion diet in the southern region to be 

 within a range of 50 to 90%. Yet our estimates fell 

 below 40% in the spring of 1979 and below 20% in the 

 summer of both 1978 and 1979, and only one 

 instance (spring 1978) did our estimates fall within 

 the range suggested by Bailey and Ainley (1982). 

 Since Bailey and Ainley (1982) based their estimates 

 on data from California sea lions in the northern 

 region, we assume our estimates more accurately 



represent the percent (weight) of Pacific whiting in 

 the diet of California sea lions south of Point Concep- 

 tion, and we recommend that additional feeding 

 studies of California sea lions be conducted 

 throughout their range. 



The percentage of estimated weight results also 

 suggests that Pacific whiting was preyed upon more 

 heavily in the spring of 1978 than in the spring of 

 1979. This is consistent with the exceptionally high 

 recruitment of the 1977 year class of Pacific whiting 

 (discussed above) which was available as yearlings to 

 California sea lions in 1978. 



Although these weight (biomass) estimates are only 

 approximate measurements, they appear instructive 

 when used in conjunction with percentage-of- 

 occurrence data. Unfortunately, there is some uncer- 

 tainty as to the accuracy of using estimates of weight 

 to estimate consumption. Our ability to make con- 

 sumption estimates awaits the resolution of several 

 questions: 1 ) What proportion of a given meal is repre- 

 sented in a single scat? 2) Are there differential 

 digestive rates of fish and squid? 3) Do sea lions of 

 different ages and sexes digest food differently? 



The results of this study suggest that the California 

 sea lions found on San Miguel Island feed oppor- 

 tunistically on prey species of changing availability, 

 and we agree with Bailey and Ainley (1982) that they 

 are behaviorally flexible enough to switch from one 

 major prey species to another, both seasonally and 

 annually. This type of flexibility in foraging appears 

 to be adaptive and may be a major factor contributing 

 to the success of the California sea lion population off 

 the coasts of California and Baja California. 



ACKNOWLEDGMENTS 



Permission to work on San Miguel Island was grant- 

 ed by the National Park Service in conjunction with 

 the U.S. Navy. Logistical assistance was provided by 

 the U.S. Coast Guard, U.S. Navy, and the Channel 

 Islands National Park. 



Superintendent W. Ehorn and his staff of the Chan- 

 nel Islands National Park frequently assisted us dur- 

 ing our research activities. Others who volunteered 

 their time and assistance during our research 

 included L. Antonelis, T. Antonelis, and K. Antonelis 

 of Seattle, Wash.; P. Collins of Santa Barbara 

 Museum of Natural History, Santa Barbara, Calif.; E. 

 Jameyson of MARIS, Seattle, Wash.; R. Morrow of 

 Oregon State University, Corvallis, Oreg.; D. Seagars 

 of NMFS Southwest Region, Terminal Island, Calif.; 

 and B. Steward of Hubbs-Sea World Research In- 

 stitute of San Diego, Calif. M. Weber of California 

 Marine Mammal Center, Fort Cronkhite, Calif., pro- 



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