Lowry et al Food habits of Zaiophus cahfornianus at San Clemente Island. California 511 



We used indices of occurrence, composition, and 

 number to quantify prey taxon consumed by sea lions. 

 Only scat samples containing otoliths, cephalopod 

 beaks, shark teeth, cartilaginous vertebrae, or pelagic 

 red crab fragments were used in the analysis. The 

 occurrence index, or percent occurrence (PO), is a 

 measure of the percentage of scat samples in which a 

 prey taxon occurred. The composition index, or per- 

 cent prey composition (PPC), is a measure of the 

 percentage of occurrences for each prey taxon from 

 a tally of occurrences from all prey taxa found in a 

 group of samples. The number index, or percent mini- 

 mum number (PMN), measures the percentage of 

 numbers of individual prey taxon. To compute the 

 number index, we used the maximum count of either 

 left or right otoliths, or upper or lower beaks, which 

 represent the minimum number of individual prey 

 taxon. The counts for all prey taxa are summed and 

 the percentage of each prey taxon is determined from 

 the sum of all counts. The number index is, then, the 

 percentage represented by each prey taxon from the 

 sum of all maximum coimts taken of all prey taxa found 

 in a group of samples. 



Interpretation of the number index is limited because 

 it excludes (1) all prey hard parts found that were 



broken or digested, and not categorized as either left 

 or right otoliths or upper or lower beaks, and (2) prey 

 that lacked otoliths or beaks (e.g., pelagic red crabs and 

 sharks). However, we think that this index is useful 

 because fish and cephalopods represented the major- 

 ity of prey in scat samples. 



Percent occurrence indicates prey consumption with- 

 out regard to other prey, and may indicate temporal 

 availability, selectivity, or ease of capture of individual 

 prey. Percent prey composition indicates the relative 

 proportions of prey consumed. Percent minimum num- 

 ber of prey indicates numbers of prey consumed and is 

 an index of the rate of consumption of individual prey. 



Statistical analysis 



We used all scats to determine what prey taxa were 

 consumed, and then limited our examination of tem- 

 poral variations in prey consumption to prey that oc- 

 curred in at least 10% of these scats, were found in 

 at least 80% of the seasons, or were occasionally very 

 abundant. Only fresh scats and dried scats represent- 

 ing one between-trip interval were used in the temporal 

 analyses of sea lion diets. We combined the 40 collec- 

 tion trips (Table 1) by season into winter (December- 



