Garcia-Rodriquez and Aurioles-Gamboa: Spatial and temporal variation in the diet of Za/ophus californianus 



55 



Discussion 



Stomach acids attack otoliths, affecting their size and 

 number and consequently the estimate of prey occurrence 

 and importance. Erosion of otoliths during digestion has 

 been analyzed in studies of pinnipeds in captivity. Bowen 

 (2000) reviewed nine studies that estimated the propor- 

 tion of otoliths recovered in scat samples to obtain a 

 prey-number correction factor (NCF). He found that NCF 

 is greater than 1.0 because many prey species are not 

 recovered in the scat samples. Additionally, the erosion 

 level can be significantly different among prey species 

 (Bowen, 2000) because of differences in the shape and 

 microstructure of otoliths. Therefore, estimates of biomass 

 based on scat analysis should be carefully interpreted 

 because the consumption of some prey species can be 

 under- or overestimated. Correction factors are needed 

 to compensate for differential erosion for the prey species 

 of each pinniped. 



In this study the most important prey of California sea 

 lions were pelagic fish with small, thin, and fragile otoliths 

 (Nolf, 1993). The lanternfish also have small otoliths — 

 perhaps smaller than those of any other prey taxa found 

 in the scats. Their true importance in California sea lion 

 feeding may be underestimated because of erosion caused 

 by stomach acids (Da Silva and Neilson, 1985; Murie and 

 Lavigne, 1985; Jobling and Breiby, 1986; Jobling, 1987; Toll- 

 it et al., 1997). Similarly, the presence of cephalopods may 

 have been underestimated because their jaws are composed 

 of chitin, which is harder to digest, and frequently are re- 

 gurgitated (Pitcher, 1980; Hawes, 1983). However, the high 

 resistence to digestion of cephalopod beaks allows recovery 

 of them in good shape. Thus they are a good choice to use in 

 such diet analyses (Lowry and Carretta, 1999). 



A numerical index of prey species importance may over- 

 or underestimate the dominance of prey species in the diet 

 because it does not consider the weight of the prey. For 

 IIMP, a numerical index that assumes a similar weight for 



all prey species, the true importance of the individual large 

 prey in the diet can be underestimated and the importance 

 of individual small prey can be overestimated. This prob- 

 lem is also present when the PO, PN, and the SSFO index 

 are used because these are all based only upon the number 

 and occurrence of otoliths and cephalopods beaks. As when 

 using PN. and the SSFO, the IIMP does not work for species 

 that cannot be enumerated, such as crustaceans. 



Given the tendencies of the trophic diversity curves, the 

 sample size was suitable in almost all cases. However, at 

 San Pedro Martir a few more samples would have been 

 useful to fully depict the diet. At Los Cantiles, except 

 during September 1995, the samplings should have been 

 more intense because the flattened portion of the diversity 

 curves are not clear. The information, therefore, that comes 

 from those samples could be biased. However, the number 

 of scats that we analyzed contained a high percentage of 

 the consumed species, especially the main prey. 



The results of this study indicate that the California 

 sea lion consumed mainly fish and some crustaceans and 

 cephalopods. According to the PN index, fish were more 

 important than cephalopods in the diet of sea lions. In ad- 

 dition, fish were more frequent (PO) than crustacean and 

 cephalopods. 



Crustaceans were represented in a similar manner in 

 scats from all rookeries. Cephalopods, however, were more 

 important at San Pedro Martir and San Esteban, prob- 

 ably because they are more common towards the southern 

 gulf. Species of the suborder Oegopsida, which includes 

 oceanic species (Roper and Young, 1975), were most com- 

 monly found in scats from these rookeries. Orta-Davila 

 (1988) and Sanchez-Arias (1992) have also noted the low 

 consumption of cephalopods at the northern rookeries. 

 Fish were the most diverse and commonly eaten prey. In 

 contrast to cephalopods, fish were slightly less important 

 in the southern region. 



The availability and abundance of the various prey 

 resources influenced the diet of the sea lions in the Gulf 



