58 



Fishery Bulletin 102(1] 



were consumed in the Canal de Ballenas region during 

 the summer (September), when they are very abundant. 

 Larger size Pacific sardines were consumed by sea lions 

 most frequently during the summer when adult sardines 

 occur more frequently in the Canal de Ballenas. As adult 

 sardine left Canal de Ballenas ( Cisneros-Mata et al., 1997 ), 

 the proportion of young individuals in the diet of sea lions 

 increased. The fish eaten by sea lions were apparently 

 smaller than those captured by the commercial fisher- 

 ies. The average estimated size of the sardines consumed 

 was 150.4 mm, whereas the average size of commercially 

 caught fish during the 1995-96 season was 162.4 mm (Cis- 

 neros-Mata et al. 3 ). This 7% difference in size may have 

 been caused by an underestimation of otolith size because 

 of digestive erosion ( Jobling and Breiby, 1986). If this is so, 

 then the size of Pacific sardines consumed by sea lions is 

 similar to the size of those captured by the fishery. 



Isla Lobos was the only rookery where Pacific sardine was 

 not consumed. This finding differs from those of Cisneros- 

 Mata et al. 3 which show the Pacific sardines present as far 

 north as Isla Lobos. However, their study period was during 

 the 1991-92 El Nino episode, whereas our study occurred 

 during normal oceanographic conditios in 1995-96. 



Less is known about the spatial 

 and temporal availability of other 

 important prey. As with commercial 

 captures (Arvizu-Martinez, 1987), 

 Pacific mackerel occurred together 

 with Pacific sardine. Similar varia- 

 tions in occurrence for both species 

 have been noticed from stomach 

 content analyses of the giant squid 

 (Dosidicus gigas) (Ehrhardt, 1991). 

 Lanternfishes were abundant north 

 of Isla Angel de la Guarda (Robison, 

 1972); however they were not im- 

 portant in the diet of the California 

 sea lion in this region. Their greater 

 importance in the diet at southern 

 rookeries was probably due to the 

 absence of more preferred prey such 

 as Pacific sardine, Pacific cutlass- 

 fish, or anchoveta. The consump- 

 tion of northern anchovy tended to 

 be less important towards Canal 

 de Ballenas, where Pacific sardine 



reached its maximum importance. The low spatial overlap 

 of these two species has also been noted in other studies. 

 The anchoveta was present only at Isla Lobos. This is 

 an estuarine-lagoon species, typical of coastal lagoons of 

 northern Sinaloa and Sonora (Castro-Aguirre et al., 1995). 

 The presence of this prey in Isla Lobos is possibly due to 

 the sandy coast (Walker, 1960), which is similar to that of 

 the Sinaloa-Sonora coast. 



The diet of California sea lions differed among rooker- 

 ies, probably due to differences in feeding sites and prey 

 availability. Antonelis et al. (1990) studied the foraging 

 characteristics of the northern fur seal (Callorhinus ur- 

 sinus) and the California sea lion at San Miguel Island 

 and found differences between foraging areas among 



0.15 



0.2 



0.25 



0.3 



0.35 



0.4 



0.45 



Figure 6 



Dendrogram of cluster analysis of seven rookeries determined with Euclidean dis- 

 tance (computed from the IIMP of the 25 prey that had on at least one occasion a 

 value >10%) and the UPGMA (unweighted pair-grouping methods) strategy. The 

 vertical lines represent the points of references to delimit the groups. 



species. The northern fur seal was found most frequently 

 foraging in oceanic water within 72.4 km from the island, 

 whereas Califorinia sea lions forgaged more often in the 

 shallower neritic zone, within 54.2 km from the island. 

 Different foraging distances in California sea lions from 

 San Miguel Island were found by Melin and DeLong 

 ( 1999). During the nonbreeding season a higher percent- 

 age of foraging locations occurred at distances less than 

 100 km, whereas during the breeding season most of the 

 foraging locations occurred at distances greater than 

 100 km. These differences are probably due to the in- 

 creased California sea lion population in San Miguel; 

 this increase in population forces sea lions to exploit new 

 areas as a density-dependent response to population 



