60 



Fishery Bulletin 102(1) 



growth. Although, these differences could also be due to 

 variability in the distribution of prey (Melin and DeLong, 

 1999), as suggested by Antonelis and Fiscus (1980), forag- 

 ing areas might change with season and annual variations 

 in prey availability and abundance. 



Foraging areas in the Gulf of California could lie closer 

 to rookeries than those recorded for San Miguel Island sea 

 lions because the diet was different among rookeries in 

 spite of the shorter distance between them (54.2 km). At 

 Los Islotes, Baja California Sur, adult females fed within 

 20 km of the colony (Duran-Lizarraga. 1998). Kooyman and 

 Trillmich (1986a, 1986b) reported similar data in sea lion 

 colonies of the Galapagos Islands. In the northern region of 

 the Gulf of California, feeding range could be shorter than 

 that at Los Islotes because of the higher concentration of 

 food at high nutrient concentrations (phosphate, nitrate, 

 nitrite, and silicate) in Canal de Ballenas that is associated 

 with strong tidal mixing (Alvarez-Borrego, 1983). 



Four foraging zones were discerned from dietary differ- 

 ences in sea lions from the seven rookeries studied. Zone 

 I, which included San Pedro Martir. San Esteban. and El 

 Rasito, was characterized by the consumption of lantern- 

 fish; zone II, which included Los Machos was characterized 

 by the consumption of Pacific sardine and Pacific mackerel; 

 zone III, which included Isla Granito, by the consumption 

 of Pacific cutlassfish and the northern anchovy; and zone 

 IV, Los Cantiles and Isla Lobos, was characterized by the 

 consumption of the plainfin midshipman and the Pacific 

 cutlassfish. These four zones may indicate differences in 

 habits used by sea lions or may indicate different oceano- 

 graphic conditions exploited by sea lions. The eastern 

 coast of the Gulf of California displays high photosyn- 

 thetic pigment concentrations, associated with upwelling 

 induced by winds from the northwest in the winter. These 

 conditions may make Canal de Ballenas one of the most 

 important for the distribution of Pacific sardine during 

 the summer. 



Trophic diversity varied spatially and temporally. San 

 Pedro Martir and Isla lobos sea lions seem to depend on a 

 more stable feeding areas compared to sea lions at rook- 

 eries on Isla Granito and Los Machos, where changes in 

 diversity of consumed species indicated that sea lions feed 

 on fewer species during certain times of the year. Similar 

 results in relation to the changes in diversity were also 

 noticed in the rookeries of the Channel Islands and Faral- 

 lon Islands, California (Bailey and Ainley, 1982; Antonelis 

 et al., 1984; Lowry et al., 1990; Lowry et al., 1991 ). Perhaps 

 the tendency to have the highest values of diversity and 

 little seasonal variation at San Pedro Martir is the result 

 of this rookery being located in a zone of transition between 

 two biogeographical areas. This geographical position con- 

 fers greater environmental heterogeneity and greater 

 ecological diversity (Walker, 1960). 



California sea lions in the upper region of the Gulf of 

 California obtain the main portion of their diet from a 

 relatively small number of species. The decrease in abun- 

 dance of any of these food resources can seriously affect the 

 population, particularly at Isla Granito and Los Machos 

 because sea lions from these rookeries depend on a few 

 species. 



Acknowledgments 



We wish to thank Secretaria de Marina, Armada de Mexico, 

 for its great support during the field activities, and the 

 Consejo Nacional de Ciencia y Tecnologia (CONACYT) 

 for funding this study under grant number 26430-N. The 

 Secretaria de Medio Ambiente, Recursos Naturales y Pesca 

 (SEMARNAP) provided permits for field work (DOO.-700- 

 (2)01104 and DOO.-700(2).-1917). We would like to thank 

 Robert Lavenberg and Jeff Siegel for allowing us the use 

 of otoliths from the collection at the Natural Museum His- 

 tory of Los Angeles County and also Lawrence Barnes for 

 his logistical support during the stay of first author at Los 

 Angeles; we also thank Manuel Nava for allowing us the use 

 of otoliths from the collection in Tecnologico de Monterrey, 

 Campus Guaymas. We are also grateful to Unai Markaida 

 for his assistance in prey identification based on the 

 examination of cephalopods beaks. We thank Mark Lowry 

 for commenting on an earlier draft of the paper, Norman 

 Silverberg for reviewing the manuscript in English, and 

 two anonymous reviewers for their valuable suggestions 

 and criticism. The first author would like to thank Centro 

 Interdisciplinario de Ciencias Marinas-IPN for a scholar- 

 ship (PIFI, Programa Institucional para la Formacion de 

 Investigadores) assigned for postgraduate studies. 



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