| I investigated aspects of the foraging ecology of immature (i.e. pup, yearling, and juvenile) California sea lions (Zalophus californianus ) at San Miguel Island, California during 2002--2007, as they transitioned from maternal dependence for nutrition to foraging independently at sea. I examined the extent to which blood oxygen storage capacity influenced the diving behaviors of these animals. Blood oxygen stores varied as a function of age and mass, and were fully developed by the end of the dependency period (∼8 to 10 months of age). However, total body oxygen stores were not developed until animals were juveniles. Satellite telemetry was used to investigate the diving behaviors and at-sea distribution patterns of young sea lions. The youngest animals instrumented were five month olds. They were the least proficient at diving (duration and depth) and distance traveled away from the island. Diving performances and distanced traveled increased with increase in age. Although performances of immature animals did not reach adult levels, they were consistent with the epipelagic foraging (primarily on the continental shelf) strategy of this species. Diet was assessed using stable nitrogen (delta 15N) and carbon (delta13C) isotope and fecal analyses. Pups had higher delta15N values compared to older conspecifics indicating they were feeding at a higher trophic level prior to weaning. Yearling delta 15N values were slightly lower than pup or indistinguishable from adult values, indicating a dietary shift from maternal dependency to independent foraging. Juveniles and adults had similar delta15N values indicating they fed at a similar trophic level. Post-weaned individuals fed in similar foraging areas, as determined from examining delta13C and satellite data. Fecal analysis indicated that adult females had a more diverse diet compared to juveniles. However, the diets of both age classes were considered similar due to the high occurrence of northern anchovy ( Engraulis mordax), Pacific sardine (Sardinops sagax), Pacific hake (Merluccius productus), rockfish (Sebastes spp.), and market squid (Loligo opalescens). The incorporation of multiple approaches (including examination of physiology, behavior data, and diet), provided useful information for a detailed examination of the ontogeny of foraging behaviors of California sea lions. |
