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About the Authors:
Sora L. Kim
* E-mail: [email protected]
Affiliation: Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, California, United States of America
M. Tim Tinker
Affiliation: Western Ecological Research Center, United States Geological Survey, Santa Cruz, California, United States of America
James A. Estes
Affiliation: Department of Ecology and Evolutionary Biology, University of California Santa Cruz, Santa Cruz, California, United States of America
Paul L. Koch
Affiliation: Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, California, United States of America
Introduction
Although diet is often treated as a species-level trait, variation in diet composition and foraging behavior occurs within most species. This variation can be attributed to at least three factors–habitat-specific variation in prey availability; differences in the cost-benefit ratios of potential prey among the sexes, or age- or size-classes of consumers; and phenotypic variation among what often appear to be otherwise similar individuals [1]–[5]. Collectively, this dietary variation influences the fitness of consumers and their ecological and evolutionary impacts on prey species, communities, and ecosystems [1]–[5].
White sharks (Carcharodon carcharias) are apex predators that can have cascading effects on marine ecosystems [6], [7], but our understanding of their foraging ecology is fragmentary and often biased by spectacular accounts, especially attacks on humans and other large mammals. In the northeastern Pacific Ocean, white sharks were once considered a nearshore species that preyed primarily on pinnipeds, a perception arising from many studies focused on coastal sites near pinniped colonies where shark foraging behavior was easy to observe [8]–[13]. This view has been challenged by recent satellite tagging data from white sharks off the coast of California and Baja California, Mexico, which revealed migration between the North American continental shelf and two offshore areas (18 to 26°N and 125 to 140°W) [14]–[17]. Isotopic data from tagged individuals corroborated offshore foraging on lower trophic level prey and indicated similar dietary preferences within this population [18]. Although observations of white shark predation on non-pinniped prey are rare, stomach contents include remains from invertebrates, turtles, fish, and sharks [19]. Here, we assess population-level diet variation, potential ontogenetic shifts in prey preferences, and individual diet specialization through analysis of carbon and nitrogen isotope variation.
The stable isotope composition...