title: A re-evaluation of the size of the white shark (Carcharodon carcharias) population off California, USA.
abstract: White sharks are highly migratory and segregate by sex, age and size. Unlike marine mammals, they neither surface to breathe nor frequent haul-out sites, hindering generation of abundance data required to estimate population size. A recent tag-recapture study used photographic identifications of white sharks at two aggregation sites to estimate abundance in "central California" at 219 mature and sub-adult individuals. They concluded this represented approximately one-half of the total abundance of mature and sub-adult sharks in the entire eastern North Pacific Ocean (ENP). This low estimate generated great concern within the conservation community, prompting petitions for governmental endangered species designations. We critically examine that study and find violations of model assumptions that, when considered in total, lead to population underestimates. We also use a Bayesian mixture model to demonstrate that the inclusion of transient sharks, characteristic of white shark aggregation sites, would substantially increase abundance estimates for the adults and sub-adults in the surveyed sub-population. Using a dataset obtained from the same sampling locations and widely accepted demographic methodology, our analysis indicates a minimum all-life stages population size of >2000 individuals in the California subpopulation is required to account for the number and size range of individual sharks observed at the two sampled sites. Even accounting for methodological and conceptual biases, an extrapolation of these data to estimate the white shark population size throughout the ENP is inappropriate. The true ENP white shark population size is likely several-fold greater as both our study and the original published estimate exclude non-aggregating sharks and those that independently aggregate at other important ENP sites. Accurately estimating the central California and ENP white shark population size requires methodologies that account for biases introduced by sampling a limited number of sites and that account for all life history stages across the species' range of habitats.
authors.affiliation: Florida Program for Shark Research, Florida Museum of Natural History, University of Florida, Gainesville, Florida, United States of America.
authors.forename: George H
authors.initials: GH
authors.lastname: Burgess
authors.affiliation: Commonwealth Scientific and Industrial Research Organization Wealth from Oceans Flagship, Marine and Atmospheric Research, Hobart, TAS, Australia.
authors.forename: Barry D
authors.initials: BD
authors.lastname: Bruce
authors.affiliation: Moss Landing Marine Laboratory, Moss Landing, California, United States of America.
authors.forename: Gregor M
authors.initials: GM
authors.lastname: Cailliet
authors.affiliation: Alaska Department of Fish and Game, Homer, Alaska, United States of America.
authors.forename: Kenneth J
authors.initials: KJ
authors.lastname: Goldman
authors.affiliation: Florida State University Coastal and Marine Laboratory, St. Teresa, Florida, United States of America.
authors.forename: R Dean
authors.initials: RD
authors.lastname: Grubbs
authors.affiliation: Department of Biological Sciences, California State University Long Beach, Long Beach, California, United States of America.
authors.forename: Christopher G
authors.initials: CG
authors.lastname: Lowe
authors.affiliation: Australian Institute of Marine Science, Townsville, QLD, Australia.
authors.forename: M Aaron
authors.initials: MA
authors.lastname: MacNeil
authors.affiliation: Moss Landing Marine Laboratory, Moss Landing, California, United States of America; Monterey Bay Aquarium, Monterey, California, United States of America.
authors.forename: Henry F
authors.initials: HF
authors.lastname: Mollet
authors.affiliation: Pelagic Fisheries Research Program, Department of Oceanography, University of Hawaii at Manoa, Honolulu, Hawaii, United States of America.
authors.forename: Kevin C
authors.initials: KC
authors.lastname: Weng
authors.affiliation: Monterey Bay Aquarium, Monterey, California, United States of America.
authors.forename: John B
authors.initials: JB
authors.lastname: O'Sullivan
mesh_terms: D000818:Animals; D002140:California; D056727:Endangered Species; D005260:Female; D008297:Male; D008962:Models, Theoretical; D010140:Pacific Ocean; D011156:Population Density; D012754:Sharks