A re-evaluation of the size of the white shark (Carcharodon carcharias) population off California, USA

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Burgess, George; Bruce, Barry; Cailliet, Gregor; Goldman, Kenneth; Lowe, Christopher; MacNeil, Aaron; Mollet, Henry; O'Sullivan, John; Weng, Kevin; Grubbs, R. Dean


2014-06-16


Journal Article


PLoS ONE


9


6


e98078


White sharks, the largest of predatory sharks, are highly migratory and segregate by sex, age and size. Unlike marine mammals, they neither surface to breathe nor frequent haulout sites, making it difficult to gather the abundance data required to estimate population size. Based on photographic identification of white sharks from two aggregation sites, the Farallon Islands and Tomales Point, a recent tag-recapture study estimated abundance in “central California” at 219 mature and sub-adult individuals. This low estimate generated great concern within the conservation community, prompting petitions for federal and state endangered species designations. We critically examine that study and find violations of model assumptions that lead to underestimates in their results. We also use a Bayesian mixture model to show that the inclusion of transients 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 population size (including all-life stages) of 2000 individuals in the California subpopulation with a predicted population growth of 7.8% year-1. Even with methodological and conceptual biases accounted for, extrapolation of sub-population results from two adjacent sampling locations to white sharks across the eastern North Pacific (ENP) is unwarranted. The true population size of white sharks in the ENP is likely several fold greater than these local and regional scale estimates as both studies exclude sharks that frequently aggregate at other important ENP aggregation sites (e.g. Año Nuevo Island, CA; Guadalupe Island, Mexico) and those that may not aggregate at all. Accurately estimating the population size of white sharks in the central California and ENP region by such techniques requires dedicated random sampling inclusive of all life history stages across all of their habitats.


Public Library of Science


Marine and Estuarine Ecology (incl. Marine Ichthyology)


https://doi.org/10.1371/journal.pone.0098078


© 2014 Authors and/or their institution


EP137196


Journal article - Refereed


English


Burgess, George; Bruce, Barry; Cailliet, Gregor; Goldman, Kenneth; Lowe, Christopher; MacNeil, Aaron; Mollet, Henry; O'Sullivan, John; Weng, Kevin; Grubbs, R. Dean. A re-evaluation of the size of the white shark (Carcharodon carcharias) population off California, USA. PLoS ONE. 2014; 9(6):e98078. https://doi.org/10.1371/journal.pone.0098078



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