Estimates of regional annual abundance and population growth rates of white sharks off central California

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Paul E. Kanive, Ecology, Montana State University, Jay Rotella, Ecology, Montana State University, Bozeman, MT, Taylor K. Chapple, Stanford University, Scot Anderson, Monterey Bay Aquarium, Timothy D. White, Hopkins Marine Station, Stanford University, Pacific Grove, CA, Barbara Block, Hopkins Marine Station, Stanford University and Salvador J. Jorgensen, UC Santa Cruz

Background/Question/Methods
Determining abundance and population trends is critical for evaluating management actions and prioritizing species protections. In this study, we used long-term empirical data to produce estimates of annual abundance and population growth rates for sub-adult and adult white sharks in central California, an iconic top predator. Sub-adult and adult white sharks seasonally aggregate at the three known locations off central California; Tomales Point, Año Nuevo Island, and the Southeast Farallon Island. White sharks were attracted to the surface using a floating seal-shaped decoy. When sharks would investigate the decoy, photographs of the dorsal fin could be taken. The pattern of notches on the trailing edge of the dorsal fin is used to identify individuals. Using the dorsal fins as a 'mark', we built a mark-recapture data set in a modified Jolly-Seber model (POPAN formulation) to estimate annual abundance estimates and then investigated population growth rates using parametric bootstrapping methods for four demographic groups; sub-adult and adult males and females.
Results/Conclusions
From 2011 to 2018, we spent over 2,500 hours attracting white sharks to the surface and collected 1,533 pictures of dorsal fins. For all demographic groups combined, we estimated 266 (95% CI = 218 to 314) sub-adult and adult white sharks in this region along the California coast. Importantly, we estimated there to be only 59 (95% CI = 41 to 77) sexually mature female sharks. For all demographic groups combined, we found equivocal evidence for a positive regional population growth (λ = 1.07 (95% CI = 0.91 to 1.23)). However, sex- and size-specific population growth rate estimates provided some evidence of population increases for sexually mature males (λ = 1.06 (95% CI = 0.99 to 1.13) and females (λ = 1.06 (95% CI = 0.95 to 1.17). For sub-adult male and female white sharks, point estimates of λ were positive but uncertainty prevents strong inference (λ = 1.07 (95% CI = 0.85 to 1.29) and λ = 1.08 (95% CI = 0.88 to 1.28)), respectively. Our findings of a potential increase in reproductive-aged white sharks in central California may be a result of regional fluxes in density or attributed in part to current protection efforts and subsequent increase in abundance of pinnipeds as well as reduced gill-net fisheries mortality of juveniles. An abundance and trend estimate for the entire northeastern Pacific white shark population will require obtaining similar data across known aggregation areas along the west coast of North America.