Estimating apparent survival of sub-adult and adult white sharks (Carcharodon carcharias) in central California using mark-recapture methods
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Date
2013
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Montana State University - Bozeman, College of Letters & Science
Abstract
Over-exploitation of sharks is a global conservation concern as losses of large apex predators will likely lead to negative consequences in marine ecosystems. The Northeastern Pacific white shark population is genetically distinct and geographically isolated from other known white shark populations in South Africa, Australia/New Zealand, and the Northwest Pacific. The Northeastern Pacific population/clade is comprised of two groups, Guadalupe Island, Mexico, and Central California, USA, that predictably aggregate at their respective coasts during late summer to early winter months. Recently, a three-year study using patterns on the trailing edge of dorsal fins to identify unique white sharks estimated an abundance of 219 (95% credible interval of 130 to 275) sub-adult and adult white sharks off Central California, assuming a closed population. However, there are no estimates for any of the population's vital rates (e.g. survival, recruitment rates). We use six years of mark-recapture data to estimate apparent survival and test for differences in survival between sexes for sub-adult and adult white sharks in central California. We collected 668 photographs that allowed us to identify 199 individual sharks over six years of sampling at three locations off Central California. Using a method developed by Nichols et al. (2004) that accounts for imperfect detection and imperfect sex assignment, we estimated that annual apparent survival was 0.90 (95% CI = 0.81 - 0.98) for males and females and throughout our study period. At this time, it is difficult to determine how this vital rate will affect population trend. Future research is needed to determine if this annual survival estimate is high enough for adult white sharks to produce enough offspring that will eventually recruit to the sub-adult demographic to balance annual mortality.