Vital rates, annual abundance, and movement of white sharks in the northeastern Pacific

Abstract

Reliable estimates of populations' vital rates and abundance are fundamental requirements for making assessments and informed management decisions regarding any species. For large marine fish species whose movements are extensive throughout ocean basins, data for individuals are difficult to acquire. Without empirical data, large assumptions must be made about a species' vital rates (i.e. survival and fecundity) to make population assessments, which can potentially lead to erroneous results. Using mark-recapture and acoustic-telemetry data, I conducted analyses estimating vital rates, annual abundance, and coastal movement for white sharks (Carcharodon carcharias) off California, US. First, I used a novel approach to estimate size-specific annual apparent survival rates and to test for differences in survival between sexes after accounting for imperfect sex assignment. Our results provide little evidence for differences in sex-specific survival rates. However, I estimated the first size-specific annual apparent survival rates for sub-adult and adult white sharks. Second, I estimated annual abundance for four white shark demographic groups off the coast of California over an eight-year study period. The estimated total annual population of sub-adult and adult white sharks increased from 180 to 266 individuals during the study. Additionally, group-specific population growth rate point estimates were all > 1.00, which indicates that all groups had positive annual positive growth during the study period, although, uncertainty around those estimates were greater for sub-adults than adults and does not rule out other possibilities for population trajectories. Finally, through collaboration between Mexico and the US, I was able to analyze a comprehensive acoustic telemetry dataset that explored connectivity between two main aggregation sites that form the northeastern Pacific population of white sharks. I found that movement between the two regions was rare and more probable to be sub-adult sharks. These analyses underscore the value of collecting and analyzing empirical data to develop reliable estimates of vital rates for a top marine predator. The work also illustrates the ongoing need to cultivate international research collaboration to include data from both the US and Mexico to make accurate population inferences for the northeastern Pacific population of white sharks.