An integrated mark-recapture and genetic approach to estimate the population size of white sharks in South Africa

Sara Andreotti, Michael Rutzen, Stéfan van der Walt, Sophie Von der Heyden, Romina Henriques, Michael Meÿer, Herman Oosthuizen, Conrad A. Matthee

Inter-Research Marine Ecology Progress Series
June 23, 2016

The loss of apex marine predators has been reported to have a cascade of detrimental effects on marine ecosystems; however, the general lack of empirical data can severely limit our understanding of the ecological interactions among marine species. In this study we propose an integrated approach using mark-recapture and genetic techniques to assess population estimates of white sharks Carcharodon carcharias. Between 2009 and 2011, 4389 dorsal fin photographic identifications were collected in Gansbaai, South Africa, from 426 white sharks and used in mark-recapture analyses. Saturation of new sightings occurred once 400 individuals were catalogued and the open population model POPAN suggested ranges between 353 and 522 individuals (95% confidence) and a point estimate of N = 438. Between 2010 and 2013, 302 biopsy samples were collected from 233 white sharks and used for a comparative genetic population estimate. Analyses of 14 microsatellite markers revealed a contemporary effective population size (CNe) of 333 individuals (95% CI = 247-487, pcrit = 0.02). These values were at least 52% less than those estimated in previous mark-recapture studies. Using this combination of techniques, we propose a Ne:N ratio of 0.76 for white sharks, which advances our ability to accurately make inferences on elasmobranch population numbers in general. Given the low population numbers of white sharks along the South African coastline, we predict a negative effect on the ecological stability of the marine environment in this region.

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Stéfan van der Walt

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