Gauging the threat: the first population estimate for white sharks in South Africa using photo identification and automated software

Towner, Alison V; Wcisel, Michelle A; Reisinger, Ryan R; Edwards, David; Jewell, Oliver J D

Gauging the threat: the first population estimate for white sharks in South Africa using photo identification and automated software

Journal Article

2013

Journal Title

PLoS One

Link to Journal

http://journals.plos.org/plosone

Publisher

Public Library of Science

Publisher

University of Cape Town

Department

Department of Biological Sciences

Faculty

Faculty of Science

License

http://creativecommons.org/licenses/by/4.0

Abstract

South Africa is reputed to host the world’s largest remaining population of white sharks, yet no studies have accurately determined a population estimate based on mark-recapture of live individuals. We used dorsal fin photographs (fin IDs) to identify white sharks in Gansbaai, South Africa, from January 2007 - December 2011. We used the computer programme DARWIN to catalogue and match fin IDs of individuals; this is the first study to successfully use the software for white shark identification. The programme performed well despite a number of individual fins showing drastic changes in dorsal fin shape over time. Of 1682 fin IDs used, 532 unique individuals were identified. We estimated population size using the open-population POPAN parameterisation in Program MARK, which estimated the superpopulation size at 908 (95% confidence interval 808-1008). This estimated population size is considerably larger than those described at other aggregation areas of the species and is comparable to a previous South African population estimate conducted 16 years prior. Our assessment suggests the species has not made a marked recovery since being nationally protected in 1991. As such, additional international protection may prove vital for the long-term conservation of this threatened species.