Gauging the Threat: The First Population Estimate for White Sharks in South Africa Using Photo Identification and Automated Software

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.

Core habitat use of an apex predator in a complex marine landscape

“…This differed from the swimming patterns at Dyer Island and Geyser Rock, where sharks spent the majority of daytime hours at or close to the Geyser Rock system, only moving to other areas at night. This difference may be attributed to varying age-classes, as sharks at Dyer Island and Geyser Rock are predominantly sub-adult to adults that primarily feed on marine mammals (Estrada et al 2006, Ferrara et al 2011, Towner et al 2013. In contrast, sharks at Mossel Bay are predominantly juvenile to sub-adult that feed on teleost, cephalopod and other reef-based prey as well as marine mammals (Estrada et al 2006, Johnson & Kock 2006, Ferrara et al 2011, Smale & Cliff 2012.…”

“…In contrast, sharks at Mossel Bay are predominantly juvenile to sub-adult that feed on teleost, cephalopod and other reef-based prey as well as marine mammals (Estrada et al 2006, Johnson & Kock 2006, Ferrara et al 2011, Smale & Cliff 2012. Interestingly, large sharks are also found inshore of Dyer Island and Geyser Rock, predominantly during summer months (Towner et al 2013) …”

“…The white shark Carcharodon carcharias is a good candidate for localised studies as it displays seasonal coastal site fidelity, often associated with pinniped colonies (Bruce 1992, Klimley et al 2001, Bonfil et al 2005, Hammerschlag et al 2006, Johnson & Kock 2006, Anderson et al 2011, Domeier 2012, Towner et al 2013. Two studies have examined fine-scale habitat use of white sharks using active acoustic telemetry and home range analysis: Goldman & Anderson (1999) at the Farallon Islands and Jewell et al (2012) at Mossel Bay.…”

“…Johnson et al (2009) used both active and passive acoustic telemetry at Mossel Bay and found distinct diurnal patterns of habitat use, with white sharks approaching the seal colony at dawn and dusk and utilising reef systems during the day and night. The Dyer Island and Geyser Rock system has been identified as an important area of white shark aggregation in South Africa (Ferreira & Ferreira 1996, Bonfil et al 2005, Jewell 2013, Towner et al 2013, Wcisel 2013), yet few telemetry studies and no regional home range estimates have been produced from the area prior to this study.…”

Tracking sharks without teeth: a non-invasive rigid tag attachment for large predatory sharks

“…Though Gleiss et al [10] found little effect from the tagging on the second dorsal fin of whale sharks, for white sharks, these biologging tags, which generally range in size from 100 to 500 g, must be placed on the first dorsal fin. These fins are biologically critical for stability while fast swimming [11] and are used scientifically as means to uniquely identify individuals across decades [12] and estimate abundance [13,14]. Additionally, some communities are dependent on white sharks for ecotourism, so there are sensitivities to the risk (or perceived risk) of damage to dorsal fins from attaching data loggers [15].…”