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Dureuil M.,Dalhousie University | Towner A.V.,Dyer Island Conservation Trust | Ciolfi L.G.,University of Sao Paulo | Beck L.A.,University of Marburg
African Journal of Marine Science | Year: 2015

Subsurface video footage can be used as a successful identification tool for various marine organisms; however, processing of such information has proven challenging. This study tests the use of automated software to assist with photo-identification of the great white shark Carcharodon carcharias in the region of Gansbaai, on the south coast of South Africa. A subsurface photo catalogue was created from underwater video footage. Single individuals were identified by using pigmentation patterns. From this catalogue, two images of the head for each individual were inserted into automated contour-recognition software (Interactive Individual Identification System Beta Contour 3.0). One image was used to search the database, the other served as a reference image. Identification was made by means of a contour, assigned using the software to the irregular border of grey and white on the shark's head. In total, 90 different contours were processed. The output provided ranks, where the first match would be a direct identification of the individual. The method proved to be accurate, in particular for high-quality images where 88.24% and 94.12%, respectively, were identified by two independent analysts as first match, and with all individuals identified within the top 10 matches. The inclusion of metadata improved accuracy and precision, allowing identification of even low-quality images. © 2015 NISC (Pty) Ltd. Source


Jewell O.J.D.,Dyer Island Conservation Trust | Jewell O.J.D.,University of Pretoria | Wcisel M.A.,Dyer Island Conservation Trust | Wcisel M.A.,University of Cape Town
South African Journal of Wildlife Research | Year: 2012

A dead, mature male leatherback turtle was sighted at Danger Point, Gansbaai on South Africa's southwest coast. Leatherback turtle sightings are rare along this coastline although the site lies between two areas of known aggregation; a tropical breeding area to the east and the highly productive Benguela upwelling ecosystem foraging area to the west. Source


Towner A.V.,Dyer Island Conservation Trust | Towner A.V.,University of Cape Town | Underhill L.G.,University of Cape Town | Jewell O.J.D.,Dyer Island Conservation Trust | And 3 more authors.
PLoS ONE | Year: 2013

The seasonal occurrence of white sharks visiting Gansbaai, South Africa was investigated from 2007 to 2011 using sightings from white shark cage diving boats. Generalized linear models were used to investigate the number of great white sharks sighted per trip in relation to sex, month, sea surface temperature and Multivariate El Niño/Southern Oscillation (ENSO) Indices (MEI). Water conditions are more variable in summer than winter due to wind-driven cold water upwelling and thermocline displacement, culminating in colder water temperatures, and shark sightings of both sexes were higher during the autumn and winter months (March-August). MEI, an index to quantify the strength of Southern Oscillation, differed in its effect on the recorded numbers of male and female white sharks, with highly significant interannual trends. This data suggests that water temperature and climatic phenomena influence the abundance of white sharks at this coastal site. In this study, more females were seen in Gansbaai overall in warmer water/positive MEI years. Conversely, the opposite trend was observed for males. In cool water years (2010 to 2011) sightings of male sharks were significantly higher than in previous years. The influence of environmental factors on the physiology of sharks in terms of their size and sex is discussed. The findings of this study could contribute to bather safety programmes because the incorporation of environmental parameters into predictive models may help identify times and localities of higher risk to bathers and help mitigate human-white shark interactions. © 2013 Towner et al. Source


Jewell O.J.D.,University of Pretoria | Wcisel M.A.,Dyer Island Conservation Trust | Wcisel M.A.,University of Cape Town | Gennari E.,Oceans Research | And 6 more authors.
PLoS ONE | Year: 2011

We present 15 individual cases of sub-adult white sharks that were SPOT tagged in South Africa from 2003-2004 and have been re-sighted as recently as 2011. Our observations suggest SPOT tags can cause permanent cosmetic and structural damage to white shark dorsal fins depending on the duration of tag attachment. SPOT tags that detached within 12-24 months did not cause long term damage to the dorsal fin other than pigmentation scarring. Within 12 months of deployment, tag fouling can occur. After 24 months of deployment permanent damage to the dorsal fin occurred. A shark survived this prolonged attachment and there seems little compromise on the animal's long term survival and resultant body growth. This is the first investigation detailing the long term effects of SPOT deployment on the dorsal fin of white sharks. © 2011 Jewell et al. Source


Wcisel M.,Dyer Island Conservation Trust | Wcisel M.,University of Cape Town | O'Riain M.J.,University of Cape Town | de Vos A.,University of Cape Town | Chivell W.,Dyer Island Conservation Trust
Behavioral Ecology and Sociobiology | Year: 2014

Refugia play an important role in shaping predator/prey interactions; however, few studies have investigated predator–prey relationships between large marine vertebrates, mainly due to the logistical challenges of studying marine species. The predictable interactions between Cape fur seals and white sharks in South Africa at two neighbouring seal colonies (Seal Island and Geyser Rock) with similar breeding conditions, but distinct adjacent seascapes, offer an opportunity to address this gap. Geyser Rock differs from Seal Island in being surrounded by abundant refugia in the form of kelp beds and shallow reefs, while Seal Island is mostly surrounded by deep open water. In this study, we compare data collected from Geyser Rock to the published data at Seal Island and ask, do seals adjust their anti-predator tactics as a function of landscape features? We found that during periods of high white shark presence, seals at Geyser Rock reduced their presence in open-water and utilized areas that contained complex landscapes around the colony. Although seals at Geyser Rock formed groups when traversing open water, neither group size (high risk median = 4, low risk median = 5) nor temporal movement patterns varied significantly with white shark presence as has been shown at Seal Island. Furthermore, recorded hourly predation rates at Seal Island were 12.5 times higher than at Geyser Rock. Together, these findings suggest that refuge use may be the more effective anti-predator response of seals to a seasonally abundant predator and that the predations at Seal Island reflect a comparative lack of refugia. © 2014, Springer-Verlag Berlin Heidelberg. Source

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