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News Article | May 18, 2017
Site: phys.org

Rare autopsies conducted on carcasses that washed ashore show how orcas dominate the marine food chain at the expense of great white sharks, whose ferocious reputation (thanks partly to the 1975 movie "Jaws") belies their vulnerability to shark nets, fishing, poaching, pollution—and the biggest member of the dolphin family. Orcas, which also face human threats, have a softer image (the 1993 film "Free Willy" helped) due to their intelligence and accessibility at water parks. Bite marks on the carcasses in South Africa were inflicted by orcas seen recently near Cape Town, conservationists said. The livers of all three sharks were removed, suggesting the orcas, also known as killer whales, targeted the nutrient-rich oil and fat in the organs and discarded the rest of their prey. Each of the sharks had a big wound on its underside, a sign of the dexterity of the orca, whose biggest males can be well over nine meters (30 feet) long—considerably larger than great white sharks. "It's just a classic sort of scenario of what orcas are capable of doing," said Alison Towner, a marine biologist with the Dyer Island Conservation Trust , which seeks to protect the fragile ecosystem in waters around the southern tip of Africa. Towner participated in autopsies on the sharks, providing rare confirmation of orca killings of the big fish off South Africa. Even if they have an advantage, orcas are unlikely to hunt great white sharks on a regular basis, said George Burgess, director of the Florida Program for Shark Research . But smaller sharks can be a routine part of an orca diet, and studies have shown that the rough texture of shark flesh grinds down the enamel on orca teeth over time, he said. Two of the sharks in South Africa were recovered in Gansbaai and another in Struisbaai. Closer to Cape Town, several cow shark carcasses were discovered with their livers removed in False Bay after orca sightings, South Africa's department of environmental affairs said . "Killer whales are apex predators and while we are accustomed to viewing great white sharks as occupying the top of the food chain in our waters, orcas are much more specialized hunters and consider almost anything in the ocean as potential prey," the department said. It said the mammals have highly developed social groups as well as a large brain, the capacity to learn and the ability to locate with the help of sound waves that bounce off objects. "Orcas are known to have culture, much like humans do, and different orca cultures specialize on different prey and different hunting strategies," said Boris Worm, a marine research ecologist at Dalhousie University in Nova Scotia, Canada. "Offshore orcas are rare, and very mobile, so it is very conceivable that they show up in an area where they were previously not seen and exploit sharks in that region," Worm wrote in an email to The Associated Press. "Of course, a white shark would be a difficult prey to tackle due to its power and size, but orcas (again like humans) use group hunting strategies that can outsmart almost any prey." Orcas have been seen attacking cow sharks off Cape Town, New Zealand and South America. Additionally, orcas were filmed attacking great white sharks around the Farallon islands off California and Neptune islands off Australia. South African cage-diving operators reported a drop in great white shark sightings around the time that the three sharks were killed, a sign that other sharks had left the area, at least temporarily, because of the orcas. Researchers are debating the size of South Africa's great white shark population and say more inquiry is needed. Human threats far outweigh any threat from orcas, they say. "We, as shark biologists, recognize that the sharks are going to get eaten sometimes by predators, just like they eat things themselves," said Burgess, the Florida expert. "It's all part of the give-and-take of the natural world." Explore further: More great white sharks appear to be visiting off Cape Cod


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 | And 6 more authors.
Marine Ecology Progress Series | Year: 2014

Manual acoustic telemetry was used to describe core habitat use of white sharks in the complex marine landscape of the Dyer Island and Geyser Rock system near Gansbaai, South Africa. We compared home range estimates and swimming pattern analyses to those established at Mossel Bay, another white shark aggregation area roughly 300 km to the east. Traditional home range estimates used in Mossel Bay did not account for movement or barriers, and were thus biased towards areas with very little shark movement (i.e. potential resting areas). We found that adapting a Movement-based Kernel Density Estimate (MKDE) could account for movement and barriers, resolving these issues. At Dyer Island and Geyser Rock, daytime shark habitat use was adjacent to the seal colony, with low rates of movement, non-linear swimming patterns and small activity areas. At night, rates of movement and linearity increased as sharks travelled further from the islands into deeper waters. MKDEs revealed 4 focal areas of habitat use: a channel between the 2 islands, an area to the south of the seal colony, another area near a kelp feature to the southwest of the seal colony and a reef system to the northwest. These results differed significantly from the habitat use at Mossel Bay, where focal areas occurred adjacent to the seal colony during the hours of dawn and dusk. We discuss possible explanations for these differences. This study is the first to make use of MKDEs in a complex marine landscape and highlights important differences in habitat use of a threatened species between 2 separate aggregation areas. © Inter-Research 2014.


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.


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.


Wcisel M.,Dyer Island Conservation Trust | Chivell W.,Dyer Island Conservation Trust | Gottfried M.D.,Michigan State University
South African Journal of Wildlife Research | Year: 2010

A live Indo-Pacific humpback dolphin (Sousa chinensis) with two bite wounds resulting from a great white shark (Carcharodon carcharias) encounter was observed from the commercial whale-watching vessel, Whale Whisperer, in Gansbaai on 23 and 24 August 2006. The most severe wound was to the dolphin's left flank area, the other to the anterior portion of its dorsal 'hump'. This is the first documented interaction and potential predatory shark attack on any cetacean species in Gansbaai, and is evidence that large great white sharks may opportunistically hunt humpback dolphins in this region of South Africa.


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.


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.


Towner A.V.,Dyer Island Conservation Trust | Towner A.V.,University of Cape Town | Wcisel M.A.,Dyer Island Conservation Trust | Wcisel M.A.,University of Cape Town | And 4 more authors.
PLoS ONE | Year: 2013

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. © 2013 Towner et al.


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.


PubMed | Dyer Island Conservation Trust
Type: Journal Article | Journal: PloS one | Year: 2013

South Africa is reputed to host the worlds 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.

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