16 research outputs found
A leatherback turtle stranding at danger point, Gansbaai, South Africa
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.VW South Africa and Marine Dynamicshttp://www.sawma.co.za/am201
Gauging the threat: the first population estimate for white sharks in South Africa using photo identification and automated software
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
Effects of Smart Position Only (SPOT) Tag Deployment on White Sharks Carcharodon carcharias in South Africa
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
Future research directions on the "elusive" white shark
White sharks, Carcharodon carcharias, are often described as elusive, with little information available due to the logistical difficulties of studying large marine predators that make long-distance migrations across ocean basins. Increased understanding of aggregation patterns, combined with recent advances in technology have, however, facilitated a new breadth of studies revealing fresh insights into the biology and ecology of white sharks. Although we may no longer be able to refer to the white shark as a little-known, elusive species, there remain numerous key questions that warrant investigation and research focus. Although white sharks have separate populations, they seemingly share similar biological and ecological traits across their global distribution. Yet, white shark’s behavior and migratory patterns can widely differ, which makes formalizing similarities across its distribution challenging. Prioritization of research questions is important to maximize limited resources because white sharks are naturally low in abundance and play important regulatory roles in the ecosystem. Here, we consulted 43 white shark experts to identify these issues. The questions listed and developed here provide a global road map for future research on white sharks to advance progress toward key goals that are informed by the needs of the research community and resource managers
Core habitat use of an apex predator in a complex marine landscape
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.Dyer Island Conservation Trust, Marine Dynamics Shark Tours and Dyer Island Cruises, VW South Africa and the National Lotteryhttp://www.int-res.comhb201
Study animal ‘Darwin’, photographed numerous times from 2007–2011.
<p>This fin clearly demonstrates how small additions/changes to notches occurring on the back of the fin can distinguish it from early photos, increasing the probability of later fin IDs being identified as multiple sharks.</p