Dyer Island is thought to host one of the most abundant populations of white sharks on the planet, this is often credited to the large (55 – 60,000) Cape fur seal colony at Geyser Rock. Yet relatively little work has ever been produced from the area. This may be attributed to the harshness in its location as a study site, exposed to wind and swell from west to east which limits research periods. This study accounts for over 220 hrs of manual tracking at Dyer Island with a further 68 added from the inshore shallow areas of the bay. Sharks focused their movements and habitat use to reefs or channels that allowed access to Cape fur seals. Movement-Based Kernel Estimates (MKDE) were used to compute home range estimates for shark movements through and around the heterogeneous structures of Dyer Island and Geyser Rock. Inshore two core areas were revealed, one being the major reef system at Joubertsdam and the other at a kelp reef where the tracked shark had fed on a Cape fur seal. At Dyer Island one core area was identified in a narrow channel, ‘Shark Alley’, here a second tracked shark foraged for entire days within meters of rafting Cape fur seals.
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... The low number of predations I observed could not be explained by an absence of sharks in these near-colony areas, as acoustic tracking data from Jewell (2012) revealed that the kelp fringes of sector four, as well as sectors five and six, are areas of high white shark presence ( Figure 24). Similar positive correlations between predator and prey spatial presence have been recorded at Shark Bay, Australia, where dugongs and bottlenose dolphins preferred predator abundant areas during times of high risk (Heithaus et al. 2008a, Wirsing et al. 2010). ...
... Figure 24: A Movement-based Kernel Density Estimation (MKDE) of spatial use from five white shark tracks at Geyser Rock. Yellow areas represent the majority of use (95% of tracks) with Red representing the core (50% of tracks) use(Jewell 2012).ConclusionsI set out to test the following predictions based on research findings from the nearby Seal Island shark-seal system, and here I present the results based on the research completed in this dissertation.Prediction: White sharks aggregate at Geyser Rock during the austral winter season to target young of the year seals that are making their first exploratory forays into open water; ...
Predators and the risk associated with their presence may affect group composition, group
size, movement patterns, and habitat use of prey species. The removal of predators, or their
reintroduction following long periods of absence, equally can have profound impacts on their
prey, triggering ecological cascades and ultimately shaping the biota of entire landscapes.
While such processes are well documented in terrestrial ecosystems, similar results are absent
in the marine realm, largely due to logistical difficulty. One exception to this is the study of
white shark and seal interactions at coastal island rookeries, where white shark presence is
seasonal, and seals exhibit marked behavioural differences between seasons. What is lacking
from these studies, however, is how subsurface habitat around the islands, specifically
refugia, may influence the behaviour of seals and their interactions with white sharks. I
address this challenge by comparing both seal movement patterns and shark-seal interactions
at Geyser Rock, Gansbaai with the established seal and shark patterns at Seal Island, False
Bay. White sharks aggregate at both islands during the austral winter and seals encounter
these aggregations when commuting to and from their respective rookeries to offshore
foraging areas. The seascape around Geyser Rock is comparably more featured, including
kelp beds and extensive shallow (5–10m depth) reef systems, whereas Seal Island is largely
featureless, with neither extensive kelp nor reefs. At Geyser Rock, predations by white
sharks were rarely observed (0.1 predations/hour) compared to Seal Island (1.24
predations/hour) and lacked the focused spatiotemporal peak at sunrise to the south of the
island. Seals at Geyser Rock did not show a relationship between group formation and
season, which was clearly demonstrated at Seal Island. This suggests that seals at Geyser
Rock may be less reliant on group formation (safety in numbers) and selfish herd tactics
within such groups to reduce predation risk. Rather, seals at Geyser Rock avoided deep open
water patches during winter and shifted their movement patterns to and from the island to
sectors with greater subsurface habitat heterogeneity. While I was limited in quantifying
spatiotemporal patterns of predation risk around Geyser Rock (predation events were rare and
widely dispersed), these results strongly suggest that seals actively avoid deep open water and
show a preference for high structural complexity sectors during the risky winter months when
shark presence is highest. This finding represents a habitat-escape tactic unidentified in
previously studied white shark/pinniped systems. Together these results provide empirical
support for both the risk-allocation hypothesis and refugia hypotheses within marine
predator-prey systems.
... As a result, signal strength/gain data for these sharks was not available and we were not able to correct positioning of these sharks. Comparing corrected position data to uncorrected position data from the same tracks provided non-significant differences for Rate of Movement (ROM), Linearity Index (LI) and Minimum Convex Polygon (MCP) calculations; distance to Geyser Rock would have lower values as sharks were able to approach the island closer than the research vessel (Jewell 2013). However, the mean distance from Geyser Rock was 318 m during daylight, compared to 1267 m during the night and it is unlikely that correcting would have affected the significance of the difference between the two (Jewell 2013). ...
... Comparing corrected position data to uncorrected position data from the same tracks provided non-significant differences for Rate of Movement (ROM), Linearity Index (LI) and Minimum Convex Polygon (MCP) calculations; distance to Geyser Rock would have lower values as sharks were able to approach the island closer than the research vessel (Jewell 2013). However, the mean distance from Geyser Rock was 318 m during daylight, compared to 1267 m during the night and it is unlikely that correcting would have affected the significance of the difference between the two (Jewell 2013). To compare the effect of light levels during daytime, night, dawn and dusk, we used Johnson et al.'s (2009) definitions: daytime – any position recorded during daylight hours, night – any position recorded during the night, dawn/dusk – any position recorded half an hour before or after dusk (Figs. ...
Both the white shark (Carcharodon carcharias) and Cape fur seal (Arctocephalus pusillus pusillus) prey on and/or attack seabirds in South Africa. The Dyer Island region abounds in these predators, as well as seabirds, including the African penguin (Spheniscus demersus), Cape cormorant (Phalacrocorax capensis), bank cormorant (P. neglectus), white-breasted cormorant (P. carbo), and crowned cormorant (P. coronatus). Between August 1999 and February 2001, predatory interactions among these taxa were quantified and qualified by the routine collection and inspection of seabird carcasses and injured birds, as well as opportunistic observations of live attacks. White sharks are infrequent predators of seabirds at Dyer Island, perhaps due to an abundance of Cape fur seals (a preferred prey), antipredator behaviour by penguins, and seabirds not being a preferred prey type. Cape fur seals were more conspicuous seabird predators, each year attacking adult penguins (1.99-2.52%), white-breasted cormorants (5.21-5.72%), and crowned cormorants (3.13%), as well as a single bank cormorant. Cape fur seals killed an estimated 0.83-1.09% of the fledgling Cape cormorants. Attacks on penguins at the island are crepuscular as birds depart to and return from foraging grounds. Fledgling Cape cormorants are frequently attacked when alighting on water following disturbance and failed flight attempts, or for bathing. Human disturbance, which may force birds to take to the water, may cause inter-annual differences in the predation impact on this taxon.
Manual acoustic telemetry techniques were used to study spatial and temporal patterns of movement of juvenile lemon sharks. Ultrasonic transmitters were implanted into the coelom of 38 sharks, yielding trackings totaling 2281 telemetry fixes. Activity space varied from 0.23 km2 to 1.26 km2 and was positively correlated with shark size. Three indices of site attachment demonstrated that juvenile lemon sharks establish a home range. An index of site defense and field observations indicated that no territoriality was observed against conspecifics.
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.
1. Fishing spans all oceans and the impact on ocean predators such as sharks and rays is largely
unknown. A lack of data and complicated jurisdictional issues present particular challenges for
assessing and conserving high seas biodiversity. It is clear, however, that pelagic sharks and rays of
the open ocean are subject to high and often unrestricted levels of mortality from bycatch and
targeted fisheries for their meat and valuable fins.
2. These species exhibit a wide range of life-history characteristics, but many have relatively low
productivity and consequently relatively high intrinsic vulnerability to over-exploitation. The
IUCN}World Conservation Union Red List criteria were used to assess the global status of 21
oceanic pelagic shark and ray species.
3. Three-quarters (16) of these species are classified as Threatened or Near Threatened. Eleven
species are globally threatened with higher risk of extinction: the giant devilray is Endangered, ten
sharks are Vulnerable and a further five species are Near Threatened. Threat status depends on the
interaction between the demographic resilience of the species and intensity of fisheries exploitation.
4. Most threatened species, like the shortfin mako shark, have low population increase rates and
suffer high fishing mortality throughout their range. Species with a lower risk of extinction have
either fast, resilient life histories (e.g. pelagic stingray) or are species with slow, less resilient life
histories but subject to fisheries management (e.g. salmon shark).
5. Recommendations, including implementing and enforcing finning bans and catch limits, are
made to guide effective conservation and management of these sharks and rays.