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Foraging Ecology of White Sharks Carcharodon carcharias at Dyer Island, South Africa

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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; ...
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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. ...
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