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Encounters between white sharks and Cape fur seals in a shallow channel

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Encounters between white sharks and Cape fur seals in a shallow
channel
R. Johnson*, T. Keswick, M.N. Bester* and W.H. Oosthuizen
*Mammal Research Institute, Department of Zoology & Entomology, University of Pretoria, Pretoria 0002, South Africa. Department of
Zoology, University of the Western Cape, Private Bag X17, Belville 7535, South Africa. Branch Marine and Coastal Management, Department
of Environmental Affairs and Tourism, Private Bag X2, Roggebaai, South Africa. Corresponding author, e-mail: ryan@sampla.org
This communication presents observations of predatory and non-predatory encounters between
white sharks and Cape fur seals in a shallow (3–6 m) channel between Geyser Rock and Dyer Island,
South Africa. Within the channel Cape fur seals raft extensively for thermoregulatory purposes, to
play, or due to terrestrial competition for space. The channel’s physical environment effectively limits
a white shark’s approach orientation to the horizontal plane, thus inhibiting it to effectively utilize
depth, and associated stealth, to capture pinnipeds. In the absence of effective camouage, sharks
may patrol this area in search of unaware, incapacitated or dead seals. Here, predator mobbing
is a behavioral strategy adopted by Cape fur seals to lower predation risk. Specic benets of
mobbing may include: (a) perception advertisement to sharks; (b) intra-specic communication of
a shark’s locality; (c) driving the shark from the area; (d) increased vigilance; (e) advertisement of a
mobber’s good health to a shark; and (f) possibly learning about a predators behavioral capabilities
by inexperienced prey. Mobbing expression is further promoted by the channels shallow nature
which enhances a seals ability to visually detect the shark, and therefore makes it easier for a seal to
evade it (reducing immediate predation risk). This environment thus promotes the widespread use
of mobbing amongst Cape fur seals when confronted with a patrolling white shark.
INTRODUCTION
Predator-prey dynamics between white sharks (Carcharodon carcharias) and pinnipeds have gained
considerable attention in South Africa (Martin et al., 2005) and elsewhere (Ainley 1985; Klimley et
al., 1992, 1996; Anderson et al., 1996; Pyle, 1996). Strong (1996) hypothesized that a shark would
optimize its hunting efciency by initiating a high speed attack sequence from depth and distance
so as to maximize the element of surprise. Consistent with Strong’s hypothesis, Martin et al. (2005)
documented that over 85 percent of attacks on Cape fur seals (Arctocephalus pusillus pusillus) in
False Bay to the east of the Cape Peninsula, South Africa, consisted of an initial high speed breach
(Martin et al., 2005). Such observations highlighted the vulnerability of pinnipeds as they traverse
between offshore foraging grounds and terrestrial rookeries, particularly in close proximity to the
pinniped rookery where their presence is highly predictable (Rand, 1956), and sharks can effectively
adapt their habitat use to optimize the likelihood of capturing seals.
Besides traveling to and from foraging grounds, pinnipeds may also enter the water adjacent to
their colony for other purposes, including thermoregulation, play or competition for space within
the colony (Rand, 1967). Frequently, seals entering water for this purpose form groups and are
said to be ‘rafting’ (the act of several seals lying together at the surface with little or no directional
movement). Little attention has been paid to: (a) the predation risk of pinnipeds involved in this
activity; (b) the behaviour strategies adopted by pinnipeds to mitigate predation risk; or (c) the
behavioural response of white sharks to the presence of such seal activity. In this communication we
report on a series of observed behavioural interactions between white sharks and Cape fur seals
adjacent to Geyser Rock, South Africa.
MATERIALS AND METHODS
Geyser Rock (34°41'S 19°25'E) lies directly opposite Dyer Island (a marine seabird sanctuary), and
hosts an estimated 55,000 Cape fur seals (Davids, personal communication). A 230 m wide channel,
between 3–6 m deep, separates the islands. Cape fur seals commonly raft in this channel adjacent
to Geyser Rock (Figure 1). White sharks are seasonally abundant in the waters surrounding Geyser
Rock between April and November (Kock & Johnson, 2006).
On 135 days from November 1999–January 2001, for 4–6 hours a day (0800–1700 h), white
sharks were attracted to a vessel anchored within the channel, as part of research being undertaken
Keywords: white shark, Cape fur seal, mobbing, predation
R. Johnson et al. Encounters between white sharks and Cape fur seals
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by R.L. Johnson for his MSc thesis (2003). During the course of
this research, opportunistic sightings of interactions (predatory
and non-predatory) between white sharks and Cape fur seals
in the channel were recorded. Vertical water visibility was
recorded on an hourly basis using a seccie disk. If the channel
bottom was visible, then visibility was recorded as greater than
water depth.
RESULTS
A total of 11 successful attacks (SP), three unsuccessful attacks
(UP), and three scavenging events were witnessed (N=14, Figure
1). Seven attacks involved living seals which were, for various
reasons, stationary in the water at the time of the attack. The
attacks on the stationary seals included four (3 SP, 1 UP) that
appeared to be sleeping (i.e. oating vertical in water column
with head at surface), two YOY (youth of the year) weakly
ailing in the current (2 SP), and one apparently moribund sub
adult (1 SP). Another three attacks (other than on stationary
seals) consisted of two on seals that were porpoising, as part
of a group, across the channel mouth (1 SP, 1 UP), and one on
a seal within a rafting group adjacent to Geyser Rock (1 UP).
The circumstances of the remaining four attacks were unknown
as they were rst sighted after feeding had commenced (4 SP).
It is, however, improbable that these four attacks represented
scavenging as no oating carcasses were seen in the area prior
the attack (i.e. 4 SP). On two occasions, sharks were observed
prior to their initial strike. On 19 January a ailing YOY seal
oated past the research vessel. A ~250 cm total length (TL)
white shark accelerated slightly (compared to normal patrolling
speed) towards the seal and gripped the seal in its mouth. The
seal was carried ~10m before the shark bit down and began
consuming it. On 10 March a juvenile drifted out from Geyser
Rock and lay resting against the research vessel’s anchor rope.
A ~250 cm TL shark slowly approached and tentatively bit the
seal (investigatory bite). The seal awoke and displayed evasive
manoeuvers—leaping and diving in multiple directions—before
escaping beneath the water’s surface towards Geyser Rock
(Figure 2A).
Twenty-one encounters were recorded that did not consist
of any obvious attempt by the white shark to attack a seal, the
details of which are described in Table 1. During only one of
these encounters were seals observed to take ight. On that
occasion, a small group of rafting seals was seen to erupt out
of the water and escape towards Geyser Rock; moments later
a white shark was observed patrolling the area vacated by the
eeing seals. Most commonly, mobbing-type behaviour was the
initial response displayed by seals to a patrolling shark (Figure 2B).
This included ‘scooting’, dened as when a seal would leave its
rafting group and approach a shark along the sea-oor to within
one or two meters, harass the shark, then return immediately
towards the rafting group. This type of harassment was typically
directed at the shark from behind, with the seals approaching
the shark’s tail region. Although inter-specic contact was not
observed, its occurrence could not be excluded. Seals were also
observed to porpoise behind the shark in a distinctive manner,
whereby they would leap high out of the water with very little
directional velocity (high porpoising). The number of seals
involved in such an interaction would vary during the course
of the shark’s progress along Geyser Rock’s shoreline, because
just as some seals ceased mobbing the shark after it had passed
them by, others would encounter the shark and begin mobbing
Figure 1. An aerial view of the channel that lies between Dyer Island (north)
and Geyser Rock (south) (courtesy of Google earth). Approximate positions
of both normal rafting and outer rafting (OR) are indicated. The approximate
positions (based on land marks) of successful attacks (SP), unsuccessful attacks
(UP), scavenging events (SC) and con-specic alerting (CA) occurring within, or
near to, the channel are indicated.
Figure 2. (A) Unsuccessful attack by a ~250 cm white shark on a sleeping
sub-adult Cape fur seal. In background is Geyser Rock, with typical rafting seals;
(B) a patrolling white shark is being mobbed (combination of HA, SCO, HP
behaviours) by a group of Cape fur seals (swimming direction of white shark
indicated by arrow).
Encounters between white sharks and Cape fur seals R. Johnson et al.
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it. Seals of all size classes (excluding YOY) and of either sex participated in these mobbing events.
Following such mobbing encounters, the distribution of the seals in the channel would change, with
a number of seals rafting further away from Geyser Rock (outer rafting) compared with prior to
the shark’s arrival. Typically not all the seals would shift location, so that two distinct rafting groups
would be apparent, one close to Geyser Rock and the other one further out. The outer rafts
would persist for 10–30 minutes following the last sighting of a shark or of a mobbing event. On
two occasions during 7 April 2000, a sleeping YOY seal (most likely a different individual on each
occasion) was sighted drifting away from Geyser Rock. Around thirty minutes before the rst of
these two sightings (which were approximately 10 minutes apart), a shark had been observed
patrolling near Geyser Rock. On each occasion, four to six seals (non YOY) swam towards and
bumped the sleeping YOY (conspecics alerting); as soon as it was aroused, all the seals swam
towards Geyser Rock. During all non predatory shark-seal encounters (N=21), the vertical visibility
of the water exceeded the channel depth (i.e. the bottom was clearly visible).
DISCUSSION
Waters adjacent to pinniped rookeries are frequently associated with the presence of white
sharks. In South Africa, white sharks appear to target Cape fur seals as seals shuttle between feeding
grounds and their island rookery (Martin et al. 2005). Our results show that such predator-prey
dynamics are not limited to a white shark’s ability to ambush pinnipeds (see Strong, 1996 and Pyle
et al., 1996) while pinnipeds traverse to and from their feeding grounds. The channel environ of this
study offers little opportunity for white sharks to ambush Cape fur seals due to its depth (maximum
6 metres) and clear visibility, yet they successfully attack, and feed off, pinnipeds here. Of interest
was the fact that 90% (N=10) of fully witnessed successful attacks/scavenging were on either dead
(N=3) or stranded (N=6) seals. Although the channel may limit predation opportunities on alert
seals, sharks patrolling directly adjacent to Geyser Rock encounter other predation opportunities
i.e. non-alert seals (sleeping), or seals that are incapable of escape (stranded YOY, injured, moribund
or dead seals). The approach pattern of two sharks (slow without breaching) witnessed prior to
them attacking seals further suggests a conscious targeting of incapacitated or unaware seals, in
which camouage is not a precursor to success. In South Africa, ontogenetic shifts in the white shark
diet exist, in which smaller sharks are primarily piscivorous, whilst pinnipeds become increasingly
important in the diet as sharks grow (Cliff et al., 1996). Targeting incapacitated or dead pinnipeds
may offer smaller sharks opportunities to exploit this resource at a comparatively earlier life stage.
Date Start Finish WS CFS Behaviours observed
TL (cm) No. YOY SUB ADU PA FL HP HA SCO OR CA
6 April 13:32 13:33 10–20 Y Y Y Y Y
6 April 14:22 14:25 300 ~15 Y Y Y Y Y Y
6 April 15:10 15:12 400 15–20 Y Y Y Y Y Y
7 April 12:17 12:36 400 30–50 Y Y Y Y Y Y Y
7 April 13:06 13:12 400 30–50 Y Y Y Y Y Y Y
7 April 13:26 250 30–50 Y Y Y Y Y Y Y
7 April 13:34 13:35 400 30–50 Y Y Y Y Y Y
7 April 14:03 5–6 Y Y Y Y Y
7 April 14:31 5–6 Y Y Y Y Y
11 April 13:24 13:26 400 ~25 Y Y Y Y Y Y
11 April 13:34 13:35 400 ~15 Y Y Y Y Y Y
12 April 12:05 12:08 20–30 Y Y Y Y Y
12 April 12:14 12:16 20–40 Y Y Y Y Y
12 April 13:06 13:08 400 5–10 Y Y Y Y Y Y Y
12 April 13:13 13:18 400 5-10 Y Y Y Y Y Y Y
12 April 13:41 13:42 400 ~20 Y Y Y Y Y Y Y
12 April 13:46 13:47 ~20 Y Y Y Y Y
12 April 13:55 Y Y Y Y Y
26 June 12:14 12:15 350 5–15 Y Y Y Y Y Y
26 June 12:16 12:17 350 5–15 Y Y Y y Y Y Y
26 June 12:20 12:22 350 5–15 Y Y Y Y Y Y Y Y
PA, patrolling; FL, eeing; HP, high porpoising; HA, harassing; SCO, scooting; OR, outer rafting; CA, conspecic alerting.
Table 1. Non-predatory (no attempted attack) encounters between white sharks (WS) and Cape fur seals (CFS) within the channel.
Approximate sizes of groups varied considerably as seals would frequently join and leave the interaction as the shark patrolled along
the coastline. Only age classes specically observed were included, although other non-observed age classes may have been involved.
R. Johnson et al. Encounters between white sharks and Cape fur seals
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In addition, such opportunistic capture of pinnipeds may have been an evolutionary precursor to
the ambush hunting described elsewhere (Martin et al., 2005). Opportunistic capture of ‘non-alert’
seals may also have the additional advantage of possibly requiring less energy and having a higher
success rate than sharks capturing seals through ambush attack.
The presence of white sharks patrolling the channel can negatively affect a seals tness directly
(predation) or indirectly (predatory fear causing avoidance of optimal habitat). As such, behavioural
adaptations that lower predatory risk are an expected consequence. Within the channel environs,
Cape fur seals appear to utilize mobbing for this purpose. Stewardson & Brett (2000) rst identied
mobbing of white sharks by pinnipeds at Plettenberg bay in South Africa (a single seal mobbing a
single shark). Although recognizing its possible function in reducing predation risk, it was discussed,
among other possibilities, that the expression of this behaviour was linked to the physiological
state of the large bull seals (e.g. heightened androgen and aggression) during the breeding season.
A second report of a white shark being mobbed, this time by the Australian fur seal (Arctocephalus
pusillus doriferus) introduced multiple mobbers, although again the group was restricted to large
male seals (Kirkwood & Dickie, 2005). Such observations (single encounters and limited to a few
adult males) lend themselves towards the conclusion that mobbing of white sharks is a dangerous
practice as mobbers immediately face possible injury. Indeed, one of the ve mobbing seals was
bitten by the white shark during observations by Kirkwood & Dickie (2005). Alternatively, in this
study mobbing seems to be a common tactic amongst seals, performed by all age classes (barring
YOY) and both sexes, thereby implying that associated risks to mobbers in the environs of the
channel is comparatively less.
Optimum environmental conditions (e.g. murky, eutrophic water) were cited as possible factors
enhancing the ability of white sharks to stay camouaged, thereby increasing the possibility of a
successful attack (Pyle et al., 1996). White sharks typically hunt in three dimensional habitats, in which
the vertical dimension (approaching from depth) is frequently used to successfully attack pinnipeds
(Strong, 1996). The channel is shallow (3–6 m depth), and combined with water visibility in excess
of depth, the channel is effectively reduced to a two dimensional habitat, in which sharks are unable
to utilize depth to surprise rafting seals. Channel depth and clarity may also affect the number of
mobbers participating. The advantages of ‘multiple mobbers’ participating in predator mobbing have
been examined in mammals (Tamura, 1989) and birds (Flasskamp, 1994). When joined by other
individuals, mobbing was intensied and prolonged, thereby suggesting that the predator is affected by
the number of mobbers (Ostreiher, 2003). We hypothesise that the favourable physical environment of
the channel signicantly reduces predation risk to a mobber, and as such facilitates regular participation
by large numbers of fur seals (increased effectiveness) when encountering a shark.
Mobbing as an anti-predation strategy arises when the costs in immediate predation risk, are
outweighed by the benets to the individuals tness (overall predation risk, social standing etc.) or to
its inclusive tness (e.g. reduced predation risk for kin; Hamilton, 1964). Specically, the components
of mobbing behaviour can potentially full a number of functions that benet the mobber’s tness.
Through scooting and harassing, the mobbers (seals) communicate their awareness of the shark’s
presence, and make the shark cognizant that the element of surprise is lost (i.e. the ‘perception
advertisement’ hypothesis, Frankenberg, 1981). Mobbers may communicate intra-specically via
‘high porpoising’ (deliberate directional swimming while porpoising at an exaggerated height) the
‘presence’ and ‘whereabouts’ of a predator to ignorant con-specics. In addition, high porpoising
may advertise to a patrolling shark the futility of attempting to attack such a ‘healthy’ seal, in the
same manner that energetic ‘stotting’ of gazelles may dissuade predators such as wild dogs (Lycaon
pictus) (Zahavi & Zahavi, 1997). Predator absence is better than a present but discouraged predator.
Harassment by seals may assist in driving the shark from the immediate rafting area, similar to
small birds mobbing owls (the ‘move on’ hypothesis, Flasskamp, 1994). Mobbing can, of course, be
innately selsh. The opportunity for seals to interact with one of their major predators, but in an
environment that effectively shackles that predator, could provide an important learning experience,
teaching individuals about predator avoidance.
The establishment of ‘outer rafts’ following sighting of a patrolling shark increases the collective
vigilance of rafting seals in what is effectively a two dimensional habitat (depth being removed).
Indeed, the establishment of outer rafts by seals may be a function of the benign environment of the
channel. The channel’s shallow, clear water promotes vigilance as a method of predator avoidance
as well as being conducive to outer rafting—it is easier for seals to outer raft in a sheltered area. It
remains unknown why outer rafting is not a permanent feature of the seal behaviour in the channel,
but it is possible that unrecognised costs associated with this behaviour exist.
Alerting YOY conspecics, via bumping, was conspicuous communication behaviour. Barlow (1972,
1974) described Galapagos sea lions Zalophus californianus wollebaeki interacting with seal pups in
the presence of Galapagos sharks Carcharhinus galapagensis (apparently to warn them), for which
there may be several explanations. Paternalism is possible though unlikely, as male pinnipeds are not
Encounters between white sharks and Cape fur seals R. Johnson et al.
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necessarily predisposed to caring for pups—they often squash them (Miller, 1974). However, such
apparent altruistic behaviour may arise if the inclusive tness benets outweigh the risks incurred.
Such benets would arise if a previous alerter, or its kin, was subsequently alerted to a predator’s
presence by one or more unrelated con-specics. Curiosity could also prompt a seal to bump a
YOY. The bumping of an object is advantageous to an individual if such bumping reveals a benet,
e.g. food.
This work formed part of a larger study funded by the Department of Environmental Affairs and Tourism
(DEAT), the University of Pretoria, Western Cape Nature Conservation Board and the International Association
of Impact Assessment – South African afliate. Permission to work from Dyer Island, assistance, and the use
of facilities by Western Cape Nature Conservation Board and Tony Venter (Dyer Island headman). Logistical
support by Marine Dynamics (supplied the research vessel). Others who assisted in this research are Deon
Sadie (formerly University of Stellenbosch) Mike Meyër (DEAT), Stephan Swanson (formerly DEAT), Deon
Kotze (DEAT), Mike Patterson (formerly DEAT), Michael C. Scholl (University of Cape Town) and Lezanne
Brits (SAMPLA).
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Submitted 5 February 2008. Accepted 10 September 2008.
... Prey may also respond to predation risk by temporally shifting activity to a safer period, such as times when predators are less active or not present (Lima & Bednekoff 1999, Turner & Montgomery 2003, Brown & Kotler 2004). It has been suggested that Cape fur seals Arctocephalus pusillus pusillus employ all 3 of the above strategies to avoid predation from patrolling white sharks Carcharodon carcharias while traversing between their colonies and their offshore feeding grounds (Hammerschlag et al. 2006, Laroche et al. 2008, Johnson et al. 2009b. Cape fur seals typically colonise small, rocky, offshore islands as well as mainland sites along the southern African coastline (David et al. 1986). ...
... During these periods of heightened foraging behaviour of lactating females and naïve pups entering the water for the first time, white sharks are consistently patrolling inshore waters, particularly within a few hundred metres of Cape fur seal colonies (Martin et al. 2005, Hammerschlag et al. 2006, Laroche et al. 2008. To avoid predation during foraging trips, Cape fur seals have been reported to use predator avoidance strategies such as grouping (safety in numbers), and diving to avoid vulnerability at the surface (Laroche et al. 2008), as well as mobbing behaviour (Johnson et al. 2009b). It has also been suggested that Cape fur seals may limit their traversing periods to particular times of day when ambient light conditions might reduce their risk of being predated (Hammerschlag et al. 2006, Laroche et al. 2008. ...
... A moonlit sky may possibly provide ambient light conditions similar to that experienced during crepuscular periods where the sharks have a visual advantage over their prey and success rate of their attacks may be greater (Hammerschlag et al. 2006). Shoaling behaviour, or seeking 'safety in numbers' is a welldocumented predator avoidance behaviour among marine prey species (Hager & Helfman 1991, Brown et al. 2001, White & Warner 2007, De Vos & O'Riain 2013, and in the case of Cape fur seals has also been observed to enable mobbing behaviour to fend off hunting white sharks (Johnson et al. 2009b). If Cape fur seals are at a greater risk of predation from white sharks during moonlit nights, then this predator avoidance strategy might explain the covariance of foraging group sizes and moonlight observed in this study. ...
Article
Full-text available
White sharks (Carcharodon carcharias) are highly visual predators, leading to the hypothesis that the predation risk for foraging Cape fur seals (Arctocephalus pusillus pusillus) might differ with ambient light conditions. This study investigated the relationship between environmental fluctuations of ambient light and the traversing behaviour of Cape fur seals in and out of their colony at Mossel Bay, South Africa to better describe potential predator avoidance strategies. A total of 12,144 traversing events were observed over a four-year period and there was an overall trend for Cape fur seals to traverse less often but in relatively larger group sizes during periods when white sharks are suggested to be more active. Specifically, Cape fur seal activity was reduced during winter when white sharks are most actively hunting, and most traversing behaviour occurred at night when Cape fur seals were less likely to be detected by white sharks. However, among nocturnal observations Cape fur seal group sizes increased significantly with moonlight. Although nocturnal predations of Cape fur seals by white sharks have been observed before in Mossel Bay, this is the first study to indicate Cape fur seals might respond to the increased risk of improved white shark visual acuity during moonlit nights by seeking safety in numbers while foraging. Further investigations are needed to assess the effect of the lunar cycle on white shark nocturnal hunting behaviour, but observations presented here suggest that white sharks may pose a bigger threat to Cape fur seals under the light of a full moon.
... The great sharks are opportunistic carnivores, which means they will attack and eat when odds are in their favor, and can palate a large variety if necessary in order to survive (Hobson 1963, Johnson et al. 2008. Other sources of food include bigger fishes such as tuna, other smaller sharks, whale carcasses, and rays. ...
... Other sources of food include bigger fishes such as tuna, other smaller sharks, whale carcasses, and rays. In cases of low food sources, sharks will prey on marine birds (Johnson et al. 2008). ...
... We need to hold the current shark populations long enough to gather more information about their importance before these species are eliminated and lost forever (Chappel et al. 2011). The subject of the research ranges from behavioral studies (Bonfil et al. 2005, Frisk et al. 2001, Johnson et al. 2008) to ecosystem roles (Baum et al. 2003, Myers et al. 2007, Ritchie et al. 2012, Sergio et al. 2008, and must incorporate all aspects of the sharks' biology and environment to be accurate. ...
Thesis
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Secondary Article reviewing the possible effects of removing of apex predator in the marine environment.
... During this time, seals become highly territorial both on land and nearby stretches of water (Bester 1990, Stewardson & Brett 2000. Bull seals experience elevated levels of androgens during breeding and their concurrent aggression triggers them to challenge anything that the bull perceives as a threat to its territory including humans, white sharks, conspecifics, or even seagulls (Stewardson & Brett 2000, Johnson 2009a, pers. obs). ...
... Adult females in particular must travel from the colony to forage offshore, returning to rest or to nurse their young (David 1987, LaRoche et al. 2008. Besides traveling to and from offshore foraging grounds, seals enter the water for other purposes such as thermoregulation, and social play, or as a result of competition for space on the island (Rand 1967, Johnson et al. 2009a). The prey of Cape fur seals is primarily schooling fish that seldom occur adjacent to seal colonies, thus seals must travel from their colonies to seek nutrient-rich patches. ...
... Juveniles feed primarily on other fish species with adults switching to marine mammals in coastal waters and cephalopods in deeper waters (Cliff et al. 1989, Compagno 2001, Smale & Cliff 2012. At Geyser Rock, white sharks are known to regularly predate on Cape fur seals during the winter months (Johnson et al. 2009a). One documented case of feeding on other marine mammals has been recorded in the area (Indo-Pacific humpback dolphin (Sousa chinensis), Wcisel et al. 2010), and various seabirds ). ...
Thesis
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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.
... White shark/Cape fur seal interactions at Seal Island have been the focus of many studies that have variously documented aspects of the spatial and temporal patterns of predation (Martin et al. 2005;Hammerschlag et al. 2006;LaRoche et al. 2008;Martin et al. 2009;de Vos 2010;de Vos and O'Riain 2010;Fallows et al. 2012;Martin and Hammerschlag 2012;Hammerschlag et al. 2012;Kock et al. 2013;Kock 2014). By comparison, only one study has attempted to quantify white shark predation risk at Geyser Rock (Johnson et al. 2009). The focus of this study was thus to first establish the seasonal patterns of white shark presence and predations around Geyser Rock and the spatiotemporal movement patterns by Cape fur seals to and from the island as in the study of de Vos (2010). ...
... Geyser Rock is a small (20 ha) rocky island lying approximately 260 m south of Dyer Island and supports an estimated 60,000 Cape fur seals throughout the year (Department of Environmental Affairs unpublished data). Geyser Rock and Dyer Island are separated by a narrow (260 m) and shallow (<6 m) channel known as 'Shark Alley' (Johnson et al. 2009), and the island system is characterized by a diversity of features, including reefs, ship wrecks, kelp forests, rock pinnacles and relatively shallow (5-20 m depth) waters. ...
... Although the shallow water does not allow for surface breach attacks, predations do still occur in both this area (Hammerschlag et al. 2006; AA Kock pers. comm.) and 'Shark Alley' at Geyser Rock (Johnson et al. 2009;Jewell et al. 2014). Thus, shallow areas are still risky, and this may explain why adult seals avoid these areas at both islands during winter when shark presence is high. ...
Article
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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.
... White shark (Carcharodon carcharias) predation on these colonies is well documented. [1][2][3] It therefore follows that there should be evidence of similar behaviour in the geological past. Diedrich 4 showed that the correlation between the presence of seal fossils and abundance of shark teeth in the Proto North Sea may indicate the earliest specialisation for seal hunting in the fossil record in the northern hemisphere. ...
... From the middle of the bone to the medial border below the broken edge of the shaft, there are six bites caused by the shark biting down but not penetrating the bone causing only surface damage (CF1a; Figure 3b). Two bites -one towards the lateral surface and the other towards the medial surfaceshow evidence of the shark biting down and leaving grooves with ridges and grooves inside (CF1b, Figure 3c, d [1][2][3] In the vicinity of Langebaanweg, 'E' Quarry, offshore islands and a protected lagoon that was open to the ocean were present 5 Ma. 11,18 The islands were surrounded by shallow water 23 , making this ideal for haul out and rookeries. ...
Article
A study of the Mio-Pliocene marine palaeoenvironment on South Africa’s west coast revealed aspects of the biology and behaviour of fossil marine mammals. Close examination showed that seals from Langebaanweg suffered from pathologies and bore marks of marine carnivore activity. This study adds to our knowledge of shark feeding behaviour in the geological past and is one of a few studies of sharks feeding on seals in the fossil record. Two incomplete seal humeri with shark tooth marks are the first documented evidence from South Africa’s Mio-Pliocene of such behaviour. These injuries show no healing, which suggests that the animals were most likely scavenged. Significance: • Fossil rich deposits at Langebaanweg contribute to the knowledge of South African Mio-Pliocene fossils by placing them in a global context. • This study is one of five globally that have documented marine carnivores feeding on seals. • This is the first description of white sharks feeding on seals from South Africa’s geological past. • This study shows this behaviour was in place on South Africa’s coast as early as 5 million years ago. • The injuries show no signs of healing, suggesting the most parsimonious explanation is that white sharks were scavenging seal carcasses.
... Baserat på att populationsstudier (Burgess et al. 2014, Towner et al. 2013, Cliff et al. 1996 utgör antalet individer vithaj enbart 1,2% av antalet individer fengångare och sälar (Johnson et al. 2009, Lowry et al. 2008, Lowry et al. 2014) efter att en kvot mellan predator och byte togs fram. Enbart 30% av populationen vithaj jagar marina däggdjur (Curtis et al. 2014). ...
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The white shark Carcharodon carcharias, is a philopatric endotermic lamnoid (Lamniformes) with one of the largest distributions among marine fishes. The white shark is an oceanodromic pelagic fish capable of migration from South Africa to Australia. In conjuction to the newly discovered oceanodromic migration behaviour and global warming there have been public speculation regarding the potential occurrence of white sharks in Northen Europe. This review article discusses the limits of the white shark distribution in a physiological and ecological context. A new linear model based on endothermic capacity of lamnids (Lamnidae) is used to determine the distribution using Sea Surface Temperature (SST). Results show a 12 °C threshold for 70 % of the white
... However reproduction is unlikely to be the cause of both sexes co-occurring at Gansbaai, because most female white sharks during this study were too small to be sexually mature (Table 3), [57] and they did not exhibit fresh bites and scarring typical of mating behaviour [58]. Predation by white sharks on Cape fur seals Arctocephalus pusillus has been observed at Dyer Island, especially from May to September (autumn and winter) [52,59606162, (AVT unpubl. data). ...
Article
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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.
Article
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The seasonal occurrence and temporal sexual segregation of great white sharks Carcharodon carcharias have been widely documented in various temperate and sub-tropical waters across the globe. Yet, there is limited understanding of the relationship between the life stages and habitat use of C. carcharias , particularly in the Southern Cape. In this study, we investigated the population dynamics of C. carcharias in Mossel Bay, South Africa, between 2009 and 2013, using skipper logbooks and citizen research data obtained by a cage-diving vessel. A total of 3064 sharks, ranging in life history stages from young-of-the-year to subadult, were sighted during 573 trips. Juveniles dominated the sightings throughout the study, and there was marked sexual segregation, with females dominating the total sightings of sharks. C. carcharias were most abundant during the cooler, winter season, with females differing in abundance seasonally and males maintaining a low abundance throughout the year but peaking in the winter. In addition, sea surface temperature was the best indicator of C. carcharias presence. Abundance was greatest when vertical water visibility exceeded 3 m, with cloud cover influencing overall abundance negatively. Likely reasoning for the aggregation of C. carcharias in Mossel Bay includes the favourable conditions and abundance of food. Juvenile sharks may also utilise this area as a training ground to learn from larger conspecifics. This research demonstrates that information on population size and structure of C. carcharias can be obtained effectively through a compilation of logbook and citizen science data to assess and identify potential critical habitats in the quest to develop appropriate management strategies. This research also shows value in commercial cage-diving operations deriving international data sets needed to assess global populations of C. carcharias .
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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.
Article
This is a report of a marine predator (the white shark) being threatened by a member of the species on which it preys (a male Cape fur seal). Although these events may be rarely observed or occur infrequently, they may have important implications for the predator and its prey. We suggest that shark mobbing by adult male Cape fur seals is adaptive for the reduction of risk of predation by sharks. Mobbing of sharks is likely to alert conspecifics to the presence of a predator, and/or reduce the shark's hunting motivation near the mobbing site.
Article
Spatial and temporal records of 146 predatory attacks by white sharks (Carcharodon carcharias) on four species of pinnipeds, one bird, and one human at the South Farallon Islands, Central California, from late Aug. to early Dec. 1986-89 are presented. During each 3.5-mo period, attacks were (1) unevenly distributed in bouts separated by hiatuses in predation, (2) paired temporally within the same day, (3) at similar times and locations on consecutive days, and (4) all during daylight hours. Predation was observed most often within 450 m of shore, with a decrease in attack frequency with increasing depth. Within this high-risk zone, predation was concentrated near coastal departure and entry points of pinnipeds, and the predatory attack positions formed linear patterns leading away from the island. Consecutive predatory attacks were often near each other, yet at times alternated between localities on either side of the island.