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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 camouage, 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. Specic benets of
mobbing may include: (a) perception advertisement to sharks; (b) intra-specic 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 efciency 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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2
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’, dened 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-specic 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-specic 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 (conspecics 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 camouage 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, conspecic 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 specically 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 identied
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 camouaged, 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 intensied 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 signicantly 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 benets to the individuals tness (overall predation risk, social standing etc.) or to
its inclusive tness (e.g. reduced predation risk for kin; Hamilton, 1964). Specically, the components
of mobbing behaviour can potentially full a number of functions that benet 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-specically via
‘high porpoising’ (deliberate directional swimming while porpoising at an exaggerated height) the
‘presence’ and ‘whereabouts’ of a predator to ignorant con-specics. 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 selsh. 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 conspecics, 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.
JMBA2 - Biodiversity Records
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5
necessarily predisposed to caring for pups—they often squash them (Miller, 1974). However, such
apparent altruistic behaviour may arise if the inclusive tness benets outweigh the risks incurred.
Such benets would arise if a previous alerter, or its kin, was subsequently alerted to a predator’s
presence by one or more unrelated con-specics. 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 benet,
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 afliate. 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.
