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Seabird predation by white shark, Carcharodon carcharias, and Cape fur seal, Arctocephalus pusillus pusillus, at Dyer Island

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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.
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Seabird predation by white shark
and Cape fur
seal at
Dyer Island
R.L. Johnson1*, A. Venter2, M.N. Bester1& W.H. Oosthuizen3
1Mammal Research Institute, Department of Zoology & Entomology, University of Pretoria, Pretoria, 0002 South Africa
2Western Cape Nature Conservation Board, Private Bag X13, Hermanus, 7200 South Africa
3Branch Marine and Coastal Management, Department of Environmental Affairs and Tourism,
Private Bag X2, Roggebaai, South Africa
Received 31 August 2004. Accepted 10 November 2005
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), anti-
predator behaviour by penguins, and seabirds not being a preferred prey type. Cape fur seals
were more conspicuous seabird predators, eachyear 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.
Key words: African penguin, Cape fur seal, cormorants, Dyer Island, predation, white shark.
INTRODUCTION
Many South African seabird populations are
declining due to various threats (Barnes 2000).
Both human-induced factors such as oil pollution,
egg harvesting, human disturbance (Hockey &
Hallinan 1981; Shelton
et al.
1984; Crawford
et al.
2000) and natural effects, including predation,
interspecific competition for nesting sites, modifi-
cation to fish stocks (Crawford
et al.
1989, 1990;
Crawford & Dyer 1995; Marks
et al.
1997; David
et al.
2003) have been implicated as possible
contributors to seabird declines. Quantitative
accounts of seabird predation are rare (
e.g.
Du Toit
et al.
2004), and therefore the relative importance
of predatory attacks in population declines is
currently unknown.
A number of apex predators attack, feed on or
induce anti-predator behaviour in seabirds in
southern Africa including the Cape fur seal
(Shaughnessy 1978; Broni 1985; Marks
et al.
1997; David
et al.
2003), the white shark (Randall
et al.
1988) and the killer whale (
Orcinus orca
)
(Randall & Randall 1990). The Cape fur seal is a
most conspicuous seabird predator and has been
recorded hunting and feeding on gannets,
Morus
capensis
(Du Toit 2002; David
et al.
2003), cormo-
rants,
Phalacrocorax
spp. (Marks
et al.
1997) and
penguins (Shaughnessy 1978; Crawford
et al.
2001; Du Toit 2002). Up to 7.1% of the fledgling
population of Cape cormorants at Dyer Island may
fall victim to seals annually (Marks
et al.
1997),
while Du Toit (2002) estimated that 0.9% of the
African penguin population on Ichaboe Island,
Namibia, succumbed to seal predation. David
et al.
(2003) calculated that 7.4% of the fledgling
Cape gannet population at Malgas Island, South
Africa, was killed by Cape fur seals during the
South African Journal of Wildlife Research 36(1): 00–00 (April 2006)
*E-mail: johnson@maxitec.co.za
November 2000 – March 2001 breeding season.
These estimates are large enough to warrant more
attention to such attacks.
Southern Africa is one of the centres of abundance
for the white shark (Compagno 1991) where it
ranges from southern Mozambique to Namibia.
White sharks were responsible for a majority of the
injured and dead African penguins recovered from
the shoreline of St Croix Island and Bird Island,
Algoa Bay, South Africa (Randall
et al.
1988).
However, only a single African penguin has been
recovered from a white shark stomach (Bass
et al.
1975).
Resident birds on Dyer Island identified as
vulnerable to attack by either the Cape fur seal
or white shark include the African penguin
(Shaughnessy 1978; Randall
et al.
1988) and the
four cormorant species (Marks
et al.
1997; Du Toit
2002; David
et. al.
2003; Crawford 2005a–d). The
African penguin is the only endemic penguin in
southern Africa, and is listed as ‘Vulnerable’by the
World Conservation Union (IUCN) due to a persis-
tent population decline (Crawford
et al.
1990;
Barnes 2000). The IUCN further classifies the
endemic crowned cormorant as ‘Near-threatened’
because of its small population size (Barnes
2000). The endemic bank cormorant is currently
classed as ‘Endangered’ due to ongoing popula-
tion declines (Crawford 2005c). The Cape cormo-
rant is classed as ‘Near-threatened’ due to a
population decrease from 277 000 pairs to
c.
72 000 pairs between 1977 and 1996 (Barnes
2000). The white-breasted cormorant is not
threatened in South Africa (Barnes 2000).
Most research efforts have been directed at
single predator-prey interactions, despite the need
for ecosystem based management (David
et. al.
2003). The diversity and abundance of predator
and prey species occurring within the Dyer Island
area enable an unprecedented array of interspecific
predatory interactions to be documented and
investigated.The aim of this paper is to determine
the comparative importance of seabird predation
by white sharks and Cape fur seals on the resident
seabird populations at Dyer Island.
STUDY AREA
The Dyer Island complex (34°41S; 19°25E) lies
off the Western Cape Province of South Africa,
and consists of two islands (Fig. 1). Dyer Island is
the largest with a surface area of 20 ha and is ex-
tensively inhabited by seabirds; Geyser Rock is
the smaller island, lying 230 m southwest of Dyer
Island. Geyser Rock is host to an estimated 55 000
2 South African Journal of Wildlife Research Vol. 36, No. 1, April 2006
Fig. 1. Map of the Dyer Island study site with zonation illustrated. Also indicated is the location of three attempted
attacks by white sharks on kelp gulls.
Cape fur seals (J.H.M. David, pers. comm.). Dyer
Island was subdivided into six zones to facilitate
the present study (Fig. 1) as outlined below.
METHODS AND MATERIALS
Predatory encounters were investigated through
opportunistic observation of attacks, collection of
bird carcasses and observation and/or collection
of injured seabirds between August 1999 and
February 2001. Daily searches for injured birds
and washed-up seabird carcasses in the intertidal
zone were carried out during a circumperambulation
of Dyer Island. All seabird carcasses discovered
were removed, examined and buried. The taxa
(species level), date, location (zone), age class,
sex (for sexually dimorphic species only) and
stage of decomposition (fresh vs skeletal remains)
were recorded postmortem. Where applicable,
injuries on the carcasses were described, and
when possible the predator identified. Depending
on the severity of the injury, and the degree of
disturbance the collection of data would cause,
injured seabirds were both collected for examina-
tion and treated, or simply examined in the field
and released. During examination the wound
characteristics were described, and when possi-
ble the predator identified. Each victim’s location
(zone), age-class, sex and the wound sever-
ity (superficial, serious, mortal) were routinely
recorded.
Live attacks were observed opportunistically
from various vantage points, including an observa-
tion tower (height 4 m), within the living compound,
during daily circum-island research patrols, and
from a research vessel anchored at various
locations in the near vicinity of the island (Fig. 1).
Predation cues included: splashing; hovering kelp
gulls (
Larus dominicanus
); a splashing seal, or
the formation of an oil slick as a result of natural
internal oils emanating from a disembowelled
seabird. The predator and prey species involved
(cormorants to genus level only) were recorded,
together with the time, location of the event, local
environmental conditions, and a description of the
encounter.
Moulting and breeding population trends in the
African penguin and four cormorant species were
established by fortnightly counts of moulting
penguins, and a monthly nest count of all birds
(Crawford & Boonstra 1993). A total of 34 counts
of moulting African penguins were completed
between August 1999 and January 2001. Moult
counts (adults only) conducted between November
1999 and October 2000, December 1999 and
November 2000, and January 2000 to December
2000 were summed independently and averaged
to estimate the breeding population size on Dyer
Island conservatively. Monthly nest counts of
all occupied cormorant nests, defined as sites
defended by adult birds or sites with eggs or
chicks, were carried out. Peak nest counts of
the Cape, bank, crowned and white-breasted
cormorant species were multiplied by two (breed-
ing pairs) to estimate the numbers of cormorants
at Dyer Island conservatively.
The predatory impact of Cape fur seals on the
African penguin population was taken as the
percentage of the adult population (established by
moult counts) killed between January 2000 and
December 2000. The fledgling population of Cape
cormorants was calculated by multiplying the peak
nest count by 2.36 (average eggs/nest) (Crawford
1992). The survival coefficients between eggs and
chicks (0.87) and chicks to fledglings (0.91) were
then used to establish the fledgling population
(Crawford 2005d).The annual impact of predators
on other cormorant species was taken as the
percentage of the population (established by peak
nest count) killed during the corresponding 12
months (January–December 2000). For each
analysis, the lower minimum estimate includes
only carcasses of birds definitely killed by Cape fur
seals, while the upper minimum includes skeletal
remains that could have come from seabirds killed
by fur seals. Both estimates are conservative, as
unknown proportions of carcasses do not wash up
on Dyer Island. Diel and seasonal trends in attack
frequency were tested using analysis of frequency
tests (χ2test), with significance set at the 5% level.
RESULTS
Seabird carcasses
Between August 1999 and January 2001 a total
of 812 seabird carcasses were collected (Table 1).
Of these the African penguin, Cape cormorant,
bank cormorant, crowned cormorant, white-
breasted cormorant and Cape gannet showed
evidence of predation.Predation by Cape fur seals
evidently caused the death of 71.6% of the birds,
while no carcasses displayed evidence of a white
shark bite. Approximately 7.5% of the carcasses
showed no evidence of predation, while advanced
decomposition prevented the establishment of the
cause of death in the remaining 20.9% of car-
casses. Some 87.7% of the corpses that displayed
Johnson
et al.
: Seabird predation by white shark and Cape fur seal at Dyer Island 3
Cape fur seal inflicted wounds were ‘degloved’ as
defined by Marks
et al.
(1997). Abdomen and neck
bites accounted for the remaining 5.2% and 7.1%
of injury to carcasses, respectively.
Injured seabirds
Injured seabirds included the African penguin
and Cape cormorant. Excluding birds contami-
nated by oil, 26 of 31 birds were African penguins,
while the remainder were Cape cormorants.
Predation was adjudged the cause of injury in only
10 birds, with three Cape cormorants and five
penguins sporting wounds inflicted by Cape fur
seals. Two penguins showed evidence of white
shark bites, the only evidence of white sharks
attacking African penguins in this study (Fig. 2a,b).
Live attacks
A total of 204 attacks by Cape fur seals on
African penguins and cormorant spp. were
observed. Of these 46.6% involved the African
penguin, 38.7% a cormorant sp., and in the
remaining 14.7% of attacks the victim was not
identified (Table 2). Additionally, three encounters
between white sharks and kelp gulls were observed
(Table 2). White sharks attacked floating gulls
twice within the channel area, adjacent to Geyser
Rock (attacks B and C, Fig. 1). During attack ‘B’
the white shark successfully hit, and killed, the gull
4 South African Journal of Wildlife Research Vol. 36, No. 1, April 2006
Tab le 1 . Cause of death in the various species of seabird carcasses collected from the intertidal region of Dyer Island
between September 1999 and January 2001.
Species Cause unknown No. injuries C.F.S. wounds W.S. wounds Total no. of
apparent present present carcasses
African penguin 45 (21.0%) 15 (07.0%) 154 (72.0%) 0 (00.0%) 214
Cape cormorant 115 (17.4%) 42 (11.3%) 396 (71.6%) 0 (00.0%) 553
Bank cormorant 0 (00.0%) 0 (00.0%) 1 (100%) 0 (00.0%) 1
Crowned cormorant 1 (07.7%) 3 (23.1%) 9 (69.2%) 0 (00.0%) 13
White-breasted cormorant 3 (12.5%) 1 (04.2%) 20 (83.3%) 0 (00.0%) 24
Gannet 0 (00.0%) 0 (00.0%) 1 (100%) 0 (00.0%) 1
Cormorant sp. 6 (100%) 0 (00.0%) 0 (00.0%) 0 (00.0%) 6
C.F.S. = Cape fur seal, W.S. = white shark.
Fig. 2. White shark-inflicted wounds on African penguins; a, a single elongated puncture is visible on the dorsal
surface; b, multiple punctures to the body cavity characterize this bite.
Tab le 2 . Live attacks by various predators on seabirds
occurring at Dyer Island. ‘Unsuccessful attempt’ repre-
sents all occasions when predator(s) made an unambig-
uous attempt to attack potential prey but failed.
White shark Cape fur seal
Successful attacks
African penguin 0 92
Cormorant sp. 0 79
Kelp gull 1 0
Unknown 0 32
Unsuccessful attacks
African penguin 0 0
Cormorant sp. 0 1
Kelp gull 2 0
Unknown 0 0
but did not return to feed on it. The third encounter
(attack A) was a breach from a white shark of
c.
380 cm total length towards two hovering gulls
that failed to make contact.
Seasonal trends in Cape fur seal attacks
Attacks on African penguins by Cape fur seals
were seasonal, with few penguin carcasses
collected during the early (October to December)
and late (January to March) summer periods
(χ2
(0.05,3) = 44.2,
P
< 0.01) (Fig. 3a). Predation on
Cape cormorants by Cape fur seals showed a sig-
nificant seasonal pattern (χ2
(0.05,3) = 232.3,
P
<
0.001) with large numbers of carcasses collected
in the early and late summer periods in both 1999
and 2000 (Fig. 3a). White-breasted cormorant car-
casses appeared aseasonally (χ2
(0.05,3) = 1.87,
P
>
0.50) (Fig. 4a). The recovery of crowned cormo-
Johnson
et al.
: Seabird predation by white shark and Cape fur seal at Dyer Island 5
Fig. 3. Seasonal trends in seabird predation at Dyer Island; a, collected seabird carcasses; b, observed attacks by
Cape fur seals.
Fig. 4. Seasonal patterns of carcass collection for three species of cormorants resident on Dyer Island; a, white-
breasted cormorant; b, crowned cormorant; c, bank cormorant.
rant carcasses was seasonal (χ2
(0.05,3) = 8.4,
P
<
0.05) with 92% of the carcasses collected in the
early (April to June) and late (July to September)
winter period (Fig. 4b). Only one carcass of a bank
cormorant was recovered in October 1999
(Fig. 4c). The seasonal trends in attacks by Cape
fur seals on seabirds mimicked trends of carcass
recovery (Fig. 3b). Live attacks on African penguins
were seasonal, with few attacks seen during the
late summer period (χ2
(0.05,3) = 23.3,
P
< 0.001). The
majority of the attacks observed involving cormo-
rant spp. occurred in early summer (χ2
(0.05,3) = 27.3,
P
< 0.001) (Fig. 3b).
Population trends of seabirds
The African penguin is a seasonal breeder at
Dyer Island, with peak nest counts recorded
between March and August (Fig. 5a), while the
peak moulting period is between October and
December (Fig. 5a). Nest counts of Cape cormo-
rants showed that in 1999 nesting began in earnest
during September, while in 2000 it began slightly
earlier in August (Fig. 5b).
Diurnal trends in Cape fur seal attacks
Significant diurnal trends in the minimum rate of
live attacks existed for both penguins (χ2
(0.005,13) =
71.13,
P
< 0.01) and cormorants (χ2
(0.005,13) = 24.21,
P
< 0.05) (Fig. 6). Differences existed in the diurnal
patterns of attack on penguin and cormorants (2 ×
14 contingency table, χ2
(0.005,13) = 45.25,
P
< 0.01).
Crepuscular peaks in penguin attacks were evi-
dent, while attacks on cormorants increased
throughout the morning period, and peaked at
around 13:00, before trailing off (Fig. 6b).
Population trends of seabirds
The African penguin is a seasonal breeder at
Dyer Island, with peak nest counts recorded
between March and August (Fig. 5a), while the
peak moulting period is between October and
December (Fig. 5a). Nest counts of Cape cormo-
rants showed that in 1999 nesting began in earnest
during September, while in 2000 it began slightly
earlier in August (Fig. 5b).
Importance of predation in seabird mortality
The African penguin breeding population was
estimated at 5081 birds. During the corresponding
12 months (Jan 2000 – Dec 2000) a total of 134
penguin carcasses were collected of which 101
showed evidence of Cape fur seal attack. The
remaining 33 birds were either too decomposed
for cause of death to be established (27 birds), or
no injuries were apparent (six birds). Therefore, a
lower minimum of 1.99% (and an upper minimum
of 2.52%) of the breeding population died from
Cape fur seal attacks during this period. White-
breasted cormorants were the most vulnerable
cormorant species to seal attack, which resulted in
6 South African Journal of Wildlife Research Vol. 36, No. 1, April 2006
Fig. 5. Nesting and moulting trends of seabirds at Dyer Island;a, African penguins; b, Cape cormorants.
the death of at least 5.72% of the population
(Table 3). Crowned cormorants (3.13%) appear
less affected and bank cormorants virtually unaf-
fected (Table 3). The peak nest count of Cape
cormorants (18 105) translates to a fledgling popu-
lation of 33 827 fledglings during the 2000/
2001-year. A lower minimum of 280 and an upper
minimum of 370 fledgling Cape cormorants were
killed by Cape fur seals during the corresponding
period (Janoury 2000 December 2000). This
suggests an annual minimum predation rate of
between 0.83% and 1.09% of the fledgling popula-
tion.
DISCUSSION
White sharks do not appear to be a major threat to
seabirds at Dyer Island, in contrast to the situation
at St Croix Island, despite similar methods of
observation used to investigate the trends in
attacks on seabirds (Randall
et al.
1988). The
most conspicuous difference between the two
ecosystems is the presence of a large Cape fur
seal colony at Dyer Island, but not at at St Croix
Island. This may have a number of direct and
indirect effects on the behaviour of penguins and
white sharks.
Pinnipeds represent a primary prey for white
sharks (Le Boeuf 1982; Klimley
et al.
1992;
Compagno
et al.
1997). The
c.
55 000 strong seal
colony may direct the attention of white sharks
towards this prey, and consequently, less effort
may be spent investigating alternative prey that is
not as energetically rewarding, or not part of their
typical diet (Bass
et al.
1975; Cliff
et al.
1989,
1996). Anti-predator behaviour (fleeing to and
from an island) of African penguins in the vicinity of
Johnson
et al.
: Seabird predation by white shark and Cape fur seal at Dyer Island 7
Tab le 3 . Predatory impact of Cape fur seals on cormorant spp.at Dyer Island between January 2000 and December
2000.
Species Peak nest count Population estimate Seal predation Annual predation
impact (%)
Bank cormorant 42 84 Min: 0 0.00
Max: 0 0.00
Crowned cormorant 128 256 Min: 8 3.13
Max: 8 3.13
White-breasted cormorant 96 192 Min: 10 5.21
Max: 11 5.72
Fig. 6. Diel patterns in live attacks on seabirds at Dyer Island.
pinnipeds, and other predators, has been well
documented (Randall & Randall 1990), and was
frequently observed at Dyer Island (this study).
Such behaviour would also make penguins less
vulnerable to investigatory bites from white
sharks.
Injured survivors offered the only evidence of
white sharks attacking penguins. The relatively
gentle nature of these bites suggests curiosity or
inspection as possible motivation, rather than
feeding.This is consistent with the wide diversity of
non-prey items that have been bitten by white
sharks (Collier
et al.
1996), as well as, infrequent
consumption of non-prey species such as humans
(
Homo sapiens
) (Burgess & Callahan 1996), sea
otters (
Enhydra lutris
) (Ames & Morejohn 1980)
and African penguins (Bass
et al.
1975) following
an attack.
Penguins were attacked by Cape fur seals
seasonally with lowest attacks during November
and December, the peak moulting period of the
penguins. Penguins remain on land during their
moult, except for preening and drinking (Cooper
1974) and restrict foraging to a minimum (Broni
1985). Penguins at Dyer Island appear to be
most vulnerable to seal predation during foraging
related movements away from, and subsequent
return to, the island. The crepuscular pattern of
such live attacks (this study) coincides with the
mean departure (08:00) and arrival time (17:30) of
foraging penguins during the breeding season at
Marcus Island, Western Cape (Wilson
et al.
1988).
Cape fur seals take at least 1.99% to 2.52% of the
adult breeding population of penguins at Dyer
Island, an underestimate as only some carcasses
end up on Dyer Island’s shore. In addition, the loss
of a breeding adult results in the loss of its clutch,
thus further compounding the predation impact.
Cape cormorants appear vulnerable to predation
by seals due to their frequent landing in the waters
surrounding Dyer Island. The presence of cormo-
rants in the inshore area of Dyer Island is not
necessarily related to feeding, as feeding occur up
to 40 km from shore (Berry 1976) in sub-surface
waters (Crawford 2005d).The alighting of cormo-
rants on waters close to shore followed failed flight
attempts and/or onshore disturbance (this study)
and the seasonal peak in attacks corresponded to
the annual fledging periods of this species. The
period between egg laying and fledging is from 10
to 12 weeks (Crawford 2005d), and in both 1999
and 2000, attacks on fledgling Cape cormorants
concomitantly rose sharply three months after
nesting began. Cape cormorants tended to be
attacked throughout the day, with a peak around
midday. Disturbance, failed flight attempts and
bathing often resulted in large numbers of fledgling
cormorants swimming in the waters adjacent to
Dyer Island. In particular when approach from
humans causes a chain reaction of panic, masses
of fledglings flee to the water. The conservative
estimate that 1.09% of the Cape cormorant fledg-
ling population succumb to seal predation is
noticeably lower than the 7.1% calculated by
Marks
et al.
(1997) for Dyer Island. One factor that
may have contributed to the large difference was
the level of human disturbance experienced by the
respective fledgling populations. Extensive obser-
vation periods, from a tower constructed on the
southwestern point of Dyer Island, were necessary
for land based observations of the channel area
during the study of Marks
et al.
(1997). This may
have contributed to more fledglings entering the
water due to human disturbance by observers
moving to and from the tower, and movement on
the tower, than during the present study.
It can be concluded that the continual decrease
in penguin numbers in South Africa requires
management to attempt to minimize all factors
contributing to the decline, including seal predation
(David
et al.
2003). Active removal of rogue seals
at Malgas Island saw significant declines in attack
rates in 1999 and 2000 (David
et al.
2003). Distur-
bance, human or otherwise, of penguins on the
island does not appear to play a major role in
increasing seal predation rate, as most penguins
are attacked when travelling to and from their feed-
ing grounds. This quantitative account of Cape fur
seals attacking and consuming bank, crowned
and white-breasted cormorants in southern Africa
shows that these attacks are comparatively infre-
quent and opportunistic. However, the impact on
the small local cormorant populations is significant
due to their low abundance and vulnerability to
stochastic disturbances. Human disturbance in
particular, should be minimized in areas and at
times when cormorants are breeding and fledg-
lings are present.
ACKNOWLEDGEMENTS
This work formed part of a larger study funded by
the Department of Environmental Affairs and
Tourism, the University of Pretoria, Western Cape
Nature Conservation Board and the International
Association of Impact Assessment – South African
affiliate. For permission to work on Dyer Island,
8 South African Journal of Wildlife Research Vol. 36, No. 1, April 2006
and the use of facilities we thank the Western
Cape Nature Conservation Board. For logistical
(research vessel) and additional support we thank
Andre Hartman and Marine Dynamics.
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Corresponding editor: C.D. McQuaid
10 South African Journal of Wildlife Research Vol. 36, No. 1, April 2006
... We found instances of bird predation by pinnipeds from across the world involving a diverse range of avifauna such as penguins (Charbonnier et al. 2010, Ryan and Kerr 2012, Morrison et al. 2017, cormorants (Marks et al. 1997, Johnson et al. 2006, gannets (Makhado et al. 2006(Makhado et al. , 2013, albatrosses (Moore et al. 2008), auks (Mallory et al. 2004), and ducks (Tallman andSullivan 2004, Lovvorn et al. 2010). Many of these interactions share similar characteristics and we identify 4 traits that appear to characterize pinniped-bird predation. ...
... While some birds appear to recognize the threat that pinnipeds present and respond appropriately (Charbonnier et al. 2010, Lovvorn et al. 2010, others make easy prey. Johnson et al. (2006) observed Cape fur seal (Arctocephalus pusillus) predation of African Penguins (Spheniscus demersus) and cormorants (Phalacrocorax sp.) and report just a single unsuccessful attempt in 204 initiated hunts. Such success rates are consistent with the naïve prey hypothesis where birds (1) fail to recognize the threat that pinnipeds present, (2) have inappropriate antipredator responses, and/or (3) their responses are appropriate but ineffective (Banks and Dickman 2007). ...
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Recovering predators can create challenges for conservation objectives when they prey on vulnerable species. Although largely uncommon, pinniped predation of birds presents one such challenge. Here, we describe the novel characteristics of this predator–prey interaction, its impact on bird populations, and possible mitigation responses. We do so both broadly, synthesizing the wider literature, and specifically, in reference to ongoing South American sea lion (Otaria flavescens) predation of Black-necked Swans (Cygnus melancoryphus) we are currently observing in southern Chile. Our review of the literature suggests that in most cases bird predation by pinnipeds is only exhibited by a small proportion of the population, spreads socially between individuals, can be temporally severe, and may rapidly threaten the viability of bird populations. We discuss feasibility and efficacy of potential mitigation measures highlighting that, as foraging specializations can be socially transmitted, any such actions need to be time conscious as bird-killing behaviors may be increasingly difficult to remove. The contrasting population trends of pinnipeds and seabirds suggests that pinniped predation of vulnerable waterbirds is going to be an increasingly common conservation challenge in the future.
... Gabriotti and De Maddalena 2004;Tricas and McCosker 1984). Generally speaking, great white shark is a voracious predator preying on bony fishes, marine mammals, sea turtles and other sharks (Fergusson 1996;Fergusson et al. 2000;Dudley et al. 2000;Compagno 2001;Celona et al. 2006;Johnson et al. 2006;De Maddalena and Heim 2012). ...
... Great white sharks are known patrolling in insular waters, densely inhabited by pinnipeds (e.g. Dyer Island, South Africa; Johnson et al. 2006). However, the paucity (even the absence) of remains of Mediterranean monk seals (Monachus monachus) in the stomach contents of great white sharks, can be due to the scarcity of this seal species in the Mediterranean Sea (Fergusson 1996). ...
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Whenever a conversation starts on the great white shark, the term “monster” has always been the most frequently used statement to describe this magnificent but misunderstood predatory fish. Agreement with the Monster, was written in the light of current knowledge to summarise the lessons we have learned from the great white shark, whose existence was denied for many years in Turkish waters. The language is English. Year of publication; 2020, 74 pages
... Furthermore, lower efficiency observed in the presence of large apex predators may also reflect predator avoidance. Indeed, African penguins have been observed coming ashore with bites from larger predators (Johnson et al. 2006 Prey capture rates indicated that Macaroni penguins continued to forage beyond the optimal give up time. However, bout-scale analysis revealed individuals terminated diving behaviour for reasons other than patch quality. ...
Thesis
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... Seals are recognised predators of penguins worldwide (e.g., Hofmeyr and Bester, 1971;Du Toit et al., 2004;Johnson et al., 2006;Lee et al., 2019;Bester et al., 2020). In this study, we found that annual little penguin numbers decreased when long-nosed fur seal numbers increased, supporting the idea that seal predation may have influenced the decline of little penguins in South Australia (Bool et al., 2007;Wiebkin, 2011). ...
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Droughts in many regions of the world are increasing in frequency and severity which, coupled with effects from anthropogenic water extraction and diversion, are reducing river discharges. Yet to date, few studies have investigated the impacts of hydrological droughts (i.e., reduced river outflows to the ocean) on seabirds. Here, we examined the consequences of the “Millennium Drought” on the local decline of an iconic Australian seabird, the little penguin ( Eudyptula minor ). We analysed monthly and annual penguin numbers in relation to river outflow, rainfall, the characteristics of the coastal waters (sea surface temperatures and chlorophyll- a concentrations), and local abundance of key predators and prey species. We found a negative association between monthly penguin numbers and both sea surface temperatures and river outflow. Annual penguin numbers were positively associated with southern garfish numbers (our local indicator of food availability) but negatively associated with annual chlorophyll- a concentrations. Our findings emphasizing the need for further research into the effect of hydrological droughts on seabird populations and for improved river management that account for potential downstream impacts on the coastal environment receiving freshwater from rivers.
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... It has been considered that interference with resting behaviour in dolphins may subject them to physiological stress or negatively influence their energy balance (Christiansen et al., 2010;Constantine et al., 2004;Tyne, Johnston, Christiansen, & Bejder, 2017),which in these and in other wild vertebrates may in turn effect population health (Parsons, 2012;Romero, 2004). Furthermore, the disturbance of wild animals by humans may desensitize or distract them, thereby increasing their vulnerability to attack by predators (Geffroy, Samia, Bessa, & Blumstein, 2015;Venter, Oosthuizen, Bester, & Johnson, 2006;Voorbergen, De Boer, & Underhill, 2012;). Such effects may be more likely given that about three-quarters of the dolphins making use of Samadai were females, many of them with dependent young (Shawky et al., 2015). ...
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• Spinner dolphins, Stenella longirostris , are the primary target for marine mammal tourism in Egypt. The present study investigated the short‐term effects of tourist presence on the behaviour of spinner dolphins at Sha'ab Samadai (Samadai Reef), in the southern Egyptian Red Sea. • The reef has a large central lagoon where a population of spinner dolphin regularly rest from mid‐morning to mid‐afternoon; visitors are permitted to snorkel in the southern part of the lagoon, but not in the northern closed zone that the dolphins mainly use. • Dolphin behaviour was monitored both on days when tourist boats were present and on days when they were absent. In the presence of tourists the proportion of time that the dolphins spent resting was reduced by two‐thirds, whereas the times spent milling, travelling, and showing avoidance behaviour all increased. • Furthermore, upon using Markov chain modelling to investigate the effect of tourist presence on the transition probabilities between dolphin activity states, significant changes were found in 10 of the 25 possible behavioural transitions, including increased probabilities of transitioning from resting to milling or travelling, from milling to travelling or avoiding, and from travelling to avoiding. • These findings raise concerns that despite the management measures in place, tourist activities affect the dolphins’ behaviour to a greater extent than was previously apparent, with potential long‐term negative effects on their energy budget. The study led to proposals for amending the zoning of the site and for strengthening the regulations for tourist vessels.
... Thus, there are good reasons to think living Pteranodon could have been within reach of predatory sharks, and the likely pterodactyloid floating posture places their head and neck close to the waters' surface (Hone & Henderson, 2014). Various modern seabirds are predated by pelagic predators, including sharks (Wetherbee, Cortés & Bizzarro, 2004;Johnson et al., 2006), and we cannot exclude this possibility for the LACM Pteranodon. Witton (2018) noted that even moderately-sized sharks, akin to the 2.5 m long Cretoxyrhina indicated by the LACM tooth, would vastly outweigh the largest Pteranodon (35-50 kg-see Paul, 2002;Witton, 2008;Henderson, 2010 for Pteranodon mass estimates), and we have little doubt that such predators could subdue these pterosaurs if they caught them (Fig. 4). ...
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A cervical vertebra of the large, pelagic pterodactyloid pterosaur Pteranodon sp. from the Late Cretaceous Niobrara Formation of Kansas, USA is significant for its association with a tooth from the large lamniform shark, Cretoxyrhina mantelli . Though the tooth does not pierce the vertebral periosteum, the intimate association of the fossils—in which the tooth is wedged below the left prezygapophysis—suggests their preservation together was not mere chance, and the specimen is evidence of Cretoxyrhina biting Pteranodon . It is not possible to infer whether the bite reflects predatory or scavenging behaviour from the preserved material. There are several records of Pteranodon having been consumed by other fish, including other sharks (specifically, the anacoracid Squalicorax kaupi ), and multiple records of Cretoxyrhina biting other vertebrates of the Western Interior Seaway, but until now interactions between Cretoxyrhina and Pteranodon have remained elusive. The specimen increases the known interactions between large, pelagic, vertebrate carnivores of the Western Interior Seaway of North America during the Late Cretaceous, in addition to bolstering the relatively small fossil record representing pterosaurian interactions with other species.
... Their lifespan is uncertain, but may be up to 40 years (Bannister 1989). They are formidable predators of marine mammals and occasionally fish, marine reptiles and marine birds (Estrada et al. 2006, Johnson et al. 2006. Although humans are sometimes attacked, this most likely occurs when the shark mistakes a person for a seal (West 2011). ...
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Singapore‘s thrust to become Asia‘s biomedical hub has indeed spurred growth for the small city-state. As part of this development, the number of animals used for scientific purposes has continually increased. Singapore is home to more than 20 research facilities where animals are used in scientific experiments. In 2003, the National Advisory Committee on Laboratory Animal Research (NACLAR) was formed to establish national guidelines for the use of animals for scientific purposes. The NACLAR Guidelines, based on the principles of the 3 Rs – Replacement, Reduction and Refinement, were set down to ensure humane and responsible care and use of animals for scientific purposes in Singapore. Under the Animals and Birds (Care and Use of Animals for Scientific Purposes) Rules 2004, all research facilities which use animals for scientific purposes have to obtain a license from the Agri-Food and Veterinary Authority of Singapore (AVA) and are required to establish their own Institutional Animal Care and Use Committee (IACUC) – the equivalent of an Animal Ethics Committee (AEC). As Singapore continues to endeavor to attract top scientific talents and serve as a melting pot for world-class biomedical research activities, the use of animals for research, teaching, and testing will continue to increase and AECs will have to keep pace with changes to maintain its crucial role in ensuring that best practices in the welfare and care of animals used for scientific purposes are met.
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Marine predators adapt their hunting techniques to locate and capture prey in response to their surrounding environment. However, little is known about how certain strategies influence foraging success and efficiency. Due to the miniaturisation of animal tracking technologies, a single individual can be equipped with multiple data loggers to obtain multi-scale tracking information. With the addition of animal-borne video data loggers, it is possible to provide context-specific information for movement data obtained over the video recording periods. Through a combination of video data loggers, accelerometers, GPS and depth recorders, this study investigated the influence of habitat, sex and the presence of other predators on the foraging success and efficiency of the endangered African penguin, Spheniscus demersus , from two colonies in Algoa Bay, South Africa. Due to limitations in the battery life of video data loggers, a machine learning model was developed to detect prey captures across full foraging trips. The model was validated using prey capture signals detected in concurrently recording accelerometers and animal-borne cameras and was then applied to detect prey captures throughout the full foraging trip of each individual. Using GPS and bathymetry information to inform the position of dives, individuals were observed to perform both pelagic and benthic diving behaviour. Females were generally more successful on pelagic dives than males, suggesting a trade-off between manoeuvrability and physiological diving capacity. By contrast, males were more successful in benthic dives, at least for Bird Island (BI) birds, possibly due to their larger size compared to females, allowing them to exploit habitat deeper and for longer durations. Both males at BI and both sexes at St Croix (SC) exhibited similar benthic success rates. This may be due to the comparatively shallower seafloor around SC, which could increase the likelihood of females capturing prey on benthic dives. Observation of camera data indicated individuals regularly foraged with a range of other predators including penguins and other seabirds, predatory fish (sharks and tuna) and whales. The presence of other seabirds increased individual foraging success, while predatory fish reduced it, indicating competitive exclusion by larger heterospecifics. This study highlights novel benthic foraging strategies in African penguins and suggests that individuals could buffer the effects of changes to prey availability in response to climate change. Furthermore, although group foraging was prevalent in the present study, its influence on foraging success depends largely on the type of heterospecifics present.
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The African Penguin Spheniscus demersus is an endangered seabird endemic to southern Africa, and killing sprees by terrestrial predators have been one of the main threats for its mainland colonies. The methods employed to manage predators may differ depending on the species involved, therefore the implementation of strategies to limit the impacts of predation relies on the correct identification of the culprit predator. We report and quantify the lesions seen in African Penguins killed by four species of terrestrial predators: Caracal Caracal caracal (52 kills), Leopard Panthera pardus (27 kills), Domestic Dog Canis lupus familiaris (10 kills), and Cape Grey Mongoose Galerella pulverulenta (4 kills). We discuss patterns of necropsy findings that can be used to identify the predator species involved. Traditional forensic methods are useful tools to direct species-specific management actions for the conservation of the African Penguin and other seabirds so that effective mitigating measures can be deployed quickly to prevent further losses. It should be borne in mind, however, that the age, size and previous hunting experience of the predator are likely to influence the pattern of lesions that will be observed, and not all carcasses will have hallmark lesions or recognisable bite marks.
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Cape fur seals (Arctocephalus pusillus pusillus) prey on Cape gannets (Morus capensis), Cape cormorants (Phalacrocorax capensis), bank cormorants (P. neglectus), crowned cormorants (P. coronatus) and African penguins (Spheniscus demersus) at Ichaboe Island (26°17′ 22″S, 14°56′36″E), Namibia. Between September 1991 and May 2000, 2774 predatory events were recorded; these involved 932 gannets, 1217 cormorants and 544 penguins. One-third of predation events noted were on seasonally abundant fledgling gannets and cormorants. Four individual seals specializing in seabird predation did not conform to this pattern of predation, differing in bird species targeted. Seabird predation may be learnt from other seals, or forms an extension of play behaviour predominantly in subadult males. Seabird predation does not seem to be a common, stereotyped behaviour in seals; rather, individuals develop their own preferences and techniques.
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Spheniscus demersus appeared to recognize killer whales Orcinus orca as predators, whereas Indian Ocean bottlenosed dolphins Tursiops aduncus, common dolphins Delphinus delphis and southern right whales Eubalaena australis were ignored. Bryde's whales Balaenoptera edeni are unlikely predators despite some evidence to the contrary. -from Authors
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Over a five-year period counts of active nest sites of jackass or African penguins at Robben island, SW Cape Province, South Africa, in May, at the middle of the breeding season, were significantly related to estimates of numbers of adult penguins obtained from counts of birds in the feather-shedding phase of moult. The number of moulting adults over a 12-month period was largely determined by a few counts conducted around the early December peak in moult. This was in agreement with a factor used in a previous study to adjust nest counts to obtain an estimate of the overall population of jackass penguins. -from Authors
Article
The inshore distribution and foraging behaviour of jackass penguins Spheniscus demersus were studied between December 1982 and August 1983 in waters close to breeding islands in Saldanha Bay, South Africa. The use of a sail-boat permitted close observation of foraging penguins with minimal apparent disturbance. Penguin numbers at sea were lowest in December, when most birds were confined to islands during moult, and highest during the winter breeding season. Although most penguin group sizes were small (one or two birds), over 44% of penguins occurred in groups of more than 10 birds. Three typical penguin group formations occurred at sea: ‘facing-search’, ‘line-abreast’, and ‘pointed-ovoid’. Penguins also foraged in association with other seabirds and marine mammals. The importance of large foraging groups suggests that the jackass penguin relies on shoals of similar sizes to those taken by the commercial purse-seine fishery, increasing the potential for "o competition.Die verspreiding en voedselbekommingsgedrag van Kaapse pikkewyne Spheniscus demersus is tussen Desember 1982 en Augustus 1983 in die see naby die broeieilande by Saldanhabaai, Suid-Afrika, ondersoek, 'n Seilboot is gebruik-ss om die kossoekende pikkewyne van naby af met minimale versteuring te ondersoek. Die minste pikkewyne is in Desember ter see, wanneer die meerderheid op die broei-QH eilande verveer, en die grootste getalle is gedurende Maart en Augustus, hulle broeipieke, ter see. Alhoewel die meeste voedselbekommingsgroepe klein was (een of twee voëls), het meer as 44% van die pikkewyne in groepe van 10 of meer voorgekom. Drie tipiese groepformasies het H voorgekom: ‘regoor-soek’, ‘langs mekaar’, en ‘spits-eiervormig’. Pikkewyne het ook saam met ander seevoëls en seesoogdiere kos gesoek. Die belangrikheid van groot voedselbekommingsgroepe dui aan dat brilpikkewyne staatmaak op skole vis van 'n gelyke grootte as dié wat deur die kommersiëie saknetvissery gevang word. As dit die: geval is, kan daar 'n groot potensiaal vir kompetisie tussen die pikkewyne en saknetvissery wees.