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THE POTENTIAL IMPACT OF A LOW-LEVEL HELICOPTER FLIGHT VS.
ENVIRONMENTAL FACTORS ON CAPE FUR SEAL MASS PUP MORTALITY IN THE
ROBBERG MARINE PROTECTED AREA, PLETTENBERG BAY
An investigational report for CapeNature, Bitou Municipality and the South African Air Force
by
JSF van der Vyver and DS Conry
For queries email [email protected] or visit www.wildocean.org.za
Submitted February 2020
Potential impact of a low-level SAAF Oryx helicopter flight on Cape fur seal
pup mortality in the Robberg Marine Protected Area 2020
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SUMMARY
The potential impact of an anomalous disturbance event caused by a low-level SAAF Oryx
helicopter flight over the Cape fur seal breeding colony on Robberg Peninsula was examined using
seal stranding records and scientific observation. Varying states of decay in carcasses washed
ashore over the November 2019 – January 2020 breeding season was used to estimate the likely
date of death in 553 newborn pups. Using this data an estimated (albeit crudely) 47 newborn pups
may have landed up in the water and drowned due to disturbance caused by the helicopter flight on
30 November 2019, although there is no causal proof. Available data and preliminary observations
suggests the other 506 newborn pups most likely drowned as a result of disturbance caused by
territorial bull fights, pups falling off steep cliffs and the onslaught of summer storms that knock
pups off the rocks – especially during periods of high easterly wind and swell. Even though summer
storms have been identified as the main factor causing high levels in pup mortality at other
breeding colonies along the south coast of South Africa, further statistical analysis into the effect of
various environmental factors are needed to fully test this hypothesis at the Robberg breeding
colony. Emerging evidence of an increasing trend in pup production at Robberg is most likely
related to a local increase in the availability of preferred prey (i.e. small pelagic fish), leading to an
increase in body condition and therefore reproductive rates of females. A possible influx of adult
females from other colonies may also be a contribution factor and could be investigated in future
using satellite tracking devices, if funding becomes available. Any further increases in the observed
pupping trend will likely be mirrored by an increase in easterly storm-related mortality, and
therefore carcasses washed ashore.
This report may be cited as:
van der Vyver JSF and Conry DS. 2020. The potential impact of a low-level helicopter flight vs. environmental factors
on Cape fur seal mass pup mortality in the Robberg Marine Protected Area, Plettenberg Bay. Investigational report
prepared for Cape Nature, Bitou Municipality and the South African Air Force. 25pp. Available online at:
http://www.wildocean.org.za.
Potential impact of a low-level SAAF Oryx helicopter flight on Cape fur seal
pup mortality in the Robberg Marine Protected Area 2020
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TABLE OF CONTENTS
ABBREVIATIONS.............................................................................................................................4
FIGURES............................................................................................................................................5
1. INTRODUCTION.......................................................................................................................6
Geographical distribution........................................................................................................6
History......................................................................................................................................7
Breeding season.......................................................................................................................8
2. AIMS AND OBJECTIVES.......................................................................................................10
3. MATERIALS AND METHODS..............................................................................................10
Study site................................................................................................................................10
Helicopter disturbance...........................................................................................................10
Recovery of carcasses.............................................................................................................11
Environmental data................................................................................................................11
Live pup counts......................................................................................................................12
Descriptive statistics...............................................................................................................12
4. PRELIMINARY RESULTS.....................................................................................................12
Helicopter disturbance...........................................................................................................12
Stranding dates.......................................................................................................................13
Carcass categorization...........................................................................................................14
Potential date of death............................................................................................................15
Comparison of spikes in pup strandings over previous seasons............................................15
Impact on live pup counts.......................................................................................................16
5. DISCUSSION.............................................................................................................................17
6. CONCLUSION..........................................................................................................................19
7. RECOMMENDATIONS..........................................................................................................20
Municipal carcass disposal....................................................................................................20
Future research and monitoring............................................................................................20
ACKNOWLEDGEMENTS.............................................................................................................21
REFERENCES.................................................................................................................................21
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ABBREVIATONS
CN – CapeNature
DEFF – Department of Environment, Forestry and Fisheries
MPA – Marine Protected Area
PEM – Port Elizabeth Museum
RNRC – Robberg Nature Reserve Complex
SAAF – South African Air Force
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FIGURES
Figure 1: The range of Cape fur seal breeding colonies from Algoa Bay in South Africa to Baia
dos Tigres in southern Angola.
Figure 2: A historical representation of the number of Cape fur seals hauled out on Robberg
Peninsula between 1833 and 2020.
Figure 3: Satellite image of Robberg Peninsula showing the extent of the seal colony at the
beginning of the recolonisation period, after 2008 and after 2010. The location of
pupping sites observed during dedicated boat-based counts between 2017 and 2020
are included.
Figure 4: a) Stranding dates of 620 Cape fur seal carcasses that washed ashore on Plettenberg
Bay beaches during the 2019/2020 breeding season, including all age classes from
pups to yearlings, subadults and adults, and b) daily wind and swell conditions over
this period.
Figure 5: The external appearance of a tagged Cape fur seal pup (GG0378) that washed ashore
a second time on Robberg beach on 12 December 2019, after drowning at the
breeding colony on 7 Dec 2019 and being submerged for ~6 days.
Figure 6: a) Estimated date of death of 620 Cape fur seal carcasses that washed ashore on
Plettenberg Bay beaches during the 2019/2020 breeding season, and b) a range of
daily environmental conditions over this period.
Figure 7: Spikes in newborn Cape fur seal pup carcasses washed ashore over three breeding
seasons between 2017 and 2020.
Figure 8: Peaks in live Cape fur seal pup counts over three breeding seasons between 2017 and
2020.
Figure 9: Emerging trends in Cape fur seal minimum pup production estimates based on a
combination of maximum live counts at the colony and total carcasses washed ashore
at the time of those counts.
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INTRODUCTION
Geographical distribution
The Cape fur seal Arctocephalus pusillus pusillus is the only resident pinniped species along the
southern African coastline. The current geographical distribution of its breeding population ranges
from Algoa Bay in South Africa to Baia dos Tigres in southern Angola (Kirkman et al. 2011). The
latest total population estimate suggests some 2 million animals, ~99% of which occurs along the
west coast of southern Africa. According to the definition of a Cape fur seal breeding colony as a
location where at least 100 pups per year are born regularly (Oosthuizen and David 1988), there are
currently 41 breeding colonies in the population (Kirkman et al. 2011, PEM unpublished data). The
majority of these are associated with the Benguela Current system located along the west coast of
the region, with only three breeding colonies occurring to the east of Cape Agulhas, in the Agulhas
Current system (Figure 1). These include Black Rocks in Algoa Bay, Robberg Peninsula in
Plettenberg Bay and Seal Island in Mossel Bay.
Fig. 1. Map showing the range of Cape fur seal breeding colonies from Algoa Bay in South Africa
to Baia dos Tigres in southern Angola. Breeding colonies along the south coast include Black
Rocks in Algoa Bay (BR), Robberg Peninsula in Plettenberg Bay and Seal Island in Mossel Bay
(SIMB). Selected breeding colonies along the west coast include Geyser Rock (GR), Seal Island
False Bay (SIFB), Vondeling Island (VI), Kleinsee (KS), Atlas Bay (AB), Hollam’s Bird Island
(HBI), Cape Cross (CC), Cape Frio (CF) and Baia dos Tigres (BT). (taken from Huisamen et al.
2011.)
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History
The current geographical distribution of the south coast Cape fur seal population differs
considerably from its historical distribution, as reconstructed from records or anecdotes of seal
hunters and early travellers (e.g. Rand 1972, Shaughnessy 1982, 1984). Historically, at least nine
seal colonies occurred to the east of Cape Agulhas, but most of these were hunted to extinction prior
to the 20th century, including at least five colonies in Algoa Bay and another two in Plettenberg
Bay (Shaughnessy 1982, Stewardson 1999a). In Plettenberg Bay specifically, Cape fur seals
historically hauled out on Seal Point at Robberg and also Beacon Island (long before being
developed and joined to the mainland) in large numbers as far back as the 15th century. This is
evident in many ship records from early travellers and seal hunters that visited the bay. In fact,
remains from Khoi middens investigated by archeologists in Nelson Bay cave suggest that Cape fur
seals have been present at Robberg Peninsula for over 8000 years (Sealy 2006). In August 1833, an
anecdotal record from an observation made by a government official suggests that there were at
least 3000 seals hauled out on Robberg Peninsula (Ross 1971). At the time Cape fur seals were
already indiscriminately hunted for their fur and blubber, both in Plettenberg Bay and elsewhere
along the east and west coast of South Africa. By 1890, due to uncontrolled harvesting, Cape fur
seals were completely exterminated from both their historical haul-out sites in Plettenberg Bay
(Ross 1971).
Following the introduction of protective legislation in 1893 (Best 1973), and the proclamation of
the Robberg Nature Reserve Complex (RNRC) in 1980, it was only in the early 1990’s that small
numbers of seals returned to Robberg Peninsula (Stewardson and Brett 2000). Numbers
subsequently increased (Stewardson 2001) and a few newborn pups were first observed in
1996/1997, leading to speculation that Robberg could eventually become a breeding colony
(Stewardson 1999, Kirkman 2010). Due to the potential for future impacts on local fisheries
(Wickens et al. 1992) and the conservation of certain other marine top predators (Kirkman 2009), as
well as the colony’s ecotourism potential, it was identified as a monitoring priority by the Robberg
management authority (CapeNature). Assisted by CapeNature and the Center for Dolphin Studies,
Huisamen et al. (2011) used routine land-based counts to describe the recolonisation process
between 2000 – 2008, including between- and within-year temporal patterns in haulout numbers
and the extent of breeding. Unpublished data from routine boat-based counts performed by
CapeNature since 2009 and independent researchers from Wild Ocean since 2018 (with the support
from the ORCA Foundation and Ocean Blue Adventures) suggests that seal haul-outs at the
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Robberg colony have been steadily increasing, despite large seasonal and annual fluctuations in
numbers (Figure 2). However, with an overall average of ~3500 seals ashore, and similar numbers
recorded during spring, it appears the Robberg colony have once again reached those historical
numbers recorded by a government official in August 1833.
Fig. 2. A historical representation of the number of Cape fur seals hauled out on Robberg Peninsula,
between 1833 and 2020.
Breeding season
The annual Cape fur seal pupping season extends from mid-November to early January with a peak
in births around mid-December. Pups are born in a rookery on land and depend on their mothers for
10-11 months. During this time mothers regularly leave their pups to forage for 2-10 days at a time,
re-uniting for a few days between trips to suckle (David & Rand 1986).
Newborn pups are notoriously susceptible to high natural mortality levels (David 1987, Roux and
de Villiers 1991, de Villiers and Roux 1992, Wickens & York 1997, Kirkman et al. 2006, Kaplan et
al. 2008). In addition, the pupping season for south coast populations coincide with summer storms
that are accompanied by high swells (>3m) (Stewardson 1999a). Whereas such conditions are not a
problem for older seals, newborn pups are not able to swim very well (Rand 1967, David 1995) and
can be swept away from the colony if they are washed off the rocks in rough weather. Many drown
and wash up on the mainland, but live ones are sometimes rescued and returned to their colony
(Stewardson 1999a, 1999b, Hofmeyr et al. 2011). Although such events have been recorded in
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Algoa Bay since 1912 (Stewardson 1999b), it has not been studied in detail at other colonies further
west.
Human disturbance can also have a major impact on the survival of young marine colonial breeding
species since they may be crushed or otherwise injured by fleeing adults, lose contact with their
mothers, be prevented to form a mother-neonate bond (leading to abandonment and starvation), or
fall into the sea and drown. Cape fur seals are no exception, being instinctively afraid of human
activity as they have evolved without human predators. At Robberg Peninsula human disturbances
over the years is thought to have resulted in changes in the distribution of their breeding locations,
leading to the present haulout distribution and pupping sites shown in Figure 3 below.
Fig. 3. Google Earth image (http://earth.google.com/) of Robberg Peninsula showing the extent of
the seal colony along the coast at the beginning of the recolonisation period (between A and B),
after 2008 (between C and D) and after 2010 (between E and D). Red sections indicate the location
of pupping sites observed during dedicated boat-based counts between 2017 and 2020. Blue
sections indicate popular shore angling locations. The most suitable access points for seals on the
north face of the peninsula are marked with arrows. Hiking trails cover vast majority of the coastal
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section along the south coast and is thought to prevent seals from hauling out in numbers along this
section. (adapted from Huisamen et al. 2011.)
AIMS AND OBJECTIVES
This report is presented in support of the investigation into a low-level South African Air Force
(SAAF) Oryx helicopter flight over the Robberg Nature Reserve Complex (RNRC) and the local
Cape fur seal breeding colony on 30 November 2019. Using seal stranding records and scientific
observation the report aims to provide 1) a descriptive overview of the potential impact of the
disturbance event on newborn pup mortality levels in the 2019/2020 breeding season, and 2) a
number of hypotheses explaining the cause of the apparent spike in mortality that was detected in
early December 2019.
MATERIALS AND METHODS
Study site
Robberg Peninsula is part of the Robberg Nature Reserve Complex and is adjoined by a marine
protected area (MPA) which was established during the late 1990s (Government Notice No. 1 of
1980, Schutte-Vlok et al. 2018). The peninsula forms the south-western extremity of Plettenberg
Bay, situated on the southeast coast of South Africa (Figure 1). There, the fast-flowing Agulhas
Current intermittently causes inshore counter currents and upwelling of colder water, especially
after periods of high easterly wind (Schumann et al. 1982, Lutjeharms et al. 2000, Lutjeharms and
Ansorge 2001, Goschen et al. 2012, 2015). However, warm water intrusion events are not
uncommon (Goschen et al. 2012, 2015). Rough sea conditions are generally associated with
westerly cold fronts in winter and strong easterly winds in summer.
Helicopter disturbance
An anomalous disturbance event that was recorded via direct observation and a video recording
provided the only factual evidence of the event. The observer was interviewed and the video
material studied to collect information of his account.
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Recovery of carcasses
As part of a broader Cape fur seal monitoring programme operating under research permits issued
to the Port Elizabeth Museum (PEM) by the Department of Environment, Forestry and Fisheries
(DEFF), and CapeNature (the Robberg management authority), data collected in Plettenberg Bay
since 2017 allow the assessment of 1) changes in the frequency of breeding season mortality levels
through the recovery of information from carcasses washed ashore, and 2) minimum pup production
estimates through a combination of dedicated boat-based counts and stranding records. In order to
obtain comparable values we have taken each breeding season as starting on 1 November and
ending on 31 January. During this period daily beach surveys and/or spot checks were performed on
Robberg, Central, and Lookout beaches. The more isolated Keurbooms beach was only surveyed 3-
4 times per week. As a result, a small portion of pup carcasses washed up on this beach may have
spent a day or two ashore before being discovered. Between 2017 and 2018 pup carcasses were
buried in order to prevent re-sampling. Since 2019 they were marked with either brightly coloured
spray-paint or plastic flipper tags. Carcasses were usually removed by municipal authorities less
than one week after washing ashore, but in some cases up to two weeks later. Carcasses found
washed up on more remote beaches such as Keurbooms were also marked with spray-paint or
plastic flipper tags, but due to the low number of beach visitors were left to decay naturally.
Live pups that washed ashore on the mainland were collected by various local authorities and
members of the public, some of which were taken to the Tenikwa Wildlife Rehabilitation and
Awareness Centre for short term treatment of injuries. Those that survived were double-tagged with
uniquely identifiable plastic roto-tags on the trailing edges of their fore flippers, and with help from
Offshore Adventures returned to the colony by boat in calm weather, usually within 1-7 days of
stranding. Any that washed ashore a second time (dead) can be used to assess the rate of decay and
therefore estimate the possible date of death. The re-stranding of the carcass of one such live pup
that was previously tagged and released at the colony was used to categorize the rate of decay in all
carcasses that washed ashore over the 2019/2020 breeding season.
Environmental data
Wind/gust speed and direction, wave height and direction, air temperature and cloud cover
conditions over the study period were extracted from archived data available on
https://www.windguru.cz/archive.php.
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Live pup counts
22 separate dedicated boat-based counts of live newborn pups ashore at the colony were made over
three consecutive breeding seasons between 2017 and 2020. Each count was carried out by an
experienced observer and black pups (defined as newborn animals less than 2 months old) were
distinguished from older animals based on their small size and the morphometric and colour
descriptions of Rand (1956). A standard sheep counter was used to keep tally of all black pups
visible to the naked eye (therefore an undercount of the true number of newborn pups ashore as the
angle of the boat most likely concealed many hiding among the rocks and boulders) while the vessel
(~9m double-hull) slowly travelled along the peninsula at a distance of 10 – 20m from the high-
water mark. Surveys started at site E near the western edge of the peninsula and ended at site D near
the eastern edge of the peninsula (Figure 3). On average counts lasted 40 minutes and were carried
out between 09:00 and 15:00. A range of environmental factors such as sea surface temperature,
wind direction, beaufort scale and cloud cover conditions were also recorded for inclusion in future
analyses on their potential effects.
Minimum pup production estimates were calculated by combining the highest count of the season
(although underestimate) with the total number of pup carcasses recorded ashore along Plettenberg
Bay beaches up until the date of that count.
Descriptive statistics
No advanced statistical analyses was applied as the aim of this report was merely to present a
summary of the data using basic figures in Excel. Data will however be analysed in future using
appropriate software, as part of broader research publications on Cape fur seal biology in the
Agulhas Current.
PRELIMINARY RESULTS
Helicopter disturbance
On Saturday 30 November 2019, at 10:00am, a CapeNature Field Ranger made a direct observation
and video recording of a low-level SAAF Oryx helicopter flight from an elevated viewpoint directly
above the ‘Meidebank’ angling site. Initially the helicopter was heard, and later seen approaching
from Beacon Island in the north heading directly towards the gap area located on the northern side
of Robberg Peninsula. After inspecting the video footage it was clear that once the helicopter
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reached the peninsula it turned east and proceeded to fly in a path parallel to the peninsula, roughly
30-50m from the coastline and below 100m above the surface of the water towards Seal Point
(Robberg Point). The footage shows the helicopter passing by two of the three known seal pupping
sites, but due to the far distance away (~2km) it was not possible to see clearly whether any
stampeding occurred that could have knocked seal pups into the water. It is worth noting that sea
conditions (i.e. wind and swell) were extremely calm on the northern side of the peninsula at the
time of the observation and for the remainder of the day.
Stranding dates
Figure 4 illustrates the stranding dates of all Cape fur seal carcasses washed ashore on Plettenberg
Bay beaches during the 2019/2020 breeding season, including daily wind and swell conditions.
Although stranding dates of older age classes are more evenly distributed throughout the breeding
season, with an increase during January, a clear peak in pup strandings is apparent roughly 9-10
days after the helicopter disturbance event took place on 30 November 2019.
Fig. 4. Illustrates a) the stranding dates of 620 Cape fur seal carcasses washed ashore on Plettenberg
Bay beaches during the 2019/2020 breeding season, including all age classes from pups to
yearlings, subadults and adults, and b) daily wind and swell conditions over this period.
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Carcass categorization
An unfortunate event involving the rescue, tagging and unsuccessful release of a live stranded
newborn pup at the breeding colony on Robberg Peninsula led to its drowning on 7 December 2019.
The carcass was found washed ashore on 12 December 2019, allowing photographic evidence of
the external appearance of a drowned newborn pup that had been submerged in seawater for ~6
days (Figure 5). As the condition of >90% of stranded carcasses were assessed on the day they
washed up, this information was used to distinguish between varying states of decay in 553
newborn pups that washed ashore over the 2019/2020 breeding season, and to estimate their likely
date of death (albeit crudely). Carcasses with very little to no fur and a similar appearance to that in
Figure 5 was scored 7 days dead, those with 2/3rds
of their fur missing 5 days dead and those with
~1/3rd
of their fur missing 3 days dead. Pups with all their fur still attached were scored with either
being one or two days dead depending on other external features associated with their eyes and
flipper tips.
Fig. 5. The external appearance of a tagged Cape fur seal pup (GG0378) that washed ashore a
second time on Robberg beach on 12 December 2019, after drowning at the breeding colony on 7
Dec 2019 and being submerged for ~6 days.
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Potential date of death
After stranding dates were corrected for estimated date of death, taking into account the state of
decay of each pup that washed ashore, the preliminary assessment (albeit crudely) suggests 47 pups
drowned on the day of the helicopter disturbance event (Figure 6).
Fig. 6. a) Estimated date of death of 620 Cape fur seal carcasses that washed ashore on Plettenberg
Bay beaches during the 2019/2020 breeding season, and b) a range of daily environmental
conditions over this period.
Comparison of spikes in pup strandings over previous seasons
Figure 7. illustrates a comparison of spikes in pup carcasses washed ashore over three breeding
seasons between 2017 and 2020. Although spikes are comparable, a major increase in strandings in
the 2018/2019 and again 2019/2020 breeding season is apparent.
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Fig. 7. Spikes in newborn Cape fur seal pup carcasses washed ashore over three breeding seasons
between 2017 and 2020.
Impact on live pup counts
Figure 8. illustrates a dates and numbers of 22 live pup counts performed over the past three
breeding seasons (2017 – 2020), showing a clear increase in maximum counts during the two most
recent breeding seasons (i.e. 2018/2019 and 2019/2020).
Fig. 8. Peaks in live Cape fur seal pup counts over three breeding seasons between 2017 and 2020.
Figure 9 shows an increasing trend in Cape fur seal pup production and newborn pup mortality over
the past three breeding seasons. The blue circle highlights the unexpected drop in live pup counts
and a higher than expected increase in dead pups washed ashore over the 2019/2020 breeding
season.
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Fig. 9. Emerging trends in Cape fur seal minimum pup production estimates based on a
combination of maximum live counts at the colony and total carcasses washed ashore at the time of
those counts.
DISCUSSION
Figure 4 shows a clear mismatch between the helicopter flight disturbance event on 30 November
and the spike in strandings of newborn pup carcasses between 9 and 10 December. Although the
mismatch could be due to the delay from disturbance to actual drowning of pups and subsequent
strandings on Robberg beach, the delay may also be influenced by the timing and strength of
easterly winds and swells which are believed to be the main factors responsible for casting
carcasses ashore on Plettenberg Bay beaches. In Figure 4 it is clear that the peak pup stranding
period over 9-10 December coincides with an extended period of storm conditions characterised by
strong easterly wind coupled with high southerly, south-easterly and especially easterly swells.
Therefore, the spike in estimated newborn pup mortality presented in Figure 6 may be attributable
to one or more of the following: (1) the helicopter disturbance had resulted in a stampede of
panicking adults that caused a large number of newborn pups to get knocked off the rocks and land
up in the water where they drowned and washed ashore concurrently, (2) pups had been washed off
Robberg Peninsula due to a major easterly storm, drowned and washed ashore concurrently, (3)
pups had drowned over a number of days previously, due to a variety of environmental conditions,
but due to a combination of easterly wind and wave conditions were washed ashore concurrently.
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The helicopter disturbance event can be ruled out as estimates of the date of death in pups that
washed ashore on 9 and 10 December suggests that the vast majority drowned between 2-6
December, some 2-6 days after the helicopter disturbance event.
It is important to remember that high rates of mortality prior to weaning are not unusual in seal pups
(Wickens & York 1997, Kaplan et al. 2008). Furthermore, catastrophic events, such as
displacement and drowning due to storms have been recorded in a number of species, including
grey seals Halichoerus grypus (Le Boeuf and Briggs 1977, Anderson et al. 1979, Barnett and
Westcott 2001), southern elephant seals Mirounga leonine (Carrick and Ingham 1962), northern
elephant seals M. angustirostris (Reiter et al. 1981, Riedman and Le Boeuf 1982), harbour seals
Phoca vitulina (Boness et al. 1992), Steller sea lions Eumetopias (Kaplan et al. 2008, Maniscalco et
al. 2008) and Subantarctic fur seals Arctocephalus tropicalis (Beauplet et al. 2005). Cape fur seal
pups are also subject to high mortality rates in the first few months of their life (David 1987, Roux
and de Villiers 1991, de Villiers and Roux 1992), including drowning of pups that are washed from
rookeries by large swells (Stewardson 1999a, Kirkman et al. 2006). Storm-related mortality is also
of importance for the conspecific Australian fur seal A. p.doriferus (Pemberton and Kirkwood 1994,
Arnould and Littnan 2000, Hume et al. 2001). Mortality of seal pups due to catastrophic
meteorological events varies with the topographic characteristics of the haulout area (Le Boeuf and
Briggs 1977, Riedman and Le Boeuf 1982, Doidge et al. 1984, Boveng et al. 1998, Baker et al.
2006); some rookeries experience very little mortality during storms because the topography of the
haulout area provides adequate protection (de Villiers and Roux 1992, Pemberton and Kirkwood
1994, Boveng et al. 1998, Gucu et al. 2004). By contrast, the pupping locations of Cape fur seals on
the northern side of Robberg Peninsula offers very little protection during easterly storms. The
location is also blocked off from cool westerly winds causing extreme air temperature conditions
during such periods in summer. Newborn pup mortality during such events is therefore expected to
be high.
Although peaks in high easterly wind/gust and swell, and high air temperatures during the
2019/2020 breeding season appears to correlate with spikes in estimated pup mortality (Figure 6),
the true effect of these factors will only be examined in future analyses (using appropriate software)
in order to fully test the other two hypotheses.
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The increasing trend in newborn pup mortality is consistent with emerging trends in minimum
estimated pup production and recent growth of the colony (CapeNature and PEM unpublished data).
Results on the potential impact of the helicopter flight are also consistent with the unexpected
decrease in 2019/2020 peak live pup counts and a large increase in the total number of pup
carcasses washed ashore, which was most likely influenced by this anomalous disturbance event.
CONCLUSION
Preliminary results presented here suggest disturbance caused by the low-level SAAF Oryx
helicopter flight on 30 November 2019 had a minor impact on overall pup mortality during the
2019/2020 Cape fur seal breeding season, potentially causing the death of up to 47 newborn pups.
Although sea conditions were calm on the day of the incident, any newborn pups that may have
landed up in the water near the steep sections of the peninsula would have had very few suitable
sites to haul out onto the safety of the rocks, and many would have inevitably drowned in their
attempts.
Based on the data it appears the peak pup stranding period of 9-10 December 2019 was most likely
related to newborn pups that had drowned over a number of days previously, mainly between 2-7
December with a peak in mortality on 4 December, and due to a combination of strong easterly
wind and swells were washed ashore concurrently. Future data analysis will investigate the effect of
various environmental factors in order to test this hypothesis.
It is important to acknowledge emerging evidence of an increase in pup production at this breeding
colony, most likely related to a local increase in the availability of the Cape fur seal’s preferred prey
species (i.e. small pelagic fish such as anchovy, sardine and horse mackerel after Huisamen et al.
2012). Previous studies have shown how the quantity and quality of prey effects the body condition
of marine top predators, including seals, which effects the timing of puberty, implantation of
embryos, mortality of foetuses, and therefore reproductive rates of females (reviewed in Hazen et
al. 2019). Based on this it is very possible that the increasing trend in Cape fur seal pups washed
ashore on Robberg beach over the past three breeding seasons (2017 - 2020) may be related to an
increase in the reproductive rates of females. However, the potential influx of adult females from
other areas may also be a contributing factor. Either way, it is suspected that any further increases in
Potential impact of a low-level SAAF Oryx helicopter flight on Cape fur seal
pup mortality in the Robberg Marine Protected Area 2020
20
pup production will naturally be mirrored by increases in easterly storm-related pup mortality, and
therefore carcasses washed ashore.
Although not related to the aims and objectives of this report, it is worth mentioning that the cause
of mortality in majority (>90%) of the older age classes (yearlings, subadults and adults; overall
76% male) since 2017 are unknown as most wash up in an advanced state of decay. However, there
are necropsy records for some fresh carcasses which report on various usual and unusual causes of
mortality and secondary symptoms. These include shark inflicted wounds, stingray inflicted
wounds, parasitic infestations, entanglement in fishing gear and plastics, and shot seals. For those
that were fresh enough to collect pathology, the samples are stored with Dr. Greg Hofmeyr at the
PEM until future investigation on toxicology and the presence of disease and virus. The results of
these investigations will be reported on in future publications.
RECOMMENDATIONS
Municipal carcass disposal
As vast majority of seal carcasses were cast ashore during periods of high easterly wind and swell,
it is advised to delay major municipal clean-up efforts until such conditions have passed.
Future research and monitoring
Continued boat-based counts of live newborn pups at the colony and land-based counts of the
number of carcasses washed ashore during the pupping season may prove crucial in studying the
importance of seal birth rate as an indicator of ecosystem health and/or changes lower down in the
food-web, especially if linked to results from future diet analysis of adult seals.
Given the potential of breeding animals moving into the region from other areas, the movement and
exchange of seals between south coast colonies should be investigated using satellite tracking
devices. Numerous funding applications have been submitted for this component over the years, but
to date none have been successful. In the meantime, relatively inexpensive flipper tagging efforts
could be intensified at all breeding colonies along the south coast. Ongoing monitoring of carcasses
washed ashore, and seals drowned in mid-water trawl nets further offshore (Reed et al. 2017, van
der Vyver personal observation) could provide valuable ‘recapture’ data.
Potential impact of a low-level SAAF Oryx helicopter flight on Cape fur seal
pup mortality in the Robberg Marine Protected Area 2020
21
ACKNOWLEDGEMENTS
This preliminary assessment was conducted as part of a broader research programme on the biology
of Cape fur seals and their interactions with fisheries in the Agulhas Current; and in terms of
permits issued to the PEM by the DEFF and CapeNature. We would like to thank Bitou
Municipality, Offshore Adventures, Tenikwa Wildlife Rehabilitation and Awareness Centre, the
Plettenberg Bay Marine Animal Stranding Network and Ocean Blue Adventures for their logistical
support. Fieldwork and equipment was partially funded by the PEM, the ORCA Foundation and
small donations from members of the public. However, a large portion of the costs were covered by
the authors. No financial compensation was received for preliminary data analysis or report writing.
We thank our research collaborator Dr. Greg Hofmeyr for his insightful comments on an earlier
version of this report.
REFERENCES
Anderson SS, Baker JR, Prime JH, Braid A. 1979. Mortality in grey seal pups: incidence and
causes. Journal of Zoology [London] 189: 407–417.
Arnould J, Littnan C. 2000. Pup production and breeding areas of Australian fur seals
Arctocephalus pusillus doriferus at Kanowna Island and The Skerries in north eastern Bass
Strait. Australian Mammalogy 22: 51–55.
Baker JD, Littnan CL, Johnston DW. 2006. Potential effects of sea level rise on the terrestrial
habitats of endangered and endemic megafauna in the northwestern Hawaiian Islands.
Endangered Species Research 4: 1–10.
Barnett J, Westcott S. 2001. Distribution, demographics and survivorship of grey seal pups
(Halichoerus grypus) in southwest England. Mammalia 65: 349–362.
Beauplet G, Barbraud C, Chambellant M, Guinet C. 2005. Interannual variation in the post-weaning
and juvenile survival of Subantarctic fur seals: influence of pup sex, growth rate and
oceanographic conditions. Journal of Animal Ecology 74: 1160–1172.
Best PB. 1973. Seals and sealing in South and South West Africa. The South African Shipping News
and Fishing Industry Review 28: 49–57.
Boness DJ, Bowen WD, Iverson SJ, Oftedal OT. 1992. Influence of storms and maternal size on
mother pup separations and fostering in the harbour seal, Phoca vitulina. Canadian Journal of
Zoology 70: 1640–1644.
Potential impact of a low-level SAAF Oryx helicopter flight on Cape fur seal
pup mortality in the Robberg Marine Protected Area 2020
22
Boveng PL, Hiruki LM, Schwartz MK, Bengtson JL. 1998. Population growth of Antarctic fur
seals: limitation by a top predator, the leopard seal? Ecology 79: 2863–2877.
Carrick R, Ingham SE. 1962. Studies on the southern elephant seal, Mirounga leonina. Population
dynamics and utilization. Commonwealth Science and Industrial Research Organisations.
Wildlife Research 7: 198–206.
David JHM. 1987. South African fur seal, Arctocephalus pusillus pusillus In: Croxall JP, Gentry
RL (eds), Status, biology and ecology of fur seals. Proceedings of an International Symposium
and Workshop, Cambridge, England, 23–27 April 1984. pp 65–71.
David JHM. 1995. Seals. In: Payne AIL, Crawford RJM (eds), Oceans of life off southern Africa.
Cape Town: Vlaeberg. pp 288–302.
David JHM, Rand RW. 1986. Attendance behaviour of South African fur seals. In: Gentry RL,
Kooyman GL (eds), Fur seals: maternal strategies on land and at sea. Princeton, New Jersey:
Princeton University Press. pp 126–141.
de Villiers D, Roux J-P. 1992. Mortality of newborn pups of the South African fur seal
Arctocephalus pusillus pusillus in Namibia. In: Payne AIL, Brink KH, Mann KH, Hilborn R
(eds), Benguela trophic functioning. South African Journal of Marine Science 12: 881–889.
Doidge DW, Croxall JP, Baker JR. 1984. Density-dependent pup mortality in the Antarctic fur seal
Arctocephalus gazella at South Georgia. Journal of Zoology [London] 202: 449–460.
Goschen WS, Schumann EH, Bernard KS, Bailey SE, Deyzel SHP. 2012. Upwelling and ocean
structures off Algoa Bay and the south-east coast of South Africa. African Journal of Marine
Science 34(4): 525–536.
Goschen WS, Bornman TG, Deyzel SHP, Schumann EH. 2015. Coastal upwelling on the far
eastern Agulhas Bank associated with large meanders in the Agulhas Current. Coastal shelf
research 101: 34–46.
Gucu A, Gucu G, Orek H. 2004. Habitat use and preliminary demographic evaluation of the
critically endangered Mediterranean monk seal (Monachus monachus) in the Cilician Basin
(eastern Mediterranean). Biological Conservation 116: 417–431.
Hazen EL, Abrahms B, Brodie S, Carroll G, Jacox MG, Savoca MS, Scales KL, Sydeman WJ,
Bograd SJ. 2019. Marine top predators as climate and ecosystem sentinels. Frontiers in
Ecology and the Environment 17(10): 565–574.
Hofmeyr GJG, du Toit M, Kirkman SP. 2011. Early post-release survival of stranded Cape fur seal
pups at Black Rocks, Algo Bay, South Africa. African Journal of Marine Science 33(3).
Potential impact of a low-level SAAF Oryx helicopter flight on Cape fur seal
pup mortality in the Robberg Marine Protected Area 2020
23
Huisamen J, Kirkman SP, Watson LH, Cockcroft VG, Pistorius PA. 2011. Recolonisation of the
Robberg Peninsula (Plettenberg Bay, South Africa) by Cape fur seals. African Journal of
Marine Science 33(3): 453–461.
Huisamen J, Kirkman SP, van der Lingen CD, Watson LH, Cockroft VG, Jewell R, Pistorius PA.
2012. Diet of the Cape fur seal Arctocephalus pusillus pusillus at the Robberg Peninsula,
Plettenberg Bay, and implications for local fisheries. African Journal of Marine Science 34(3):
431–441.
Hume F, Arnould J, Kirkwood R, Davis P. 2001. Extended maternal dependence by juvenile
Australian fur seals (Arctocephalus pusillus doriferus). Australian Mammalogy 23: 67–70.
Kaplan CC, White GC, Noon BR. 2008. Neonatal survival of Steller sea lions (Eumetopias
jubatus). Marine Mammal Science 24: 443–461.
Kirkman SP, Oosthuizen WH, Meÿer MA. 2006. The seal population of Seal Island, False Bay. In:
Nel DC, Peschak TP (eds), Finding a balance: white shark conservation and recreational safety
on the inshore waters of Cape Town, South Africa. Proceedings of a specialist workshop, Cape
Town. WWF SA Report Series 2006/ Marine/001. Stellenbosch: World Wide Fund for Nature
(WWF) South Africa. pp 83–94.
Kirkman SP. 2009. Evaluating seal-seabird interactions in southern Africa: a critical review.
African Journal of Marine Science 31: 1–18.
Kirkman SP. 2010. The Cape fur seal: monitoring and management in the Benguela Current
ecosystem. PhD thesis, University of Cape Town, South Africa.
Kirkman SP, Oosthuizen WH, Meÿer MA, Seakamela SM, Underhill LG. 2011. Prioritising range-
wide scientific monitoring of the Cape fur seal in southern Africa. In: Kirkman S, Elwen SH,
Pistorius PA, Thornton M, Weir CR (eds), Conservation biology of marine mammals in the
southern African subregion. African Journal of Marine Science 33: 495–509.
Le Boeuf BJ, Briggs KT. 1977. The cost of living in a seal harem. Mammalia 41: 167–195.
Lutjeharms JRE, Cooper J, Roberts M. 2000. Upwelling at the inshore edge of the Agulhas Current.
Continental Shelf Research 20: 737–761.
Lutjeharms JRE, Ansorge IJ. 2001. The Agulhas return current. Journal of Marine Systems 30:
115–138.
Maniscalco J, Calkins D, Parker P, Atkinson S. 2008. Causes and extent of natural mortality among
Steller sea lion (Eumetopias jubatus) pups. Aquatic Mammals 34: 277–287.
Potential impact of a low-level SAAF Oryx helicopter flight on Cape fur seal
pup mortality in the Robberg Marine Protected Area 2020
24
Oosthuizen WH, David JHM. 1988. Non-breeding colonies of the South African (Cape) fur seal in
southern Africa. Investigational Report, Sea Fisheries Research Institute, South Africa 132: 1–
17.
Pemberton D, Kirkwood RJ. 1994. Pup production and distribution of the Australian fur seal,
Arctocephalus pusillus doriferus, in Tasmania. Wildlife Research 21: 341–352.
Rand RW. 1956. The Cape fur seal Arctocephalus pusillus (Schreber). Its general characteristics
and moult. Investigational Report, Division of Sea Fisheries, South Africa 21: 1–52.
Rand RW. 1967. The Cape fur seal Arctocephalus pusullus pusillus 3. General behaviour on land
and at sea. Investigational Report, Sea Fisheries Institute, South Africa 60.
Rand RW. 1972. The Cape fur seal Arctocephalus pusillus. 4. Estimates of population size.
Investigational Report, Sea Fisheries, South Africa 89: 1–28.
Reed JR, Kerwath SE, Attwood CG. 2017. Analysis of bycatch in the South African midwater trawl
fishery for horse mackerel Trachurus capensis based on observer data. African Journal of
Marine Science 39(3): 279–291.
Reiter J, Panken KJ, Le Boeuf BJ. 1981. Female competition and reproductive success in northern
elephant seals. Animal Behaviour 29: 670–687.
Riedman ML, Le Boeuf BJ. 1982. Mother-pup separation and adoption in northern elephant seals.
Behavioral Ecology and Sociobiology 11: 203–215.
Ross GJB. 1971. Notes on seals and sealing in the Eastern Cape. The Eastern Cape Naturalist 44:
6–8.
Roux J-P, de Villiers DJ. 1991. Annex G: Field measurements of early pup mortality in Namibia.
In: Butterworth DS, Punt AE (eds), Report of the Benguela Ecology Program Workshop on
Seal-Fishery Biological Interactions 22.
Schumann EH, Perrins LA, Hunter IT. 1982. Upwelling along the South Coast of the Cape
Province, South Africa. South African Journal of Science 78: 238–242.
Schutte-Vlok AL, Huisamen J, Nieuwoudt H, Cleaver-Christie G. 2018. Robberg Nature Reserve
Complex Management Plan, 2013-2018. 135pp.
Sealy J. 2006. Diet, mobility, and settlement pattern among Holocene hunter-gatherers in
Southernmost Africa. Current Anthropology 47(4): 569–595
Shaughnessy PD. 1982. The status of seals in South Africa and South West Africa. Mammals of the
seas. FAO Fisheries Series 5: 383–410.
Potential impact of a low-level SAAF Oryx helicopter flight on Cape fur seal
pup mortality in the Robberg Marine Protected Area 2020
25
Shaughnessy PD. 1984. Historical population levels of seals and seabirds on islands off southern
Africa, with special reference to Seal Island, False Bay. Investigational Report, Sea Fisheries
Research Institute, South Africa 127.
Stewardson CL. 1999a. The impact of the fur seal industry on the distribution and abundance of
Cape fur seals Arctocephalus pusillus pusillus on the Eastern Cape Coast of South Africa.
Transactions of the Royal Society of South Africa 54: 217–246.
Stewardson CL. 1999b. South African Air Force wildlife rescue: Cape fur seal pups (Pinnipedia:
Otariidae) washed from Black Rocks, Algoa Bay, during heavy seas, December 1976.
Canberra: Australian National University.
Stewardson CL. 2001. Biology and conservation of the Cape (South African) fur seal. PhD thesis,
Australian National University, Australia.
Stewardson CL, Brett M. 2000. Aggressive behaviour of an adult male Cape fur seal
(Arctocephalus pusillus pusillus) towards a great white shark (Carcharodon carcharias).
African Zoology 35: 147–150.
Wickens PA, Japp DW, Shelton PA, Kriel F, Goosen PC, Rose B, Augustyn CJ, Bross CAR,
Penney AJ, Krohn RG. 1992. Seals and fisheries in South Africa – competition and conflict. In:
Payne AIL, Brink KH, Mann KH, Hilborn R (eds), Benguela trophic functioning. South African
Journal of Marine Science 12: 773–789.
Wickens P, York AE. 1997. Comparative population dynamics of fur seals. Marine Mammal
Science 13: 241–292.