African Journal of Marine Science 2011, 33(3): 523–534
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Population dynamics of southern elephant seals: a synthesis of three
decades of demographic research at Marion Island
PA Pistorius1*, PJN de Bruyn2 and MN Bester2
1 Department of Zoology, Nelson Mandela Metropolitan University, PO Box 77000, Port Elizabeth 6031, South Africa
2 Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria, Private Bag X20, Hatfield 0028,
* Corresponding author, e-mail: email@example.com
Manuscript received February 2011; accepted May 2011
Southern elephant seal Mirounga leonina numbers declined precipitously throughout most of their
circumpolar distribution since the 1950s. A long-term intensive demographic programme was initiated
in 1983 on the relatively small population of southern elephant seals at sub-Antarctic Marion Island in an
attempt to identify causative mechanisms associated with this decline. Weaned pups have been tagged
annually since 1983, and this has produced a large number of individuals of known identity. A regular
resighting programme yielded a mark-recapture dataset that has been subjected to numerous survival-
based models. This ongoing programme produced a substantial body of scientific literature on population
growth patterns, vital rates (survival and fecundity) and population regulation in southern elephant seals,
which are reviewed in this synthesis. We briefly describe the analytical framework common to much of the
demographic research, highlight important conclusions concerning population regulation of elephant seals
at Marion Island, and discuss priorities for future research.
Keywords: environmental change, fecundity, growth, mark-recapture, Mirounga leonina, population regulation, survival
The study of life-history characteristics that govern popula-
tion growth is firmly entrenched in the discipline of popula-
tion ecology, and is of both applied value and theoretical
interest (Gaillard et al. 1998, Saether et al. 2002, Baker and
Thompson 2007). Understanding the means of population
regulation, and discerning which vital rates and population
components are most responsive to environmental change,
have obvious merit in applied fields such as wildlife manage-
ment (Schwarz and Stobo 2000, Sibly et al. 2005, Lande et
al. 2006, Hone and Clutton-Brock 2007). Population growth
is furthermore disproportionally influenced by certain popula-
tion parameters (Gaillard et al. 1998, Pistorius et al. 2001a,
McMahon et al. 2005a). Identifying the key determinants of
population changes, including vital rates and affected age
categories, is key to the management of wildlife populations,
particularly when dealing with species that are of conserva-
tion concern or that impact on human welfare (Jorgenson et
al. 1997, Holmes et al. 2007). The potential for longitudinal
studies of survival and fecundity to yield robust estimates
that are free of a number of biases associated with cross-
sectional or short-term studies has long been recognised
(Caughley 1977). Possibly the most important source of
bias in cross-sectional population models is related to the
assumption that the age distribution in the population under
study is stationary, an unlikely scenario for most wildlife
populations (Caughley 1977). However, longitudinal studies
are frequently subject to stringent assumptions and are
labour intensive and lengthy in duration, necessitating a
long-term commitment to the study (Lebreton et al. 1992).
A longitudinal study of southern elephant seals Mirounga
leonina, which commenced at sub-Antarctic Marion Island
in 1983, has produced what is arguably one of the most
comprehensive demographic datasets for a large mammal
population (Bester et al. 2011). The high levels of philopatry
shown by these animals during obligatory terrestrial phases
(Condy 1979, Hofmeyr 2000), together with their relative
immobility on land, facilitates the identification of uniquely
marked individuals (Pistorius et al. 2000, de Bruyn et al.
2008) and makes this species a prime candidate for studies
of population ecology (e.g. Pistorius et al. 1999a, McMahon
et al. 2003), life-history theory (Pistorius et al. 2004, 2008a)
and large-scale environmental change (McMahon and
The Marion Island population of this circumpolar species
is relatively small and its numbers declined precipitously for
about four decades, beginning in the mid-1950s (Pistorius
et al. 2004, McMahon et al. 2009), like most other southern
elephant seal populations in the southern Indian and Pacific
oceans (Burton 1986, Hindell and Burton 1987, Guinet et
al. 1992, McMahon et al. 2005a, Authier et al. 2011). The
Pistorius, de Bruyn and Bester
concern raised by this general decline stimulated the initia-
tion of several long-term ecological studies on southern
elephant seal populations (Bester 1988, Hindell 1991,
Bester and Wilkinson 1994). The focus was on monitoring
further changes in population size (Hindell and Burton
1987, Guinet et al. 1992, 1999, Bester and Wilkinson 1994,
Pistorius et al. 1999a) and investigating causal factors of
these changes — both proximate and ultimate (Hindell 1991,
Bester and Wilkinson 1994, Pistorius et al. 1999b). Between
1951 and 1994, the Marion Island population declined by
83% (Pistorius et al. 1999b). Between 1986 and 1994, it
declined from about 2 120 to 1 330 individuals, which is an
absolute decrease of 37.2% and an annual average rate
of 5.8% (Pistorius et al. 1999a). The population appeared
stable between 1994 and 1999 (Pistorius et al. 2004), but
a recent analysis with additional years of population counts
indicated that these years were associated with an inflexion
in population growth (McMahon et al. 2009). De Bruyn
(2009) identified an inflexion in population survivorship for
the year 1994 and postulated that this resulted in a popula-
tion trend inflexion around 1998. Either way, the relatively
stable population between 1994 and 1999 allowed for the
comparison of age- and state-specific vital rates during two
distinct population trajectories (e.g. Pistorius et al. 2008b).
Therefore, inferences regarding causative factors associ-
ated with population trends became possible.
The Marion Island southern elephant seal mark-recapture
database has been subjected to numerous analyses,
resulting in several publications concerned with specific
aspects of survival and fecundity, particularly in relation to
population regulation and life-history theory. This review aims
to provide an in depth synthesis of this work and explain our
current understanding of population parameters and their
role in regulating the southern elephant seal population at
Marion Island. Future research priorities are also discussed.
The mark-recapture framework
Sub-Antarctic Marion Island (46°54′ S, 37°45′ E; Figure 1)
is located in the Prince Edward Island group in the southern
Indian Ocean, approximately 2 180 km south-east of Cape
Town, South Africa. It is 300 km2 with a coastline of approxi-
mately 90 km with varied physiognomy but predominated by
cliff faces (Chown and Froneman 2008). Southern elephant
seals regularly haul out on 54 beaches dispersed along a
51.9 km stretch of the coastline, mainly on the eastern side
of the island between Storm Petrel Bay and Goodhope Bay
(Figure 1). Neighbouring Prince Edward Island (~22 km to
the north-east), the only other island in the archipelago, is
about a quarter of the size of Marion Island. The closest land
mass to the Prince Edward Islands group is Île aux Cochons
of the Crozet archipelago — a French possession — about
950 km to the east. Chown and Froneman (2008) provide
further ecological information on Marion Island.
Data collection and analyses
A total of 13 400 recently weaned southern elephant seal
pups have been tagged since 1983 — on an annual basis
— in both of their hind flippers (annual average 479, range
389–700). Virtually all pups that survived to weaning age
were tagged, particularly in the past 15 years (Figure 2).
The tag Dal 008 Jumbotags® (Dalton Supplies Ltd, Henley-
on-Thames, UK) was used, which had a unique number
for each individual in a cohort (year of birth), and the
colour combination of the tags varied by cohort. Initially,
pups were tagged in either of the two inner inter-digital
webbings of each hind flipper (see Pistorius et al. 2000 for
details). However, the tag site was changed from the year
2000 onwards, to the upper, outer inter-digital webbing of
each hind flipper (Oosthuizen et al. 2010) to facilitate the
sighting of tags. Date, location and the sex of the pup was
recorded at tagging. In addition, since 2006, Supersmall®
tags (Dalton Supplies Ltd, Henley-on-Thames, UK) were
deployed in the inner inter-digital webbing of the right hind
flipper in unweaned pups prior to the usual tagging regime
at weaning age (de Bruyn et al. 2008). As a result, between
2006 and 2010, this protocol allowed for identification of
mother–pup relatedness of an average of 148 pups (range
109–176) annually (de Bruyn et al. 2008).
Since 1983, all the beaches where southern elephant
seals haul out regularly were checked for the presence of
tagged individuals. The searches were conducted weekly
during the breeding season (mid-August to mid-November)
and every 10 days during the rest of the year. For each
tagged seal encountered, efforts were made to read the tag
number and record the colour combination, as well as the
number of tags remaining.
The large majority of demographic studies covered here
made use of broadly similar analytical procedures, which are
briefly described below. Encounter-history matrices, which
are required for mark-recapture analyses, were constructed
from resight data. Multiple sightings during any given year
were treated as a single sighting. The peak haulout date for
females during the breeding season is 15 October (Condy
1978, Bester and Wilkinson 1994) and animals were assumed
to age on this date. The software program MARK (G White,
Colorado State University; White and Burnham 1999), which
is an application for the analysis of encounter-history matrices
of marked individuals, was used to obtain likelihood estimates
of annual survival and resighting (capture) probabilities. The
software program provides parameter estimates under the
essential Cormack-Jolly-Seber (CJS) model, but also under
a range of models that appear as special cases of this model
(Lebreton et al. 1992).
The two fundamental parameters in these models are:
the survival probability for all animals between the ith and
(i + 1)th encounter occasion, and the resighting probability
for all animals in the ith encounter occasion. The survival
probability incorporates both death and permanent emigra-
tion of individuals and can therefore be referred to as
apparent survival. It was assumed that southern elephant
seals show a strong site fidelity to their natal grounds
and that permanent emigration from Marion Island is
minimal (Hofmeyr 2000). Although these assumptions are
generally applicable, recent work provides evidence that
the assumption must be applied with caution for future
work (de Bruyn 2009, Oosthuizen et al. 2009, in press a,
in press b [see below]).
Model fit to the data were mostly assessed using the
program RELEASE goodness-of-fit procedure (Burnham
et al. 1987), implemented in program MARK to ascertain
African Journal of Marine Science 2011, 33(3): 523–534 525
whether the assumptions pertaining to the model were
met (see Lebreton et al. 1992). An information theoretic
approach was used for model selection (Burnham and
Anderson 2002) during which a set of models, including fully
parameterised models and models with various constraints
on survival and resight probability, were compared. We
used the small sample-corrected Akaike’s information
criterion (AICc) to select the most parsimonious model
that adequately described the data (Lebreton et al. 1993,
Anderson et al. 1994). The survival models used rely on the
resight probability at each resight occasion in order to allow
for survival estimation. This parameter in itself is also of
interest and was used in the estimation of reproductive rates
and costs associated with reproduction in southern elephant
seals at Marion Island (e.g. Pistorius et al. 2001a, but see
Bradshaw et al. 2002).
37°34' E 37°38' E 37°42' E 37°46' E 37°50' E 37°54' E
Storm Petrel Bay
37°45' E37°30' E 38° E
Tristan da Cunha
South Sandwich Is.
South Orkney Is.
South Shetland Is.
Figure 1: Location of Marion Island of the Prince Edward Island group, in the southern Indian Ocean
Pistorius, de Bruyn and Bester
The double tagging of study animals (i.e. one tag in
each hind flipper) allowed the potential for bias in survival
rate estimates as a result of tag loss to be assessed.
Correction factors based on this have consistently been
applied to compensate for tag loss in survival estimates
(Wilkinson and Bester 1997, Pistorius et al. 2000,
Oosthuizen et al. 2010). Independence of tag-loss rates
between the flippers of the same individual has been
assumed in the estimation of tag loss correction factors
(Pistorius et al. 2000). McMahon and White (2009) demonst-
rated in their study at Macquarie Island that this assumption
is incorrect. Due to low rates of tag loss at Marion Island
(Pistorius et al. 2000, Oosthuizen et al. 2010), it is consid-
ered likely that any biases in demographic parameters that
may have been caused by violation of this assumption were
Synthesis of demographic research
Important in the assessment of mechanisms associated with
population regulation is the availability of robust estimates
of vital rates that potentially influence population growth,
namely survival (or its inverse, mortality) and fecundity
(Pistorius et al. 2001b, McMahon et al. 2005a, Harting et al.
2007, Holmes et al. 2007, Bradshaw and McMahon 2008;
Figure 3). In gauging the influence of the environment on
population growth, it is furthermore important to be able to
assess temporal variability in these vital rates (Beauplet et
al. 2005, Baker and Thompson 2007). These are ideally
studied at an age- or state-specific level so that popula-
tion components that are proximately related to observed
population change can be identified (Wisdom et al. 2000,
Coulson et al. 2005). Such life-history parameters and their
potential contribution to population regulation in southern
elephant seals at Marion Island are dealt with below.
Although successful reproduction is an obvious condition
for population maintenance in long-lived animals such
as southern elephant seals, its role is generally thought
to be inferior to that of survival in governing population
growth (Lima and Paez 1997, Saether 1997, Gaillard et
al. 1998, Pistorius 2001, McMahon et al. 2005a; Figure 3).
Notwithstanding, its demographic function, together with
the information that can be gleaned regarding intrinsic and
extrinsic drivers of population change (Gaillard et al. 1992,
Jorgenson et al. 1993, Saether 1997, Bowen et al. 2006,
Harting et al. 2007, de Little et al. 2007), warrants the study
of reproductive rates in animal populations. For example,
a decrease in age of maturity and an increase in age-
specific fecundity were observed for the declining Marion
Island southern elephant seal population, suggesting a
density-dependent response (Pistorius et al. 2001, but
see Bradshaw et al. 2002). Body size is generally thought
to be the fundamental criterion affecting age of primiparity
(Laws 1956, Reimers 1983, Boyd 2000). It is likely that
per capita availability of prey, which comprises various
fish and squid species (Rodhouse et al. 1992, Daneri
et al. 2000, Daneri and Carlini 2002, van den Hoff et al.
2002, 2003, Hindell et al. 2003), increased as the southern
elephant seal population declined (Pistorius et al. 2001b).
Associated with this, accelerated growth and improved body
condition, in all probability stimulated the aforementioned
shifts in reproductive rates, a response that has also been
observed elsewhere (Huber et al. 1991, Gaillard et al. 1992,
Saether 1997). However, at Marion Island, growth and body
condition of individuals could not be measured directly. The
assessment of these measures and others associated with
individual experience and status represents an important
covariate along with vital rates for assessment in future
studies (de Bruyn et al. 2009).
Age of primiparity varied between three and six years
for this population, with fecundity on average being 0.16
(SE 0.04), 0.40 (SE 0.07), 0.45 (SE 0.07) and 0.50 (SE 0.06)
respectively (Pistorius et al. 2001a). Life table analyses
and stochastic growth models of this population indicate
that fecundity plays a relatively minor role in population
growth, with unrealistic adjustments to this variable being
required to significantly change the population trajectory
(Bester and Wilkinson 1994, Pistorius 2001, McMahon et
al. 2005b). In a food-limited environment, reproduction may,
however, be expected to be the first demographic variable
NUMBER OF PUPS BORN
NUMBER OF PUPS THAT
Total escaped tagging
Number of pups born
Total pups tagged
Figure 2: Numbers of southern elephant seal pups born (1986–2010) and tagged at weaning age (1983–2010) at Marion Island
African Journal of Marine Science 2011, 33(3): 523–534 527
to be compromised (Fowler 1987). This indeed appears
to be the case for southern elephant seals, with shifts in
fecundity having preceded changes in survival concurrent
with apparent changes in food availability at Marion Island
(Pistorius et al. 2001a, Pistorius et al. 2004), rendering
fecundity a useful variable to monitor in order to assess
changes in the environment.
Using age-specific resight probabilities during the breeding
season as a rational index of age-specific breeding probabil-
ities assumes that females breed virtually every year
after the age of primiparity. This has recently been shown
not to be the case at Marion Island; therefore, the validity
of this approach has been questioned (de Bruyn et al.
2011). Several studies have based conclusions regarding
demographics of adult female southern elephant seals on
this tenuous assumption (e.g. Hindell 1991, Pistorius and
Bester 2002a, McMahon et al. 2003, 2005b, Pistorius et al.
2004, 2008b). Reproductive estimates derived from such
studies should be viewed with some caution, although the
potential bias in point estimates of fecundity is likely to be
similar over time and between different adult female age
groups, thereby validating temporal and group-specific
comparisons to some degree. Moreover, at Marion Island,
detectability of seals present at the island approaches 100%,
justifying the use of such an index under certain circum-
stances (de Bruyn et al. 2011).
In colonial breeding animals, particularly those in which
breeding activities are compressed within a relatively narrow
window of opportunity, mortality of immature seals during
the course of the breeding season has the potential to have
a considerable impact on a population’s trajectory (Boveng
et al. 1998). This is particularly true in seabirds and seals in
which relatively short periods of food shortage could hamper
provisioning rates or adult body condition, leading to high
juvenile mortality levels (Soto et al. 2004). Weather conditions
(most notably storms and heavy rainfall), breeding activi-
ties associated with male dominance, and topography and
substrate of breeding habitat, could also influence levels
of early juvenile mortality in seals (Chilvers et al. 2005,
Frederikson et al. 2008). In southern elephant seals breeding
at Marion Island, pup mortality, however, does not appear
to play a large role in governing population growth. Pup
mortality during the course of the breeding season is about
4% at Marion Island with no relationship to population density
(Wilkinson 1992, Pistorius et al. 2001b), similar to (McCann
1985, Hindell and Burton 1987) or slightly higher than
(Galimberti and Boitani 1999) that reported at other breeding
sites. In contrast to some populations of northern elephant
seals, in which storms potentially have a severe impact on
pup survival (Stewart 1992), weather conditions do not appear
to play a large role in pup survival at Marion Island.
1st, 2nd, 3rd
Figure 3: A probable model depicting population regulation in southern elephant seals at Marion Island. Level of importance of parameters
(at each organisational level) in terms of governing population growth is indicated by the tone of the linking arrow (--- = low; — = medium;
= high). * Note that this refers to proportion of three-year-old females that have not commenced breeding
Pistorius, de Bruyn and Bester
Juvenile survival, or post-weaning survival over the first
three years of life prior to sexual maturity (Pistorius et al.
2001a, McMahon et al. 2005a), is a key component of
population dynamics in many large-mammal populations
(Eberhardt and Siniff 1977, Eberhardt 1981, Promislow and
Harvey 1990, Hindell et al. 1994, York 1994, Benton et al.
1995, Jorgenson et al. 1997, Hastings et al. 1999). This
parameter has furthermore been purported to be particularly
sensitive to environmental variability (Fryxell 1987, Gaillard
et al. 1998), and first year survival in particular has been
shown to be related to weaning mass of cohorts (McMahon
et al. 1999, 2000, 2003, McMahon and Burton 2005, de
Little et al. 2007).
At Marion Island, however, greater variability has been
evident in adult rather than in juvenile survival (Pistorius
et al. 1999b, Pistorius and Bester 2002a). It is uncertain
whether juvenile survival at Marion Island was proximately
related to the decline of southern elephants there (Pistorius
and Bester 2002a, McMahon et al. 2003), as has been found
for several other pinniped populations that have shown a
decline (Trites and Larkin 1989, Hindell 1991, York 1994).
However, juvenile survival did not show significant change
concurrent with the change in population growth, in contrast
to that found for adult female survival (Pistorius et al. 2004,
2008a). Indeed, based on population matrix models (Caswell
2001) applied to the Marion Island population, Pistorius
(2001) found population growth to be more sensitive to
proportional changes in adult female survival than in juvenile
survival (Figure 3). McMahon et al. (2005a), however, found
a marginally higher elasticity in juvenile survival for the same
population. As noted by de Bruyn (2009), the different result
was probably due to different delineations between juveniles
and adults. Females in their fourth year, a proportion of
which pup every year (Bester and Wilkinson 1994, Pistorius
et al. 2001a), were classified as juveniles by McMahon et al.
(2005a) and as adults by Pistorius (2001).
Irrespective of whether growth in the Marion Island
elephant seal population is most sensitive to equivalent
change in juvenile or in adult survival, the fact that juvenile
survival (at least during the first three years of life) was
stationary (Pistorius and Bester 2002a) during a period
of significant change in population growth (a fact initially
questioned by Bradshaw et al. (2002) but since confirmed
by Pistorius et al. 2004 and McMahon et al. 2009) argues
against it having been the primary driver associated with
population change at Marion Island (Figure 3). During the
first three years of life, survival in this population was 0.60
(SE 0.01), 0.81 (SE 0.02) and 0.78 (SE 0.02), without there
being significant influence by gender (Pistorius and Bester
2002a; see also McMahon et al. 1999, Pistorius et al.
1999b, de Bruyn 2009).
A proportion of juvenile southern elephant seals haul
out during winter. No particular function has as yet been
ascribed to this haulout, which is referred to as the winter
or the resting haulout (Kirkman et al. 2001). An investigation
into the fitness implications associated with this behaviour
revealed that participation in this haulout bears no survival
implications (Pistorius et al. 2002a). It is, however, likely to
be related to differential levels of philopatry among individ-
uals (Pistorius et al. 2002a).
With regard to levels of sexual dimorphism and polygamy
in marine mammals, southern elephant seals occupy the
extreme end of the scale on both accounts (Laws 1956).
This has resulted in marked structural and functional differ-
ences between the sexes and a pronounced disparity in
sex-specific adult survival (Promislow 1992, Pistorius et al.
1999b, de Bruyn 2009). Greater susceptibility to nutritional
stress and aggressive behaviour associated with male
dominance results in relatively low survival in adult males
(Toigo et al. 1997) of 0.69 (averaged over 6–11th year;
Pistorius et al. 1999b), yet males are not a limiting resource
when it comes to ensuring fertilisation of females during the
breeding season (Wilkinson and van Aarde 1999). However,
adult female survival appears to be important in governing
growth in this population (Pistorius et al. 2004, de Bruyn
2009). A comparison of adult female survival at Marion
Island during two population trajectories (declining and
stable) with that of females at the increasing population at
Peninsula Valdés, Argentina, demonstrated positive relation-
ships between adult female survival and population growth
of 0.77 (SE 0.01), 0.83 (SE 0.01) and 0.84 (SE 0.03) for the
three population trajectories respectively (Pistorius et al.
2004). The importance of adult female survival was further
supported by the fact that juvenile survival between the
Marion Island population when it was in decline and the large
stable population at South Georgia, was equivalent, whereas
adult female survival was substantially higher for the latter
(88%) (McCann 1985, Pistorius and Bester 2002a).
Young adult female southern elephant seals that are
still growing somatically (Laws 1956) are especially likely
to be susceptible to nutritional stress during their first
pregnancy and soon after primiparity (Oftedal et al. 1987,
Carlini et al. 1997, Hastings et al. 1999). In this regard,
high energetic requirements resulting from foetus nourish-
ment and accumulation of sufficient fat reserves required
for lactation may explain the pronounced changes in adult
rather than juvenile survival associated with a presumed
increase in food availability (Pistorius and Bester 2002a,
Pistorius et al. 2004). Indeed, a relationship between adult
female survival rates and the decline in southern elephant
seal numbers at Marion Island was apparent (Pistorius et al.
2004, Pistorius et al. 2008b; Figure 3), whereas an increase
of 6.2% in their survival rate was associated with a change
in population growth rate, which is thought to have facilitated
the levelling off in population numbers after the long-term
decline (Pistorius et al. 2004). A similar pattern to that
described above for adult females has also been reported for
pubescent southern elephant seal males. These males have
unusually high energetic demands and their survival is also
related to population growth (Pistorius et al. 2005). These
findings therefore provide further support in favour of the
food limitation hypothesis (Pistorius et al. 1999b, McMahon
et al. 2005a).
Except for mortality incurred soon after birth during the
breeding season (Pistorius et al. 2001b), mortality on land
in southern elephant seals at Marion Island is negligible
(Pistorius et al. 1999). Therefore, mortality mainly occurs
at sea between the time of the two distinct and highly
African Journal of Marine Science 2011, 33(3): 523–534 529
synchronised terrestrial phases that characterise the
life cycle of adult elephant seals, namely breeding and
moulting (Condy 1979, Kirkman et al. 2003). Using resight
data collected during the moulting and breeding seasons
allowed for an investigation into the seasonal components
of adult female survival during the two intervening pelagic
phases (Pistorius et al. 2008a). The estimated post-breeding
survival rate (duration of 62 days) of primiparous females
was 0.83 (SE 0.02) compared to 0.92 (SE 0.02) for more
experienced females. This indicates a cost associated with
first reproduction, which is dependent on population status
(Pistorius et al. 2008b), and may be governed by food availa-
bility (Tavecchia et al. 2005, Hadley et al. 2007, de Bruyn
2009). Post-moulting survival (255 days) was 0.85 (SE
0.01) and independent of reproductive history. Per unit time,
this implies much lower survival during the post-breeding
pelagic phase. This is perhaps not surprising as a complete
separation from food resources takes place during the three
and four weeks while on land during breeding and moulting
respectively (Condy 1979, Kirkman et al. 2003). In conjunc-
tion with the costs associated with weaning of a pup during
the breeding season, this is likely to render post-breeding
females at a high risk of starvation relative to post-moulting
Adult female survival during the two pelagic phases has
been shown to vary independently (Pistorius et al. 2008b).
This highlights the importance of studying survival in this
species at a seasonal scale, with consideration of covari-
ates such as body mass that can be expected to be associ-
ated with foraging ecology (McIntyre et al. 2010, Tosh 2010).
This would be most relevant when ascertaining the effect of
environmental variability on survival.
Research on ageing is fundamental to the understanding of
life-history parameters and their consequences on population
demography. Senescence, generally viewed as the rate of
increase in age-specific mortality or reduction in reproduct ive
rates with age, which results from degenerative changes in
the organism (Abrams 1991, Promislow 1992, Nussey et al.
2008), is widely encountered in wild populations (Caughley
1966, Promislow 1992, Gaillard et al. 1993, 1994, Jorgenson
et al. 1997). One of the first long-term studies of senescence
in a marine mammal based on mark-recapture data did
not demonstrate a senescence effect (reduction in either
survival or fecundity) for southern elephant seals at Marion
Island (Pistorius and Bester 2002b). However, a subsequent
analysis, including an additional decade of mark-recapture
data, provided evidence for reproductive senescence in
females older than 12 years (de Bruyn 2009). Interestingly,
this effect was skewed towards individuals that had
undergone primiparity at a relatively young age (de Bruyn
Despite a dataset encompassing 25 years of longitudinal
data, which extends beyond the observed longevity of the
species (Hindell and Little 1988), no evidence for actuarial
senescence (increased mortality with age) was found for
the Marion Island elephant seals (de Bruyn 2009). It has
therefore been argued that a consequence of the observed
age-specific mortality is that no individuals survive to the age
where physiological decline results in mortality (Pistorius
and Bester 2002b, de Bruyn 2009). This may be an indica-
tion of the toll that food limitation takes on survival because
previous studies that detected senescence dealt with
populations in which adults that were unconstrained by food
availability demonstrated high annual survival rates (Gaillard
et al. 1993, Jorgenson et al. 1997). Conversely, the analyt-
ical procedures employed by Pistorius and Bester (2002b)
and de Bruyn (2009) may not be optimal for describing the
presence/absence of senescence (PJNdB unpublished
data), which may be described best using continuous
models (e.g. Loison et al. 1999). Mark-recapture procedures
for fitting Gompertz and Weibull models explicitly may also
provide a more reliable test of senescence (e.g. Gaillard et
Density dependence and food limitation
Density dependence is a widely recognised form of popula-
tion regulation (Brook and Bradshaw 2006). In essence, it
entails a density feedback on population parameters as a
result of per capita resources being dependent on popula-
tion density. Density dependence has most commonly
been demonstrated in terms of food availability (e.g.
Festa-Bianchet et al. 2003). As population numbers change,
so does the rate of food consumption, ultimately leaving
more or less food per individual. This in turn impacts on
either survival, reproduction, or both, altering population
growth and ultimately population numbers. In southern
elephant seals at Marion Island, density-dependent popula-
tion regulation was initially inferred when increased rates of
reproduction and earlier onset of maturity were observed for
females (see above). Although this inference was criticised
because of premature speculation regarding a change in
population growth rate (Pistorius et al. 2001a, Bradshaw
et al. 2002), these changes were subsequently confirmed
(Pistorius et al. 2004, 2008a, McMahon et al. 2009). On
land, density dependence does not seem to limit popula-
tion growth at Marion Island. Smaller harems tend to have
the highest level of pup mortality, probably on account of
the presence of younger, less-experienced females, and
possibly due to suboptimal harem sites (Pistorius et al.
Food limitation is thought to have been ultimately account-
able for the changes in population growth of the Marion
Island southern elephant seal population during the study
period (Pistorius et al. 1999a, 2008a, McMahon et al. 2009).
Because of their extensive foraging distributions (Bester
1989, Jonker and Bester 1997, Campagna et al. 1999,
Hindell et al. 2003, Bradshaw et al. 2004, Biuw et al. 2007,
Tosh et al. 2009, Tosh 2010), the life-history param eters
of elephant seals are expected to reflect the productivity of
large ocean ecosystems, rendering them important potential
indicator species of large-scale environmental change
(McMahon et al. 2003). However, whereas it is likely that
changes in food availability have been associated with
large-scale environmental changes (Hindell 1991, Pistorius
et al. 1999a, Reid and Croxall 2001, McMahon et al. 2003,
Weimerskirch et al. 2003, de Bruyn 2009), evidence of
associations between proxies of environmental change,
such as sea surface temperature and the El Niño Southern
Oscillation expressed as the Southern Oscillation Index,
remains limited for southern elephant seals (McMahon and
Pistorius, de Bruyn and Bester
Burton 2005, de Little et al. 2007; also see Nevoux et al.
2007, 2010 for examples of other species). For example,
during the analyses of seasonal resight data of southern
elephant seals (Pistorius et al. 2008b), inclusion of either
of the above proxies as predictive variables associated
with survival did not improve model fit (∆AIC > 2; PAP
unpublished data). McMahon et al. (2009) also found little
evidence of large-scale environmental changes on southern
elephant seal population numbers or population growth,
although McMahon and Burton (2005) showed a significant
relationship between pup survival at certain locations and
the Southern Oscillation Index.
As the only confirmed predator of southern elephant seals
at Marion Island, killer whales Orcinus orca could have the
potential to impact growth in this population (Pistorius et
al. 2002b, McMahon et al. 2003, Reisinger et al. 2011a).
However, a recent study by Reisinger et al. (2011b) showed
that predation pressure by killer whales does not impact
significantly on the seal population. The fact that changes
in survival observed throughout the mark-recapture study
were not uniform across demographic groups also does not
support the notion that predation has had a major impact on
the status of this population. Furthermore, first-year survival
of elephant seals at Marion Island has been relatively high
and constant during the past three decades (Pistorius
and Bester 2002a). This would not be expected if killer
whale predation was having a major impact on population
growth as they primarily target post-weaning juvenile seals
(Pistorius et al. 2002b, Reisinger et al. 2011b).
Priorities for future research
Underpinning much of the demographic work that has been
carried out on southern elephant seals is the assumption
that the respective populations are negligibly influenced by
permanent immigration and emigration. In the estimation of
survival rates, emigration is particularly important in survival
models as it functionally translates into mortality which leads
to negatively biased estimates (Lebreton et al. 1992). The
assumption of low levels of dispersal (especially emigration)
in the southern elephant seal population at Marion Island is
based on the high level of philopatry of the species at this
locality (Hofmeyr 2000), and the appreciable genetic differ-
ences between the major global stocks of southern elephant
seals indicating minimal cross-dispersal (Slade 1997,
McMahon et al. 2005b).
However, dispersal across stocks is not required for an
animal to be lost to a mark-recapture study if more than one
haulout locality (island) is available within the geographic
limits of one ‘stock’ (de Bruyn 2009). Whereas Hofmeyr
(2000) found that native tagged individuals hauled out at
Marion Island demonstrated high levels of fidelity to the
beaches on the island where they were born, the author
could not identify temporary emigration of tagged individuals
(e.g. to neighbouring Prince Edward Island or further afield)
(de Bruyn 2009). If such temporary emigration is significant,
capture probability estimates and consequently survival
estimates from the mark-recapture programme would be
compromised (Lebreton et al. 1993). Field observations of
large numbers of unmarked seals present at Marion Island
(Oosthuizen et al. in press a) have cast further doubt on the
veracity of the closed-population assumption, given that tag
loss could not account for these unmarked seals (Oosthuizen
et al. 2010). The question arises: if so many immigrants haul
out at Marion Island, how many native tagged seals could
similarly be hauling out elsewhere? Even low levels of
migration in a small population can potentially have a signifi-
cant influence on population growth (McMahon et al. 2005a).
There is also the issue of whether the tagged and untagged
components of the local population adhere to similar regula-
tory parameters, being of different origin and subject to
differing local behaviour and haulout patterns (Oosthuizen et
al. in press a).
Some temporary emigration occurs from Marion Island to
nearby (~22 km distant) Prince Edward Island (Oosthuizen
et al. 2009), with several studies having reported on more
widespread movement of marked elephant seals within the
region (Bester 1988, Guinet et al. 1992, Reisinger and Bester
2010, Oosthuizen et al. in press b). These findings suggest
that emigration from Marion Island does in all likelihood have
some impact on demographic variables that are derived from
tagged individuals. It may be reasonable to assume similar
rates of emigration throughout the study period. Under this
assumption, age-specific temporal comparisons would not
be expected to have been adversely affected by temporary
emigration, although point estimates of survival may have
been negatively biased. It is recommended that a multistate
modelling approach, including an ‘unobservable’ state that
takes into account Markovian temporary emigration of seals
from the study site (Kendall and Nichols 2002, Schaub et
al. 2004), be implemented in future mark-recapture-based
life-history studies of the elephant seal population at Marion
Population growth is a function of survival and fecundity
but also of migration in and out of the population. It is unlikely
that immigration balances emigration at Marion Island, partic-
ularly when considering the presence of much larger colonies
within the Kerguelen stock of elephant seals (McMahon
et al. 2005a, Authier et al. 2011). Accordingly, in future
demographic studies, caution should be taken when relating
vital rates to population growth. Where possible, modelling of
population growth should incorporate possible scenarios of
emigration and immigration (Oosthuizen et al. in press a, in
Individual variation in seal body size, regardless of age, will
affect foraging ability (Weise et al. 2010) and consequently the
ability to survive periods of food limitation and the demands of
pup provisioning (Wheatley et al. 2006). Therefore, assessing
the relationship between body size fluctuations of known-age
females over time, their survival and their offspring survival,
has been promoted as a research priority for Marion Island
(de Bruyn et al. 2008). Recent advances in the use of
photogrammetry that facilitate relatively simple but accurate
mass estimation of adult elephant seals will allow this
covariate to be incorporated into future survivorship modelling
(de Bruyn et al. 2009).
Although the mechanism(s) involved remain poorly
understood, it seems likely that changes in southern elephant
seal numbers can be attributed to large-scale environmental
processes (Hindell 1991, Pistorius et al. 1999b, McMahon et
African Journal of Marine Science 2011, 33(3): 523–534 531
al. 2008). It may be associated with a large-scale ecosystem
‘regime shift’ in the Southern Ocean (Weimerskirch et al.
2003), a shift that could potentially be linked to the extent
of sea-ice in this region, which impacts strongly on ocean
productivity (Atkinson et al. 2004, McMahon and Burton
2005). Should large-scale environmental change be instru-
mental in regulation of southern elephant seal populations,
it is important to be able to quantify the relationship between
large-scale physical processes (e.g. Antarctic Circumpolar
Wave and the Southern Oscillation Index) and southern
elephant seal demographic rates (de Little et al. 2007),
mediated through their food resources. Consequently, it
is imperative that efforts be made to better understand the
mechanisms involved so as to allow information on life-history
variability to be useful for assessing the state of the Southern
Ocean ecosystem. Such studies, particularly when relating
survival to large-scale processes, are likely to be most useful
if they are investigated at a seasonal rather than an annual
scale (Pistorius et al. 2008a).
Acknowledgements — The Department of Environmental Affairs
provided both financial and logistical support for research at Marion
Island in earlier years. More recently the Department of Science
and Technology provided the funding, managed by the National
Research Foundation. We are indebted to numerous field personnel
for their dedicated marking and resighting of elephant seals on
Marion Island when we were not in the field. Clive McMahon and
an anonymous reviewer are thanked for their comments on the
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