P.A. Pistorius á M.N. Bester á S.P. Kirkman

Survivorship of a declining population of southern elephant seals,Mirounga leonina, in relation to age, sex and cohort

Received: 14 December 1998 /Accepted: 14 June 1999

Abstract This study quanti®ed both the age- and sex-speci®c survival rates of juveniles and adults, and testedfor interannual di�erences in age-speci®c survival rates ofthe southern elephant seal population at Marion Island.Pups were tagged on an annual basis from 1983 onwardsat Marion Island, and a consistent recapture programyielded data that was analysed using the software packageMARK to obtain maximum-likelihood estimates of sur-vival and capture probability. On average, 1st-year sur-vival was 0.58 and 0.62, and survival rate averaged overthe ®rst 3 years of life, 0.69 and 0.74 for males and fe-males, respectively. Fromyears 4 to 9, the average survivalrate was 0.66 and 0.75 formales and females, respectively.Survival estimates for elephant seals in their 10th±13thyear are also presented, although these are based on verysmall sample sizes. Averages of age-speci®c survival esti-mates from the earlier (mostly 1983±1987 cohorts) andlater (mostly 1988±1992 cohorts) periods were comparedand considerable reductions were observed in 4th- and5th-year male survival, and 4th-year female survival. Thecomparatively low adult survival is suggested as theproximate cause, and food limitation as deduced from thedecline in survival of elephant seals with comparativelyhigh energetic demands as the ultimate cause behind thepopulation decline atMarion Island. Although not tied inwith the decline of the population, 1987, 1990 and 1993were identi®ed as high-mortality years.

Key words Survival á Decline á Mark-recapture áElephant seals á Marion Island

Introduction

Fundamental to the study of animal population biologyis an understanding of population dynamics (Lebretonet al. 1993). The study of population dynamics, andhence life history parameters, is becoming increasinglyimportant in the ®eld of ecology and a major objective isto detect and analyse di�erences in life history traitsamong groups of individuals through space and time(Lebreton et al. 1992). These di�erences often inducechanges in survival and fecundity, which in turn governrates of population change (Bowen et al. 1981).

As is the case with several other southern elephantseal populations, the Marion Island population has de-clined over the past few decades (Barrat and Mougin1978; Condy 1978; Bester 1980; Skinner and Van Aarde1983; Pascal 1985; Burton 1986; Hindell and Burton1987; Guinet et al. 1992; Bester and Wilkinson 1994),with no single cause being unequivocally identi®ed.Birth, death, immigration and emigration are the fourfundamental demographic parameters that in¯uence thesurvival of a population (Brewer 1994). While the for-aging ranges of several elephant seal populations over-lap, there appears to be very limited immigration andemigration between the di�erent populations as a con-sequence of the high level of philopatry characteristic ofthis species (Hindell and Little 1988; Bester 1989). Thedecline of the Marion Island population is therefore aresult of mortality rate exceeding birth rate.

The most reliable method to determine age-speci®csurvival and fecundity is to follow the fate of a group ofindividuals, all born during the same time interval(Begon and Mortimer 1986). Long-term monitoring isrequired to yield the necessary data to test for age-, sex-and time-dependent e�ects on survivorship. A compre-hensive elephant seal tagging programme commenced atMarion Island in 1983, rendering consistent long-termrecapture data. The aim of this study was to assess age-and sex-speci®c survival rates in ten cohorts of southernelephant seals at Marion Island. This was undertaken to

Oecologia (1999) 121:201±211 Ó Springer-Verlag 1999

P.A. Pistorius (&) á M.N. Bester á S.P. KirkmanMammal Research Institute,Department of Zoology and Entomology,University of Pretoria,Pretoria 0002, South Africae-mail: ppistorius@zoology.up.ac.za,Fax: +27-12-3625242

investigate possible proximate causes of the observedpopulation decline, which would permit speculation asto the ultimate cause.

Survival parameters are available for the decliningMacquarie Island and Marion Island populations(Hindell 1991; Wilkinson 1991; McMahon et al. 1999),the stable South Georgia and Falkland Islands popula-tions (McCann 1985; Galimberti and Boitani 1999), andthe increasing northern elephant seal population at AnÄ oNuevo, California (Le Boeuf et al. 1994). Severalbranded cohorts of weaned pups at Macquarie Islandwere followed over their entire lifespan (Hindell 1991;McMahon et al. 1999) and survival estimates were basedon the minimum number of individuals alive at each age(Hindell 1991), whereas survival data for the SouthGeorgia population were based on the age structure ofshot samples. While these studies were mostly based onlimited data sets, their ®ndings have made possible somecomparison between populations.

Materials and methods

Mark-recapture methods are widely used to obtain data for esti-mations of the various parameters of animal populations, wherethese data consist of a record of the captures and recaptures ofmarked animals obtained over a period of time (Manly 1970; Be-gon and Mortimer 1986; Lebreton et al. 1992). Between 1983 and1997, a total of 7391 (average: 499 tagged annually, range: 394±700) of recently weaned elephant seal pups were double tagged atMarion Island (46°54¢S, 37°45¢E), with uniquely numbered, colour-coded Dal 008 Jumbotags (Pistorius et al. 1999a). Date, locationand pup sex were recorded at the time of tagging. All the studybeaches (32 along a 51.9-km coastline) were checked weekly duringthe breeding season (mid-August to mid-November), and every10 days outside the breeding season, for tagged elephant seals ex-cept prior to 1990, when no censuses were conducted during thewinter months (June, July and August). The reason for the morefrequent censuses during the breeding season was related to theincreased di�culty of reading tags in harems. All seals were as-sumed to age on 15 October, which is the peak haul-out date foradult females on all the Indian Ocean breeding sites (Condy 1978;Bester and Wilkinson 1994). For each tagged seal that was re-sighted, attempts were made to record the tag number and colourcombination, the number of tags remaining (one or two), locationand date of sighting. The data were entered into the DBASE IV(Ashton Tate) software package for future analysis. Where possible,tags were read without physical contact tominimize disturbance, butwhen necessary the hind ¯ippers were spread. All the beaches that areregularly frequented by elephant seals are accessible on foot. Whenindividuals hauled out onto inaccessible beaches, attempts weremade to read the tags with binoculars from the cli�-tops.

Capture-history matrices were constructed using the recapturedata from the 1983±1992 cohorts, treating multiple sightings withina year as a single sighting. Fourteen years of capture history wereavailable for the 1983 cohort and 5 years for the 1992 cohort. Thesematrices were used as input ®les for the computer software packageMARK (G. White, University of Colorado), designed to obtainmaximum-likelihood estimates of survival and capture probabilityrates from the resighting of marked individuals. The softwareprogram provides parameter estimates under the essential Cor-mack-Jolly-Seber (CJS) model (Cormack 1964; Jolly 1965; Seber1965), but also under several models that appear as special cases ofthis model (Lebreton et al. 1992).

The two fundamental parameters in these models are: / � thesurvival probability for all animals between the ith and (i+1)th

sample (i � 1, . . ., k±1), and p � the capture probability for allanimals in the ith sample (i � 1, . . ., k).

Goodness-of-®t (GOF) tests of the CJS model, which is a fulltime-dependent model, were performed to check the assumptionspertaining to the model using the RELEASE program (Burnhamet al. 1987). As age dependence was assumed, and the di�erentcohorts were treated separately, Test 3.Sm was retained (see Le-breton et al. 1992). In the present study it was impossible to dis-tinguish between time-dependent and age-dependent survival (sincetime and age intervals were equivalent), and this prevented us fromperforming pseudo-GOF tests as described in Lebreton et al.(1992) and applied by Gaillard et al. (1993).

The CJS model is often criticised as being too general, due tothe fact that separate parameters are included for each survival andcapture probability (Cormack 1979; Pollock et al. 1990). Themodels that appear as special cases of this model are generallyreferred to as constrained models, the constraints mostly involvingequality between parameters. If a reduction in parameters is justi-®ed by the data, it results in a more accurate estimation of theremaining parameters (Lebreton et al. 1992).

Proper model selection, which is the primary issue in the anal-ysis of capture-mark-recapture (CMR) data (Anderson et al. 1994),was accomplished using the Akaike Information Criterion (AIC).AIC is a standard procedure for model selection in a CMR context,and it weighs the quality of ®t (deviance) and the precision (via thenumber of parameters), so as to select the most parsimoniousmodel that adequately describes the data (Lebreton et al. 1993;Anderson et al. 1994; Loison et al. 1994). This method was used toselect between the full-time dependent model and the constant-capture-probability model (assuming no year-to-year changes incapture probability) for each cohort. The model with the lowestAIC value was selected for each cohort. Survival rates in largemammals are expected to plateau as animals achieve adult status(Caughley 1977). All the females at Marion Island are recruitedinto the adult population at 6 years of age (Wilkinson 1991). Al-though puberty in southern elephant seal males is reached betweenabout 4 and 6 years of age (Laws 1956; Carrick et al. 1962), socialmaturity is only attained at about 8 years of age (Laws 1984; S.P.Kirkman, unpublished results). Survival probabilities were there-fore maintained constant from age 8 and 6 onwards for males andfemales, respectively, and if this reduced the AIC value further, themodel was selected. AIC values for the simplest models (bothcapture and survival probabilities constant over time) were alsoprovided. As a major aim of the study was to determine which agecategories are most closely associated with the population decline,estimates from the latter model were, however, not provided evenwhen lowered AIC values were observed.

Age-speci®c tag retention rates, estimated from double-taggedindividuals (Pistorius et al. 1999a), were used to adjust the survivalestimates to compensate for tag loss.

The probability of surviving to a given age (lx) was calculatedfrom the product of all the survival values (/) prior to that age. A lifetablewas drawnup, formales and for females, using survival data (/)averaged across the cohorts 1983±1992. This yielded a general rep-resentation of the survival schedule for the elephant seal populationatMarion Island, aswell as reducing the possible e�ects of short-termenvironmental and demographic stochasticity on the survival esti-mates. It is important to di�erentiate between cohort and stationarylife tables (Caughley 1977; Krebs 1985). The cohort life table (above)provides the proportion of individuals surviving to each age, whereasthe stationary life table provides the proportion of animals, relativeto the number of newborns, in each age class at a particular time.

The lx values were used to determine the probability of dying(dx � lx ) lx + 1), and mortality rate (qx � dx/lx), whereas sur-vival rates were taken from the MARK survival estimates(/ � px).

Age-speci®c survival estimates from the ten consecutive co-horts were plotted against each other, ®rst to illustrate interan-nual changes in survival estimates, and second, in an e�ort toidentify years of poor survival. The cohorts for which comparableage-speci®c survival estimates were available were separated intotwo groups, to relate estimate means of the earlier (mostly 1983±

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1987) and later periods (mostly 1988±1992). The median valuewas included into both periods in cases where means were cal-culated using uneven numbers of estimates.

Results

Table 1 shows the results of the GOF tests (programRelease) for males and females separately for each co-hort. Eight of the 20 data sets showed departures fromthe CJS model (1983, 1985, 1986, 1987 and 1990 males,and 1986, 1987 and 1992 females), and it is suggestedthat between-age di�erences in survival could have ac-counted for these departures (the CJS model fails toaccount for age dependence).

Table 1 Goodness-of-®t tests of the Cormack-Jolly-Seber (CJS)model by southern elephant seal cohort

Cohort Males Females

v2 df P v2 df P

1983 8.37 3 0.039 1.32 3 0.7241984 1.79 4 0.774 3.61 1 0.0581985 6.25 2 0.044 8.51 5 0.1301986 12.08 3 0.007 9.90 4 0.0421987 12.51 1 0.001 6.35 2 0.0421988 0.301 3 0.960 7.66 4 0.1051989 0.001 2 0.999 1.09 3 0.7801990 7.225 1 0.007 1.23 2 0.5401991 0.001 1 0.999 4.88 2 0.0871992 0.001 1 0.999 1.24 1 0.027

Cohort Males Females

Model np DEV AIC Model np DEV AIC

1983 (1) (/t, pt) 24 213.60 1207.74 (/t, pt) 24 228.28 1287.28(2) (/t, pc) 12 224.61 1201.65 (/t, pc) 14 243.49 1281.20(3) (/c, pc) 2 237.14 1193.57 (/c, pc) 2 265.49 1278.43(4) (/1±9, pc) 10 232.66 1205.52 (/1±7, pc) 8 250.76 1275.95

1984 (1) (/t, pt) 23 144.16 1187.62 (/t, pt) 23 277.55 1370.52(2) (/t, pc) 13 162.80 1184.81 (/t, pc) 13 291.63 1363.39(3) (/c, pc) 2 173.82 1173.17 (/c, pc) 2 337.12 1386.30(4) (/1±9, pc) 10 164.55 1180.29 (/1±7, pc) 8 297.39 1358.79

1985 (1) (/t, pt) 21 267.67 1931.19 (/t, pt) 21 363.63 2182.07(2) (/t, pc) 12 280.09 1924.87 (/t, pc) 12 395.59 2195.33(3) (/c, pc) 2 298.31 1922.75 (/c, pc) 2 413.70 2193.10(4) (/1±9, pc) 10 280.19 1920.87 (/1±7, pt) 18 366.70 2178.86

1986 (1) (/t, pt) 19 206.14 1465.05 (/t, pt) 19 394.34 2077.61(2) (/t, pc) 11 216.87 1459.00 (/t, pc) 11 402.60 2069.29(3) (/c, pc) 2 224.63 1467.21 (/c, pc) 2 431.77 2080.16(4) (/1±9, pc) 10 216.87 1456.94 (/1±7, pc) 8 408.05 2068.60

1987 (1) (/t, pt) 17 208.94 1507.28 (/t, pt) 17 297.27 2014.57(2) (/t, pc) 10 213.91 1497.68 (/t, pc) 10 320.66 2023.50(3) (/c, pc) 2 241.99 1509.47 (/c, pc) 2 344.66 2031.27(4) (/1±7, pt) 16 297.57 2012.79

1988 (1) (/t, pt) 15 174.13 1597.08 (/t, pt) 15 149.74 1505.00(2) (/t, pc) 9 176.65 1587.19 (/t, pc) 9 181.50 1524.84(3) (/c, pc) 2 187.12 1583.42 (/c, pc) 2 45.09 1550.03(4) (/1±7, pt) 15 149.74 1505.48

1989 (1) (/t, pt) 13 155.50 1209.73 (/t, pt) 13 165.99 1426.97(2) (/t, pc) 8 157.57 1201.39 (/t, pc) 8 182.47 1431.05(3) (/c, pc) 2 37.30 1236.64 (/c, pc) 2 17.11 1444.08

1990 (1) (/t, pt) 11 136.03 1135.37 (/t, pt) 11 170.01 1423.97(2) (/t, pc) 7 145.46 1136.53 (/t, pc) 7 177.93 1423.67(3) (/c, pc) 2 158.12 1138.99 (/c, pc) 2 5.46 1427.26

1991 (1) (/t, pt) 9 64.67 926.99 (/t, pt) 9 58.02 1136.88(2) (/t, pc) 6 68.63 924.76 (/t, pc) 6 64.46 1137.16(3) (/c, pc) 2 81.39 929.38 (/c, pc) 2 15.28 1152.16

1992 (1) (/t, pt) 7 40.09 706.69 (/t, pt) 7 47.27 862.72(2) (/t, pc) 5 43.87 706.34 (/t, pc) 5 48.89 860.23(3) (/c, pc) 2 77.85 734.20 (/c, pc) 2 82.42 887.66

Table 2 Elimination of non-signi®cant e�ects from the full CJSmodel in modelling survival in southern elephant seals at MarionIsland: for each model, the number of estimable parameters (np),the deviance (DEV), and the Akaike Information Criterion (AIC)

are given. The italicised AIC value represents the selected model (/t

time-dependent survival rate, pt time-dependent capture prob-ability, /1±7/9 survival probability constant after year 6/8, pc con-stant capture probability)

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The constant-capture-probability model was selectedfor the majority of cohorts, and in several cases, theconstant-adult-survival model proved to be moreparsimonious than the full time-dependent model(Table 2). Survival estimates with standard errors, andcapture probability estimates are presented in Appen-dix 1. Cohort life tables were constructed from theage-speci®c survival estimates averaged over the1983±1992 cohorts, for males and for females, and arepresented in Tables 3, 4.

The most conspicuous peak in mortality for bothsexes occurred in the 1st year of life. There were subse-quent peaks in the 4th- and 9th-year mortality amongmales. Although a 100% mortality is observed in the13th year, this is likely to be an artefact of the very smallsample size. After year 1, relatively high mortality rateswere evident among 2-, 5- and 8-year-old females.Nevertheless, females in their 2nd±8th year had rathersimilar mortality rates, with a range of only 7.9%.

Mortality rates were higher among males than amongfemales in all the age classes (Tables 3, 4). The most

remarkable sexual di�erences in mortality were foundfor ages 3, 8, 9 and 10.

A reduction in survival was seen for several age classeswhen comparing the averages of the age-speci®c survivalestimates from the earlier (mostly 1983±1987 cohorts)and later (mostly 1988±1992 cohorts) periods (Table 5).The decrease in survival of 3- and 4-year-old males isparticularly noteworthy. Among females, 3-year-oldsexperienced the most severe reduction in survival.

The high mortality of 3-year-old females in the 1983cohort (Table 2A, Fig. 1d) corresponds to a high mor-tality of 2-year-old females in the 1984 cohort(Table 3A, Fig. 1c). Both these groups therefore expe-rienced high mortalities in 1987. This further corre-sponds to a high 2nd- and 3rd-year male mortality(Fig. 1b, c) in 1987. A high female mortality in the 4thyear for the 1989 cohort similarly corresponds to a highfemale mortality in the 5th year for the 1988 cohort(Fig. 1d, e), hence a high mortality in 1993. This issupported by the high 1st-year mortality (Fig. 1a), 3rd-year male mortality (Fig. 1c), 4th-year male mortality(Fig. 1d), 5th-year male mortality, and 6th- and 7th-yearmale mortality in 1993. Although not as clear, a similarpicture holds for 1990. Females of the 1987 cohort intheir 3rd year and females of the 1985 cohort in their 5thyear experienced high mortalities, although females intheir 4th year in the 1986 cohort experienced extremelylow mortality. In addition, in 1990, high mortalities werealso experienced by 1st-year males (Fig. 1a), 2nd-yearfemales (Fig. 1b), 3rd-year females (Fig. 1c), 4th-year

Table 3 Life table for male southern elephant seals (Miroungaleonina) from Marion Island (survival estimates averaged over co-horts 1983±1992)

Age (x) Survival(lx)

Mortality(dx)

Mortalityrate (qx)

Survivalrate (px)

0 1.000 0.414 0.414 0.5861 0.586 0.144 0.246 0.7542 0.442 0.114 0.258 0.7423 0.328 0.119 0.364 0.6364 0.209 0.057 0.274 0.7265 0.151 0.038 0.252 0.7486 0.113 0.035 0.305 0.6957 0.079 0.020 0.253 0.7478 0.059 0.024 0.414 0.5869 0.034 0.012 0.346 0.65410 0.023 0.007 0.322 0.67811 0.015 0.004 0.244 0.75612 0.012 0.012 1.000 0.00013 0.000

Table 4 Life table for female southern elephant seals (M. leonina)from Marion Island (survival estimates averaged over cohorts1983±1992)

Age(x)

Survival(lx)

Mortality(dx)

Mortalityrate (qx)

Survivalrate (px)

0 1.000 0.372 0.372 0.6281 0.628 0.121 0.193 0.8072 0.507 0.110 0.217 0.7833 0.397 0.075 0.190 0.8104 0.321 0.074 0.229 0.7715 0.248 0.060 0.242 0.7586 0.188 0.042 0.222 0.7787 0.146 0.037 0.251 0.7498 0.109 0.029 0.263 0.7379 0.081 0.020 0.245 0.75510 0.061 0.014 0.238 0.76211 0.046 0.010 0.219 0.78112 0.036 0.011 0.290 0.71013 0.026

Table 5 Comparison of mean age-speci®c survival estimates forthe elephant seal population on Marion Island over two discretetime periods

Age Cohorts Males Females

Survival Di�erence(%)

Survival Di�erence(%)

0 1983±1987 0.611 0.6391988±1992 0.561 )5.0 0.616 )2.3

1 1983±1987 0.736 0.7981988±1992 0.773 +3.7 0.815 +1.7

2 1983±1987 0.704 0.7841988±1992 0.779 +7.5 0.781 )0.3

3 1983±1987 0.684 0.8561988±1992 0.588 )9.6 0.764 )9.2

4 1983±1987 0.775 0.7671987±1991 0.703 )7.2 0.767 0

5 1983±1986 0.747 0.7641987±1990 0.749 +0.2 0.752 )1.2

6 1983±1986 0.679 0.7551986±1989 0.684 +0.5 0.790 +3.5

Fig. 1a±f Survival of southern elephant seals (Mirounga leonina) fromMarion Island. a Inter-cohort comparison (1983±1992) of 1st-yearsurvival. b Inter-cohort comparison (1983±1992) of 2nd-year survival.c Inter-cohort comparison (1983±1992) of 3rd-year survival. d Inter-cohort comparison (1983±1992) of 4th-year survival. e Inter-cohortcomparison (1983±1991) of 5th-year survival. f Inter-cohort compar-ison (1983±1990) of 6th-year survival

c

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males (Fig. 1d), 5th-year males and females (Fig. 1e),and 6th-year males (Fig. 1f). The elephant seal popula-tion on Marion Island therefore experienced exception-ally high rates of mortality in 1987, 1990, and 1993.

Variation in survivorship among cohorts was sub-stantially lower up to age 3 than for older animals [meancoe�cient of variation (CV) is 9.1% compared to 14.6%for ages 3±6; CV estimates for older animals are notconsidered due to a likely increase as a function ofsmaller sample sizes].

Discussion

A decline in numbers of a population that is not subjectto immigration or emigration is caused by a decrease insurvival and/or reproductive success (York 1994). Theknown dispersion and dispersal of elephant seals fromMarion Island has been summarized (Bester 1989; Jon-ker and Bester 1998), with the only noteworthy emigra-tion being to Ile de la Possession, where they are mostlyobserved during the moulting phase. A single 3-year-oldfemale from Marion Island gave birth on Possession Is-land in 1988, and a 5-year-old female with a pup wassighted on Hog Island in 1989 (Guinet et al. 1992).Movement of elephant seals (mostly juveniles) has beenrecorded between Marion Island and Prince Edward Is-land, 23 km to the north-east (Panagis 1981; Bester1989). Nevertheless, emigration appears to be limitedand is assumed to have a negligible e�ect upon the dy-namics of the Marion Island elephant seal population.

Fecundity is well recognized as an important param-eter relating to the dynamics of a population (Caughley1977; Krebs 1985). Population growth rate in largemammals is, however, more in¯uenced by survival thanby fecundity (Choquenot 1991; Lima and Paez 1997;Saether 1997). Bester and Wilkinson (1994) manipulatedlife table data from the Marion Island elephant sealpopulation, and demonstrated that even when fecunditywas set at a maximum, the population growth rate wasstill negative. Furthermore, the mean age at ®rst pupping(1983 cohort) was similar to that of the stable SouthGeorgia population (McCann 1985; Bester and Wilkin-son 1994), and the apparent 100% pupping rates of fe-males age 6 and older were higher than the 85% and97.8% recorded for southern and northern elephant seals,respectively (McCann 1985; Le Boeuf and Reiter 1988;Bester andWilkinson 1994). The population decline musttherefore be a consequence of either a decrease in adultsurvival, juvenile survival or a combination of these.

Age-speci®c survival

A previous long-term study on life history parameters ofsouthern elephant seals on Macquarie Island, was re-ported by Hindell (1991). While the accuracy of the studywas reduced by the acknowledged violation of severalassumptions, including that of consistency of search ef-

fort, it is still useful for comparison. Another recognizedweakness in the study, which was based on the number ofanimals known to be alive in each age class for each co-hort, is that juvenile survival may have been underesti-mated due to lower levels of philopatry, relative to adults.This may explain, to some extent, the low 1st-year sur-vival estimates found during that study (0.459 and 0.425for males and females, respectively, in the 1950s; 0.292and 0.241, respectively, in the 1960s). More recently, therenewed mark-recapture programme at Macquarie Is-land revealed 1st-year survival estimates (65.6%:McMahon et al. 1999) falling within the range recordedin the present study (Fig. 2). With the exception of 9- and12-year-old males and 3-, 8- and 11-year-old females,Hindell's (1991) estimated survival rates for all post year1 age categories of Macquarie Island elephant seals werehigher than those for the Marion Island population.Males aged 3 up to 7 exhibited survival rates in excess of6% higher than those found for equivalent age categoriesat Marion Island. Considering that these are minimumestimates (calculated from `known to be alive' animals), itseems likely that adult survival is substantially higher atMacquarie Island than at Marion Island.

The survival rates found in the present study are rep-resentative of a declining population. Comparison with astable population could highlight parameters that areincisive in the decline. Survival data for the stable SouthGeorgia elephant seal population are available in theform of stationary life tables that are based on age-structure data (McCann 1985). Although this method ofobtaining survival data has in recent years been increas-ingly criticised because of the assumptions that are made(e.g. stability of age structure, random sampling, andstationary population size over time) (Gaillard et al.1993; Loison et al. 1994), it is often the only practicalmeans of obtaining survival estimates for a population.By utilising the standing age distribution for the MarionIsland elephant seal population over the period 1989±1991 (Pistorius et al. 1999b), it was possible to obtainsurvival data comparable to those for the South Georgiapopulation (by dividing the number of individuals in eachage category by those in the preceding age category � pxand averaging it over the four age distributions). Juvenilemortality rate, equated over the ®rst 3 years of life, wasslightly higher for the South Georgia population (24.1%and 22.3% compared to 22.7% and 20.5%, for males andfemales, respectively). This militates against the likeli-hood that juvenile mortality is the driving force behindthe Marion Island elephant seal population decline.When considering survival from ages 3 to 8, the converseis true (19.2% versus 30.1% annual mortality for males,and 12% versus 21.7% annual mortality for females).The comparison emphasises that adult mortality, ratherthan juvenile mortality, is an important parameter in thepopulation decline of elephant seals at Marion Island.

Other studies reporting on survival, in which largemammals were followed through time, include Boydet al. (1995), who calculated an average survival rate of0.83 for female Antarctic fur seals, and Croxall and

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Hiby (1983) and Testa and Sini� (1987) who reportedsurvival rates of 0.80 and 0.85, respectively, for adultfemale Weddell seals. Toigo et al. (1997) found an av-erage adult survival of 0.97 for Alpine ibex (Capra ibexibex), and Loison et al. (1994) described juvenile andadult survival rates of 0.58 and 0.96, respectively, forfemale chamois (Rupicapra rupicapra). These resultsfurther emphasise the relatively high juvenile and lowadult survival of elephant seals on Marion Island.

The elephant seals on Marion Island were only taggedonce they were successfully weaned. Pre-weaning mor-tality were therefore not included in the mark-recaptureestimates, similar to other 1st-year survival estimatesbased on the resightings of marked elephant seals (LeBoeuf et al. 1994). Pre-weaning mortality at Marion Is-land (4.0%; 1983±1997, present study) is lower than thatfor the stable South Georgia population (4.5%; McCann1985) suggesting thatmortality at this stage is negligible interms of the population decline at Marion Island.

Several studies have addressed one of the funda-mental problems in population biology, namely whatimpact the di�erent vital rates have on the overallvariability in population growth rate. Eberhardt (1981)suggested that juvenile survival is a key component ofpopulation dynamics and a potential indicator of pop-ulation status in large-mammal populations. Increases inpopulation size have been shown to occur among fourdi�erent pinniped species when juvenile survival washigh, while prolonged periods of low juvenile survivalwere documented during periods of population declines,for the same species (Eberhardt and Sini� 1977). Thiscontrasts with the results of the present study. The ele-phant seal population at Marion Island has been de-clining steadily despite its relatively high juvenilesurvival rate. Adult survival has also been considered tobe one of the most important demographic parametersof large-mammal populations, and their growth rateshave been recorded as being consistently more sensitiveto adult survival than to any other vital rate (Eberhardt1985; Walsh et al. 1995; Toigo et al. 1997). Adult femalesurvival is considered particularly in¯uential in relationto population growth (Eberhardt 1985; Taylor et al.1987; Walsh et al. 1995). Although adult female survivalis substantially higher than that of adult males in thepresent study, it is still considerably lower than thesurvival estimates of adult females in the studies men-tioned above. It would appear that changes in theMarion Island population have been a�ected by adult,in particular adult female, rather than by juvenile sur-vival. Adult survival could conceivably be a�ected bythe availability of food which has been discussed as oneof the possible reasons for the decline of elephant sealpopulations (see Hindell 1991; Hindell et al. 1994).

Sex-related survival

The principal disparity in adult survival in many species isoften sex related (Toigo et al. 1997). In many mammals,

males su�er greater mortality rates than females (Ro�and Bowen 1983). This is especially true for sexually di-morphic, polygynous mammals, among which malemortality is likely to increase with the attainment of sex-ual maturity (McCann 1985). The di�erences in survivalbetween sexes of various mammals appear positivelycorrelated to their degree of sexual dimorphism (Promi-slow 1992). Reasons for this disparity in survival havebeen attributed to male-male competition and greatersusceptibility of large-bodied males to nutritional stress(Toigo et al. 1997). Considering that southern elephantseals are the most sexually dimorphic mammals, and thatsexual dimorphism becomes pronounced after 12 monthsof age (Bell et al. 1997), it is not surprising that males ofthis species have higher mortality rates than females.

Sini� et al. (1977) suggested that wounds receivedand energy expended in the defence of underwater ter-ritories could lead to the higher mortality observed formale Weddell seals. While no evidence exists to suggestthat wounds received by male-male interactions result inthe mortality of elephant seal males from Marion Island,increased energy expenditure during the breeding seasonmay be a contributing factor. In addition, the greateryear-round absolute energy requirements of males maymake them more prone to starvation during periods offood shortage. Survival costs incurred by the sexuallydimorphic Alpine ibex male in the maintenance of theirlarge-bodied phenotype have been shown to be negligi-ble in the absence of resource limitation (Toigo et al.1997). This supports the notion that the increased foodrequirements among males of sexually dimorphic speciescauses a discrepancy in survival between the sexes, andcould explain the sexual di�erences in survival ofsouthern elephant seals from Marion Island. This ar-gument, however, does not hold for the stable SouthGeorgia population, where the sex ratio showed an evenstronger bias towards the female component (McCann1985; Pistorius et al. 1999b).

Inter-cohort survival

Although the pattern of variation in survival rates con-stitutes an important determinant of vertebrate popula-tion dynamics (Gaillard et al. 1993), the degree to whichsurvival among mammals varies between years remainslargely unknown (Jorgenson et al. 1997). Jorgenson et al.(1997) showed that yearling survival among bighornsheep ismore sensitive than adult survival tobetween-yearenvironmental ¯uctuations. A similar pattern was ob-served for populations of 14 other species of large herbi-vore (Gaillard et al. 1998). It has been argued thatjuveniles are more susceptible to stress induced by envi-ronmental ¯uctuation due to immature immune systems,small size, inexperience, or a combination of these, andthus have more variable mortality rates than adults(Promislow and Harvey 1990). While there is much evi-dence indicating that juveniles of large mammal specieshavemore variable survival rates than adults, the opposite

207

is the case amongMarion Island elephant seals. Althoughsubstantial year-to-year variation in survival in the 1styear was apparent, it was much lower than in subsequentage classes. This seems to indicate a relative insensitivity of1st-year survival to any environmental ¯uctuations thatoccurred during the study period. Juvenile survival ishighly sensitive to limiting factors (Gaillard et al. 1998),and with the relatively high and constant survival of sealsin their 1st year on Marion Island, it would appear thatthey are not constrained by any lack of resources withintheir, as yet undetermined (Bester 1989), foraging range.This, furthermore, provides a possible indication of sep-arate foraging grounds, or foraging strategies, for 1st-yearand adult elephant seals, although Slip (1997) presentedsome evidence to the contrary.

The natural rates of population increase of large-mammal species are in¯uenced more by changes insurvival than in fecundity (Bester and Wilkinson 1994;Lima and Paez 1997; Saether 1997), and are often moresensitive to prime-age adult than juvenile survival(Eberhardt 1985; Escos et al. 1994). In most marinemammals, only a small change in adult survival is re-quired to alter population growth (Eberhardt and Sini�1977). It has been demonstrated that a mere 5% increasein adult survival could stabilize two rapidly decliningpopulations of Spanish ibex (C. pyrenaica), whereas a5% decrease could destabilize a stable population (Escoset al. 1994). Bearing in mind: (1) the importance of adultsurvival in determining population change, (2) the rela-tively high, and stable nature of Marion Island juvenilesurvival, and (3) the comparatively low and still de-creasing adult survival rates of southern elephant sealson Marion Island, it would seem that the proximatefactor responsible for the elephant seal decline at Mar-ion Island is the decrease in adult survival.

Bester and Wilkinson (1994) showed that there wasan increase in mortality of 3-year-old females in the 1983cohort on Marion Island. A re-analysis of the cohort,with additional recapture data, illustrated the same in-crease, although it was not evident when averaging thesurvival values from all the cohorts. The average mor-tality of 3-year-old females has, however, increased sig-ni®cantly over the study period (Table 5). This issigni®cant since at this stage a number of 3-year-oldshave been recruited to the breeding population. Theremaining 3-year-olds are, therefore, on the verge ofentering the adult population, and an increase in mor-tality at this stage would impact negatively upon thepopulation. This is also a time when females are exposedto increased physiological stress and energetic demandsimposed by gestation and the postpartum lactation pe-riod (Bester and Wilkinson 1994), which would be ex-acerbated by the phase of continuing body growth whichwould also increase energetic demands relative to theolder females (Laws 1953; Reiter et al. 1981; Reiter andLe Boeuf 1991). Furthermore, a decrease in 4th- and5th-year survival of elephant seal males occurred overthe study period. During this period, southern elephantseal males undergo a spurt of secondary growth (Ling

and Bryden 1981; Laws 1984), and food requirementsare greatly increased. It is therefore suggested that thedecline in survival of males in their 4th and 5th year oflife, and females in their 4th year, strongly points to foodlimitation as the ultimate cause behind the overallpopulation decline at Marion Island.

Acknowledgements Formerly the South African Department ofEnvironment A�airs and latterly the Department of EnvironmentalA�airs and Tourism provided ®nancial and logistical support onthe advice of the South African Scienti®c Committee for AntarcticResearch (SASCAR) and SACAR, respectively. We are indebted toCraig Saunders, Steve Atkinson, Anton Hunt, Peter Bartlett, IanWilkinson, Charlie Pascoe, Jaco Swart, Rory Heather-Clark,Sampie Ferreira, Andre La Cock, Hendrik Pansegrouw, FrancoisRoux, Johan Fourie, Johannes de Lange, Greg Hofmeyr, JohannesKlopper, Frans Jonker and Derrick Shingwenyana for their dedi-cated marking and resighting of elephant seals on Marion Islandwhen we were not in the ®eld. Frances Taylor assisted with thepreparation of the original data for which we are extremelygrateful, and Peter Boveng kindly helped in deriving tag loss cor-rection factors.

Appendix 1

Table 1A Survival of southern elephant seals born at Marion Is-land averaged over the cohorts 1983±1992 (survival adjusted for tagloss)

Age (x) Males Females

/ SE (/) P / SE (/) P

1 0.586 0.037 0.684 0.628 0.035 0.7142 0.754 0.051 0.700 0.807 0.048 0.7143 0.742 0.062 0.686 0.783 0.054 0.7084 0.636 0.073 0.705 0.810 0.064 0.6995 0.726 0.088 0.687 0.771 0.068 0.7466 0.748 0.091 0.693 0.758 0.073 0.7327 0.695 0.094 0.687 0.778 0.069 0.7998 0.747 0.089 0.687 0.749 0.066 0.7099 0.586 0.106 0.687 0.737 0.075 0.79510 0.654 0.128 0.687 0.755 0.081 0.73311 0.678 0.091 0.687 0.762 0.030 0.81912 0.756 0.077 0.687 0.781 0.211 0.72813 0.000 0.000 0.687 0.710 0.050 0.731

Table 2A Survival of southern elephant seals born at Marion Is-land in 1983 (survival adjusted for tag loss)

Age (x) Males Females

/ SE (/) P / SE (/) P

1 0.651 0.045 0.592 0.647 0.042 0.6482 0.710 0.061 0.592 0.727 0.054 0.6483 0.752 0.074 0.592 0.881 0.063 0.6484 0.708 0.085 0.592 0.674 0.068 0.6485 0.713 0.098 0.592 0.915 0.078 0.6486 0.830 0.119 0.592 0.732 0.040 0.6487 0.748 0.134 0.592 0.710 0.050 0.6488 0.747 0.154 0.592 0.710 0.050 0.6489 0.926 0.201 0.592 0.710 0.050 0.64810 0.821 0.258 0.592 0.710 0.050 0.64811 0.664 0.124 0.592 0.710 0.050 0.64812 0.672 0.146 0.592 0.710 0.050 0.64813 0.000 0.000 0.592 0.710 0.050 0.648

208

Table 3A Survival of southern elephant seals born at Marion Is-land in 1984 (survival adjusted for tag loss)

Age (x) Males Females

/ SE (/) P / SE (/) P

1 0.605 0.042 0.619 0.545 0.036 0.7252 0.664 0.060 0.619 0.817 0.051 0.7253 0.549 0.066 0.619 0.675 0.057 0.7254 0.764 0.089 0.619 0.844 0.061 0.7255 0.847 0.113 0.619 0.831 0.067 0.7256 0.618 0.126 0.619 0.812 0.075 0.7257 0.515 0.147 0.619 0.851 0.035 0.7258 0.682 0.207 0.619 0.851 0.035 0.7259 0.839 0.127 0.619 0.851 0.035 0.72510 0.839 0.127 0.619 0.851 0.035 0.72511 0.839 0.127 0.619 0.851 0.035 0.72512 0.839 0.127 0.619 0.851 0.035 0.725

Table 4A Survival of southern elephant seals born at Marion Is-land in 1985 (survival adjusted for tag loss)

Age (x) Males Females

/ SE (/) P / SE (/) P

1 0.644 0.033 0.647 0.645 0.032 0.6882 0.706 0.044 0.647 0.789 0.051 0.5483 0.717 0.052 0.647 0.828 0.066 0.5154 0.784 0.065 0.647 0.927 0.103 0.4565 0.584 0.068 0.647 0.626 0.086 0.5106 0.833 0.088 0.647 0.745 0.033 0.5337 0.867 0.114 0.647 0.726 0.037 0.7098 0.493 0.115 0.647 0.726 0.037 0.6199 0.530 0.113 0.647 0.726 0.037 0.93610 0.530 0.113 0.647 0.726 0.037 0.87911 0.530 0.113 0.647 0.726 0.037 1.000

Table 5A Survival of southern elephant seals born at Marion Is-land in 1986 (survival adjusted for tag loss)

Age (x) Males Females

/ SE (/) P / SE (/) P

1 0.584 0.035 0.655 0.703 0.034 0.6302 0.808 0.050 0.655 0.759 0.042 0.6303 0.748 0.061 0.655 0.853 0.044 0.6304 0.559 0.064 0.655 0.948 0.051 0.6305 0.871 0.087 0.655 0.732 0.059 0.6306 0.706 0.104 0.655 0.765 0.069 0.6307 0.584 0.123 0.655 0.732 0.041 0.6308 0.605 0.172 0.655 0.732 0.041 0.6309 0.425 0.159 0.655 0.732 0.041 0.63010 0.425 0.159 0.655 0.732 0.041 0.630

Table 6A Survival of southern elephant seals born at Marion Is-land in 1987 (survival adjusted for tag loss)

Age (x) Males Females

/ SE (/) P / SE (/) P

1 0.569 0.033 0.683 0.656 0.032 0.6622 0.792 0.047 0.683 0.899 0.055 0.5543 0.756 0.057 0.683 0.685 0.053 0.6574 0.603 0.062 0.683 0.885 0.059 0.6335 0.861 0.083 0.683 0.732 0.060 0.8456 0.609 0.094 0.683 0.712 0.068 0.7507 0.564 0.114 0.683 0.667 0.058 0.8758 0.956 0.187 0.683 0.667 0.058 0.7849 0.211 0.131 0.683 0.667 0.058 0.960

Table 7A Survival of southern elephant seals born at Marion Is-land in 1988 (survival adjusted for tag loss)

Age (x) Males Females

/ SE (/) P / SE (/) P

1 0.618 0.033 0.690 0.603 0.036 0.6892 0.757 0.045 0.690 0.727 0.050 0.6813 0.745 0.053 0.690 0.800 0.054 0.6834 0.692 0.064 0.690 0.857 0.060 0.7785 0.649 0.075 0.690 0.652 0.061 0.8806 0.746 0.094 0.690 0.774 0.041 0.9197 0.671 0.108 0.690 0.893 0.054 1.0008 1.000 0.166 0.690 0.805 0.060 0.805

Table 8A Survival of southern elephant seals born at Marion Is-land in 1989 (survival adjusted for tag loss)

Age (x) Males Females

/ SE (/) P / SE (/) P

1 0.550 0.036 0.635 0.616 0.036 0.6402 0.898 0.051 0.635 0.884 0.061 0.5633 0.865 0.069 0.635 0.716 0.061 0.7784 0.432 0.062 0.635 0.651 0.062 0.7615 0.629 0.096 0.635 0.732 0.075 0.8286 0.888 0.125 0.635 0.626 0.084 0.8187 0.917 0.181 0.635 0.867 0.087 0.849

Table 9A Survival of southern elephant seals born at Marion Is-land in 1990 (survival adjusted for tag loss)

Age (x) Males Females

/ SE (/) P / SE (/) P

1 0.607 0.040 0.657 0.664 0.033 0.7322 0.701 0.053 0.814 0.813 0.041 0.7323 0.741 0.063 0.679 0.754 0.048 0.7324 0.730 0.073 0.867 0.777 0.053 0.7325 0.628 0.097 0.684 0.841 0.061 0.7326 0.754 0.083 0.736 0.895 0.089 0.732

209

References

Anderson DR, Burnham KP, White GC (1994) AIC model selec-tion in overdispersed capture-recapture data. Ecology 75:1780±1793

Barrat A, Mougin JL (1978) L'Elephant de mer Mirounga leoninade l'ile de la Possession, archipel Crozet. Mammalia 42:143±174

Begon M, Mortimer M (1986) Population ecology: a uni®ed studyof animals and plants. Blackwell, Oxford

Bell CM, Burton HR, Hindell MA (1997) Growth of southernelephant seals,Mirounga leonina, during their ®rst foraging trip.Aust J Zool 45:447±458

Bester MN (1980) The southern elephant seal Mirounga leonina atGough Island. S Afr J Zool 15:235±239

Bester MN (1989) Movements of southern elephant seals andsubantarctic fur seals in relation to Marion Island. MarMammal Sci 5:257±265

Bester MN, Wilkinson IS (1994) Population ecology of southernelephant seals at Marion Island. In: Le Boeuf BJ, Laws RM(eds) Elephant seals: population ecology, behaviour, andphysiology. University of California Press, Berkeley, pp85±97

Bowen WD, Capstick CK, Sergeant DE (1981) Temporal changesin the reproductive potential of female harp seals (Pagophilusgroenlandicus). Can J Fish Aquat Sci 38:495±503

Boyd IL, Croxall JP, Lunn NJ, Reid K (1995) Population de-mography of Antarctic fur seals: the costs of reproduction andimplications for life-histories. J Anim Ecol 64:505±518

Brewer R (1994) The science of ecology. Harcourt Brace, LondonBurnham KP, Anderson DR, White GC, Brownie C, Pollock KH

(1987) Design and analysis methods for ®sh survival experi-ments based on release-recapture. Am Fish Soc Monogr 5

Burton H (1986) A substantial decline in numbers of the southernelephant seal at Heard Island. Tasmanian Nat 86:4±8

Carrick R, Csordas SE, Ingham SE (1962) Studies on the southernelephant seal, Mirounga leonina (L.). IV. Breeding and devel-opment. CSIRO Wildl Res 7:161±197

Caughley G (1977) Analysis of vertebrate populations. Wiley,London

Choquenot D (1991) Density-dependent growth, body condition,and demography in feral donkeys: testing the food limitationhypothesis. Ecology 72:805±813

Condy PR (1978) The distribution and abundance of southernelephant seals, Mirounga leonina, on the Prince Edward Islands.S Afr J Antarct Res 8:42±48

Cormack RM (1964) Estimates of survival from the sighting ofmarked animals. Biometrika 51:429±438

Cormack RM (1979) Models for capture-recapture. In: CormackRM, Patil GP, Robson DS (eds) Sampling biological popula-tions. International Co-operative, Fairland pp 218±249

Croxall JP, Hiby L (1983) Fecundity, survival and site ®delity inWeddell seals, Leptonychotes weddellii. J Appl Ecol 20:19±32

Eberhardt LL (1981) Population dynamics of the Pribilof fur seals.In: Fowler CW, Smith TD (eds) Dynamics of large mammalpopulations. Wiley, New York, pp 197±220

Eberhardt LL (1985) Assessing the dynamics of wild populations.J Wildl Manage 49:997±1012

Eberhardt LL, Sini� DB (1977) Population dynamics and marinemammal management policies. J Fish Res Bd Can 34:183±190

Escos J, Alados CL, Emlen JM (1994) An application of the stage-projection model with density-dependent fecundity to thepopulation dynamics of Spanish ibex. Can J Zool 72:731±737

Gaillard JM, Delorme D, Boutin J, Van Laere G, Boisaubert B,Pradel R (1993) Roe deer survival patterns: a comparativeanalysis of contrasting populations. J Anim Ecol 62:778±791

Gaillard JM, Festa-Bianchet M, Yoccoz NG (1998) Populationdynamics of large herbivores: variable recruitment with con-stant adult survival. Trends Ecol Evol 13:58±63

Galimberti F, Boitani L (1999) Demography and breeding biologyof a small, localized population of southern elephant seals(Mirounga leonina). Mar Mamm Sci 15:159±178

Guinet C, Jouventin P, Weimerskirch H (1992) Population changesand movements of southern elephant seals on Crozet andKerguelen archipelagos in the last decades. Polar Biol 12:349±356

Hindell MA (1991) Some life-history parameters of a decliningpopulation of southern elephant seals,Mirounga leonina. J AnimEcol 60:119±134

Hindell MA, Burton HR (1987) Past and present status of thesouthern elephant seal (Mirounga leonina) at Macquarie Island.J Zool (Lond) 213:365±380

Hindell MA, Little GJ (1988) Longevity, fertility and philopatry oftwo female southern elephant seals (Mirounga leonina) atMacquarie Island. Mar Mamm Sci 4:168±171

Hindell MA, Slip DJ, Burton HR (1994) Possible causes of thedecline of southern elephant seal populations in the southernPaci®c and southern Indian oceans. In: Le Boeuf BJ, Laws RM(eds) Elephant seals: population ecology, behaviour, andphysiology. University of California Press, Berkeley, pp 66±84

Jolly GM (1965) Explicit estimates from capture-recapture datawith both death and immigration-stochastic models. Biome-trika 52:225±247

Jonker FC, Bester MN (1998) Seasonal movements and foragingareas of adult southern female elephant seals,Mirounga leonina,from Marion Island. Antarct Sci 10:21±30

Jorgenson JT, Festa-Bianchet M, Gaillard JM, Wishart WD (1997)E�ects of age, sex, disease, and density on survival of bighornsheep. Ecology 78:1019±1032

Krebs CJ (1985) Ecology: the experimental analysis of distributionand abundance. Harper & Row, New York

Laws RM (1953) The elephant seal (Mirounga leonina Linn.). I.Growth and age. Falkland Isl Depend Surv Sci Rep 8:1±62

Laws RM (1956) The elephant seal (Mirounga leonina Linn.). III.The physiology of reproduction. Falkland Isl Depend Surv SciRep 15:1±66

Laws RM (1984) Seals. In: Laws RM (eds) Antarctic ecology.Academic Press, London, pp 621±715

Le Boeuf BJ, Reiter J (1988) Lifetime reproductive success innorthern elephant seals. In: Clutton-Brock (ed) Reproductivesuccess. University of Chicago Press, Chicago, pp 344±362

Le Boeuf BJ, Morris P, Reiter J (1994) Juvenile survivorship ofnorthern elephant seals. In: Le Boeuf BJ, Laws RM (eds) Ele-phant seals: population ecology, behaviour, and physiology.University of California Press, Berkeley, pp 121±136

Table 10A Survival of southern elephant seals born at MarionIsland in 1991 (survival adjusted for tag loss)

Age (x) Males Females

/ SE (/) P / SE (/) P

1 0.601 0.035 0.848 0.642 0.034 0.7712 0.730 0.044 0.848 0.808 0.042 0.7903 0.775 0.053 0.848 0.824 0.045 0.8384 0.543 0.064 0.848 0.724 0.050 0.9125 0.752 0.094 0.848 0.876 0.042 0.868

Table 11A Survival of southern elephant seals born at MarionIsland in 1992 (survival adjusted for tag loss)

Age (x) Males Females

/ SE (/) P / SE (/) P

1 0.430 0.035 0.808 0.558 0.035 0.8212 0.777 0.053 0.808 0.844 0.040 0.8213 0.769 0.066 0.808 0.811 0.048 0.8214 0.545 0.086 0.808 0.809 0.068 0.821

210

Lebreton J, Burnham KP, Clobert J, Anderson DR (1992) Mod-eling survival and testing biological hypotheses using markedanimals: a uni®ed approach with case studies. Ecol Monogr62:67±118

Lebreton J, Pradel R, Clobert J (1993) The statistical analysis ofsurvival in animal populations. Trends Ecol Evol 8:91±95

Lima M, Paez E (1997) Demography and population dynamics ofSouth American fur seals. J Mammal 78:914±920

Ling JK, Bryden MM (1981) Southern elephant seal Miroungaleonina Linnaeus, 1758. In: Ridgway SH, Harrison RJ (eds)Handbook of marine mammals. Academic Press, London,pp 297±327

Loison A, Gaillard JM, Houssin H (1994) New insight on survi-vorship of female chamois (Rupicapra rupicapra) from obser-vation of marked animals. Can J Zool 72:591±597

Manly BFJ (1970) A simulation study of animal population esti-mation using the capture-recapture method. J Appl Ecol 7:13±39

McCann TS (1985) Size, status and demography of southern ele-phant seal (Mirounga leonina) populations. In: Ling JK, BrydenMM (eds) Sea mammals of south latitudes: proceedings of asymposium of the 52nd ANZAAS congress in Sydney, May1982. South Australian Museum, North®eld, pp 1±17

McMahon CR, Burton HR, Bester MN (1999) First year survivalof southern elephant seals, Mirounga leonina, at sub-antarcticMacquarie Island. Polar Biol 21:279±284

Panagis K (1981) Local movement of southern elephant seal pupsMirounga leonina (Linn.) at Marion Island. S Afr J Antarct Res10:28±31

Pascal M (1985) Numerical changes in the population of elephantseals (Mirounga leonina) in the Kerguelen Archipelago duringthe past 30 years. In: Beddington JR, Beverton RJH, LavigneDM (eds) Marine mammals and ®sheries. Allen & Unwin,London, pp 170±186

Pistorius PA, Boveng PL, Bester MN, Kirkman SP (1999a) Age-dependent rates of tag loss in southern elephant seals. J WildlManage

Pistorius PA, Bester MN, Kirkman SP (1999b) Dynamic age-dis-tributions in a declining population of southern elephant seals.Antarct Sci

Pollock KH, Nichols JD, Brownie C, Hines JE (1990) Statisticalinference for capture-recapture experiments. Wildl Monogr107:1±97

Promislow DEL (1992) Costs of sexual selection in natural popu-lations of mammals. Proc R Soc Lond B 247:203±210

Promislow DEL, Harvey PH (1990) Living fast and dying young: acomparative analysis of life-history variation among mammals.J Zool (Lond) 220:417±437

Reiter J, Le Boeuf BJ (1991) Life history consequences of variationin age at primiparity in northern elephant seals. Behav EcolSociobiol 28:153±160

Reiter J, Panken KJ, Le Boeuf BJ (1981) Female competition andreproductive success in northern elephant seals. Anim Behav29:670±687

Ro� DA, Bowen WD (1983) Population dynamics and manage-ment of the northwest Atlantic harp seal (Phoca groenlandica).Can J Fish Aquat Sci 40:919±932

Saether B (1997) Environmental stochasticity and population dy-namics of large herbivores: a search for mechanisms. TrendsEcol Evol 12:143±149

Seber GAF (1965) A note on the multiple-recapture census. Bio-metrika 52:249±259

Sini� DB, DeMaster DP, Hofman RJ, Eberhardt LL (1977) Ananalysis of the dynamics of a Weddell seal population. EcolMonogr 47:319±335

Skinner JD, Van Aarde RJ (1983) Observations on the trend of thebreeding population of southern elephant seals, Mirounga leo-nina, at Marion Island. J Appl Ecol 20:707±712

Slip DJ (1997) Diving and foraging behaviour of juvenile southernelephant seals from Heard Island. In: Hindell M, Kemper C(eds) Marine mammal research in the southern hemisphere.Surrey Beatty, Chipping Norton, pp 114±124

Taylor MK, DeMaster DP, Bunnel FL, Schweinsburg RE (1987)Modeling the sustainable harvest of female polar bears. J WildlManage 51:811±820

Testa JW, Sini� DB (1987) Population dynamics of Weddell seals(Leptonychotes weddelli) in McMurdo Sound, Antarctica. EcolMonogr 57:149±165

Toigo C, Gaillard JM, Michallet J (1997) Adult survival pattern ofthe sexually dimorphic Alpine ibex (Capra ibex ibex). Can JZool 75:75±79

Walsh NE, Gri�th B, McCabe TR (1995) Evaluating growth of theporcupine caribou herd using a stochastic model. J WildlManage 59:262±272

Wilkinson IS (1991) Factors a�ecting reproductive success ofsouthern elephant seals, Mirounga leonina, at Marion Island.PhD thesis, University of Pretoria

York AE (1994) The population dynamics of northern sea lions,1975±1985. Mar Mamm Sci 10:38±51

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