Observations on the Trend of the Breeding Population of Southern Elephant Seals, Mirounga leonina, at Marion Island

Skinner, J. D.; Aarde, Rudi J. van

doi:10.2307/2403121

Cited by 31 publications

(36 citation statements)

References 9 publications

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“…As is the case with several other southern elephant seal populations, the Marion Island population has declined over the past few decades (Barrat and Mougin 1978;Condy 1978;Bester 1980;Skinner and Van Aarde 1983;Pascal 1985;Burton 1986;Hindell and Burton 1987;Guinet et al 1992;Bester and Wilkinson 1994), with no single cause being unequivocally identi®ed. Birth, death, immigration and emigration are the four fundamental demographic parameters that in¯uence the survival of a population (Brewer 1994).…”

Section: Introductionmentioning

confidence: 98%

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

1999

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This study quantified both the age- and sex-specific survival rates of juveniles and adults, and tested for interannual differences in age-specific survival rates of the southern elephant seal population at Marion Island. Pups were tagged on an annual basis from 1983 onwards at Marion Island, and a consistent recapture program yielded data that was analysed using the software package MARK to obtain maximum-likelihood estimates of survival and capture probability. On average, 1st-year survival was 0.58 and 0.62, and survival rate averaged over the first 3 years of life, 0.69 and 0.74 for males and females, respectively. From years 4 to 9, the average survival rate was 0.66 and 0.75 for males and females, respectively. Survival estimates for elephant seals in their 10th-13th year are also presented, although these are based on very small sample sizes. Averages of age-specific survival estimates from the earlier (mostly 1983-1987 cohorts) and later (mostly 1988-1992 cohorts) periods were compared and considerable reductions were observed in 4th- and 5th-year male survival, and 4th-year female survival. The comparatively low adult survival is suggested as the proximate cause, and food limitation as deduced from the decline in survival of elephant seals with comparatively high energetic demands as the ultimate cause behind the population decline at Marion Island. Although not tied in with the decline of the population, 1987, 1990 and 1993 were identified as high-mortality years.

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Section: Introductionmentioning

confidence: 98%

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

1999

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“…Declines in this and several other populations in the southern Indian Ocean have been evident since the 1970s (Condy 1978; van Aarde 1980a;Skinner and van Aarde 1983;Burton 1986). Condy (1978) suggested that killer whales, Orcinus orca, may have contributed to the decline at Marion Island, as orca are abundant there and have regular annual visitation patterns coinciding with elephant seal breeding .…”

Section: Introductionmentioning

confidence: 90%

“…The southern elephant seal (Mirounga leonina) population at Marion Island has been studied extensively since 1973 (Condy 1977;Skinner and van Aarde 1983;Wilkinson 1992;Bester and Wilkinson 1994). Declines in this and several other populations in the southern Indian Ocean have been evident since the 1970s (Condy 1978; van Aarde 1980a;Skinner and van Aarde 1983;Burton 1986).…”

Section: Introductionmentioning

confidence: 99%

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Marion Island elephant seals: the paucity-of-males hypothesis tested

Wilkinson¹,

Aarde²

1999

Can. J. Zool.

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The southern elephant seal, Mirounga leonina, population at Marion Island has declined since the start of research activities there in 1973, as have populations at other Indian Ocean breeding sites. One suggested mechanism for the decline at Marion Island is a shortage of males, resulting in low insemination rates. We tested this "paucity-ofmales" hypothesis by looking at (i) adult sex ratios, (ii) male sexual activity, and (iii) the relationship between levels of sexual activity and the probability of a cow pupping in the following season. Sex ratios were similar to those at other sites where populations are either stable (South Georgia) or increasing (Peninsula Valdes). Dominant bulls monopolised mating opportunities, achieving 98.1% of 629 observed copulations over three seasons. Of the 138 cows coming ashore, only 4 (2.9%) were not seen mating and 89% copulated with only one bull; dominant bulls copulated with all but 1 of the 134 (99.3%) cows that did mate. Cows were mated more frequently than at sites with stable and increasing populations. There was no difference in the observed level of sexual activity between cows seen pupping in the following season and those who failed to pup. This indicates that bulls monopolising harems are capable of inseminating all the cows. Such findings refute the paucity-of-males hypothesis.Résumé : La population d'éléphants de mer, Mirounga leonina, dans l'île Marion a diminué depuis le début des activités de recherche en 1973, à l'instar d'autres populations aux sites de reproduction dans l'océan Indien. Une hypothèse avancée pour expliquer le déclin dans l'île Marion est une pénurie de mâles qui entraîne un taux réduit de fécondation. Nous avons testé cette hypothèse de « rareté des mâles » par évaluation (i) du rapport mâles : femelles, (ii) de l'activité sexuelle des mâles et (iii) de la relation entre le degré d'activité sexuelle et la probabilité de mise-bas de la femelle au cours de la saison suivante. Les rapports mâles : femelles sont semblables à ceux enregistrés à d'autres sites où les populations sont stables (Géorgie du Sud) ou en croissance (péninsule de Valdes). Les mâles dominants monopolisent les chances de s'accoupler et ils ont été responsables de 98,1% des 629 accouplements observés au cours de trois saisons. Seulement 4 des 138 femelles (2,9%) qui ont gagné la terre ferme n'ont pas été observées en train de s'accoupler, 89% se sont accouplées à un seul mâle et les mâles dominants se sont accouplés à toutes les femelles sauf une sur les 134 (99,3%) qui se sont accouplées. Les femelles de l'île Marion se sont accouplées plus souvent qu'à des sites où les populations sont stables ou en croissance. Nous n'avons pas observé de différence dans l'importance de l'activité sexuelle chez les femelles qui ont mis bas au cours de la saison suivante et celles qui n'ont pas eu de petit. Cela indique que les mâles qui monopolisent des harems sont capables de féconder toutes les femelles, ce qui contredit l'hypothèse de la « rareté des mâles ».[Traduit par la Rédaction] 1...

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“…Hypotheses for the decline varied from density-dependent mechanisms such as paucity of males, population "overshoot" and pandemic disease to extrinsic factors such as predation, competition with fisheries concerns, interspecific competition, environmental change and human 7 disturbance (McMahon et al 2005a). The "paucity of male" hypothesis (Skinner and Van Aarde 1983) was found wanting Wilkinson 1994, Wilkinson andVan Aarde 1999) and the "prey limitation" hypothesis increasingly found support (Pistorius and Bester 2002, McMahon et al 2003, Pistorius et al 2005. The controversy that arose e.g.…”

Section: Population Biology and Ecologymentioning

confidence: 99%

The Marine Mammal Programme at the Prince Edward Islands: 38 years of research

Bester

Bruyn²,

Oosthuizen³

et al. 2011

African Journal of Marine Science

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The Marine Mammal Programme (MMP) conducts research on pinnipeds and killer whales at Marion Island, Prince Edward Islands, under the auspices of the Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria. The early history of the MMP is described, emphasizing the contributions in subsequent years, ending with current research. The setting up of long-term studies such as the mark-resighting of southern elephant seals Mirounga leonina was implemented in 1983. The varying fates of the three seal populations are described, with the Marion Island elephant seal population declining by 87% from 1951 to 2004, followed by a change in growth rate in 1997 and a current increase. Sympatric populations of Subantarctic fur seals Arctocephalus tropicalis and Antarctic fur seals A. gazella breed at Marion Island and the recovery (following cessation of commercial sealing) and subsequent rapid increase of these populations are documented. Insights into many aspects of elephant seal and fur seal biology, including life-history, demography, diet, growth, foraging and ranging behaviour are described.Ancillary work on morphology, genetics, anthropogenic influences and rare events are mentioned, as well as the extent of current research that addresses population dynamics in an ecosystem context, i.e. to relate survival, fecundity and foraging success to environmental conditions. Presently, the response of the populations to increased abundance is gauged and investigations into the foraging ecology, maternal provisioning and diet of both fur seal populations reflect changes in the environment. Opportunistic, land-based photographic identification of killer whales (Orcinus orca) provide baseline population size, seasonal abundance and sociality data for the Marion Island population. Recent dedicated observations are used to quantify observer effort, improve sighting rates and to further our understanding of the 3 potential impact of the killer whale population on resident pinniped populations. The MMP benefits from collaboration with leading national and international researchers. The programme has a long track record of training postgraduate students, as evidenced by the reference list to the manuscript. Their endeavours in the field and completion of postgraduate studies make them ideal candidates to contribute to marine mammal science in South Africa. October 1951-April 1952 published the first scientific papers on the seals of the island (Rand 1955(Rand , 1956(Rand , 1962, followed by Le Grange (1962) (Condy et al. 1978) was elaborated on in an opportunistic fashion (Keith et al. 2001). Throughout, personnel were also alert to seal vagrants (Kerley 1983a) or transients hauling out, sightings of cetaceans (Condy and Burger 1975, Ryan and Bester 2008, fur seal females successfully rearing twin pups Kerley 5 1983, de Bruyn et al. 2010) and extraordinary leucistic individuals (de Bruyn et al. 2007, Reisinger et al. 2009). Terrestrial feeding events by fur seals (Hofmeyr and Bester 1993, ...

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Observations on the Trend of the Breeding Population of Southern Elephant Seals, Mirounga leonina, at Marion Island

Cited by 31 publications

References 9 publications

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

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

Marion Island elephant seals: the paucity-of-males hypothesis tested

The Marine Mammal Programme at the Prince Edward Islands: 38 years of research

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