Population status, trends and a re-examination of the hypotheses explaining the recent declines of the southern elephant seal Mirounga leonina

Authors

  • CLIVE R. MCMAHON,

    Corresponding author
    1. Australian Antarctic Division, Channel HWY, Kingston, Tasmania 7050, Australia,
    2. Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria, Pretoria, Gauteng, Republic of South Africa,
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  • MARTHÁN N. BESTER,

    1. Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria, Pretoria, Gauteng, Republic of South Africa,
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  • HARRY R. BURTON,

    1. Australian Antarctic Division, Channel HWY, Kingston, Tasmania 7050, Australia,
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  • MARK A. HINDELL,

    1. Antarctic Wildlife Research Unit, School of Zoology, University of Tasmania, Private Bag 05, Hobart, Tasmania 7001, Australia,
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  • COREY J. A. BRADSHAW

    1. Antarctic Wildlife Research Unit, School of Zoology, University of Tasmania, Private Bag 05, Hobart, Tasmania 7001, Australia,
    2. Key Centre for Tropical Wildlife Management, Charles Darwin University, Darwin, NT 0909, Australia
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  • Editor: RM

Clive R. McMahon. School of Biological Sciences, University of Wales Swansea, Swansea SA2 8PP. E-mail: C.R.McMahon@swansea.ac.uk

ABSTRACT

1. Between the 1950s and 1990s the southern elephant seal Mirounga leonina underwent large decreases in population size throughout most of its breeding range in the Southern Ocean. While current population estimates suggest a recent recovery, some breeding populations have continued to decrease in recent years (Macquarie and Marion Islands), others have either remained stable (South Georgia, Kerguelen and Heard Island) or have increased (Peninsula Valdés, Argentina).

2. Intrinsic hypotheses for patterns of regional decline include factors that are affected by density-dependent mechanisms: (i) paucity of males, (ii) population ‘overshoot’ and (iii) pandemic disease. Extrinsic hypotheses include (iv) predation, (v) competition with fisheries concerns, (vi) interspecific competition, (vii) environmental change and (viii) human disturbance. Of the eight hypotheses proposed and examined here, we conclude that three can be discounted (i, v, viii), three are unlikely, but may require more testing (ii, iii, iv) and two are plausible (vi, vii).

3. The interspecific competition hypothesis is difficult to test because it requires the simultaneous monitoring of species that overlap directly with elephant seals, many of which have not been identified or little is known. However, an analysis of the relationship between log variance and log abundance (Taylor's power law) for populations of southern and northern elephant seals suggests that interspecific competition is not a significant factor in the decline of the southern elephant seal.

4. The hypothesis that decreases in southern elephant seal populations between the 1950s and 1990s were caused by the environmental change is the easiest to test and most plausible of the hypotheses. We propose a framework by which to test this hypothesis to determine how food availability affects individual survival.

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