Elephant seals on Macquarie Island

 
Mr Harry Burton, Australian Antarctic Division

For more information, email: seals@antdiv.gov.au

Approximately 70,000 elephant seals inhabit Macquarie Island. It provides their birthsites and their moulting and breeding sites. The island is only an essential place in their lives at these times. It is the ocean around the island which is the sustaining area for their foraging, if indeed they are to be sustained at all. For many do not "make it". The Antarctic Division is supporting a long term (~10 years) study into the numbers of seals surviving in each year following their birth after having discovered that there are only about half the number now that were there in the 1950's. This came as a surprise for the biologists, who had lived on the island in the intervening decades, because no evident decrease in the numbers of beach animals was evident to their passing glances. It was only dedicated censuses in the 1980's which showed up the losses. For it is difficult to distinguish 70,000 from 140,000 seals on already overcrowded beaches and the decrease was only a few percent a year. Any one biologist would usually have only a term or two and memories of "crowding" are very subjective. Recent analyses of the survivorship of pups from the 1950's show variable results. In some years 60% lived through to their first year of life and in other years only 30 to 40% did. In the 1960's almost none survived. Despite the superficial impression that all the seals were fat and evidently successful in their foraging, these results are unequivocal data showing that food restrictions were indicated. The current study is into its third year and the results are only beginning to arrive. Perhaps the best way of summarizing the study's approach is to list the questions being addressed:-
  1. Measure changes in population by annual whole island censuses.
  2. Discover the foraging grounds of the age and sex classes by attachment of position fixing devices to samples of the animals.
  3. Interpret dive records from animals in "2." above as indications of their foraging strategies and success.
  4. Collect diet samples from newly beached seals by stomach lavaging.
  5. Mark ~2,000 pups /year for survivorship determinations.
  6. Weigh ~1,000 pups /year as a measure of maternal mass and condition and hence "foraging success".
  7. Reweigh these known-mass pups whenever they return so that growth-rates may be calculated.
  8. Weigh known-age animals so that lifetime growth-rates may be calculated.
  9. Develop life-tables for this M.I. population for comparison with other island populations.
  10. Utilise and develop new methods of mass and condition estimation eg ultrasound and photogrammetry.
  11. Use the population as a "sentinel herd" for disease monitoring.
  12. Attempt to correlate annual pup mass means with "oceanic events".
Some of the results to date are clear. The different age and sex classes do forage in different areas of the ocean and they range from Antarctica to Australia and from New Zealand to west of Perth. What is not clear is how "faithful" an animal is to a foraging site from year to year. Whilst it is clear that Macquarie Island pups have lower wean masses than pups from the Atlantic Sector of the Southern Ocean the extent of annual variation is not. All in all, the elephant seals of Macquarie Island are poised in sensitive balance with their natural environment and appear to be stabilising in numbers following a catastrophic decline. A local fishery is likely, a priori, to affect that balance.
This document (ellie_seals.html) last updated Monday, 30-Jun-97 10:40:00 EST.