Part I: Seals teeth and whales ears - Scott Polar Research Institute ...

population. This could only be a crude approximation owing to uncertainty about
several parameters of the herd, but it could be shown that even large differences in
the initial assumptions led to rather similar results. The values I used were all based,
to some extent, on actual counts I had made or samples I had taken. Initially I
assumed that the population was more or less stable, for an industry had been
prosecuted for forty years, without obvious decline. [The apparent uniformity of the
age composition of the catch in recent years supports this [???]
With these reservations I drew up three [cross sectional] life tables, based on
different assumptions. Human life tables present a tabular numerical representation
of mortality and survivorship of a cohort at birth at each subsequent year of life. It
comprises an array of measures, including probabilities of death, probabilities of
survival and life expectancies at various ages. The original data are to be found in
archives of births, deaths and marriages, etc. In 1951 life tables were well established
for people, but novel for wild animals, particularly large wild mammals. A paper by
E S Deevey (1947) is now a ‘citation classic’ and I had been fortunate to come across it
when I was in Cambridge in 1950 after my first two year Antarctic sojourn. Life table
notation had not previously been used in field ecology: to paraphrase it tabulates
survivorship at each age x, from birth to x max (the maximum age for the cohort). If
ages of wild animals (alive, found dead or extinct) are known, “fates of oysters and
song birds can be quantitatively compared with the fates of people and fruit flies”.
Now it guided my thoughts to ways of analysing the population dynamics,
management and conservation of the world’s largest elephant seal population. In this
case I had age data from my collected samples, data on sex ratios at birth, ages at
maturity and an estimate of pregnancy rates. I was working in isolation, but my
approach and presentation was similar to that later published by American biologists
working on the northern fur seal (Kenyon, Scheffer and Chapman (1954) as part of a
large, well-funded programme of research on that species. My whole programme,
including other studies, cost my annual salary (£400), plus limited travel to and from
South Georgia, food and accommodation on the island – no more than £1500 in all.
That is equivalent today allowing for inflation, to some £25 -30,000.
My three life table models were for females exposed only to natural mortality, for
males exposed to natural mortality and for males exposed to both natural and
hunting mortality. The following basic data were used: a) total number of pups at
birth estimated to be 102,000; b) sex ratio at birth estimated to be 54% male; c) age
composition of 74 mature females, from my earlier studies; d) a pregnancy rate of
82.5% annually from the third year; e) potential maximum longevity of 20 years,
from ages from teeth; f) an average assumed catch of 6,000 males (later revised to
5,818 males per year (actuals, 1955-58), having an age composition based on
examination of tooth layers.
The female life table model was constructed from the first five of these values. A
crude time-specific survival curve was drawn up for the small sample of females I
had collected, assuming it to be random, and the female population to be stable in
time. A straight line, on a logarithmic scale, representing annual mortality rate of
about 13% was fitted from 2 -11 years. A peak of females estimated at 12-years-old,
included older females (thought due to ageing inaccuracies) was extended to later
age groups (up to 18 years) to produce a more regular curve. Recruitment to the
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