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

population – the initial cohort – was calculated from this smoothed curve, the
estimated pregnancy rate and the sex ratio at birth. The survival was completed by
interpolation so that annual mortality in the first two years was 40% and 20%
respectively. To draw up an approximate female life table the initial cohort was
taken to be 46,920 female pups (46% of 102,000).
In drawing up a male life table the initial cohort was taken to be 55,080 (54% of
102,000), and it was assumed that due to exploitation there was no escapement above
14 years, since no older males were found. The samples from the catch showed a
uniform total annual male mortality of 65% from 8 years upwards. A survival curve
was initially constructed by working back from 14 years age, assuming similar
annual mortality rates as for the female. However even in those days it was generally
thought that male natural mortality rates are higher than in females, so I adjusted the
life table accordingly.
A potential natural life table for males was drawn up using these initial rates,
extended to a maximum age of 20 years.
As a check on the accuracy of the estimate for the total annual recruitment, I
applied the provisional natural mortality rates to the estimated recruitment for
divisions I and II separately, taking into account the size of the annual catch and its
age composition in each of these divisions. For division I my calculations showed no
escapement beyond 14 years, so the life table fitted that parameter. For division II a
1.4% downward adjustment of estimated recruitment gave the same result. This
check suggested that the accuracy of the estimate of pups born annually in all
divisions is about the same as the estimates for divisions I and II, which were based
on adjusted counts of pups.
These very crude life tables suggested that the size of the herd in the 1950 s was
about 370,000 animals, but this maximum figure was never actually attained. The
average mid-year population is probably about 310,000. The potential life tables for
an unexploited herd suggested that the corresponding figures should be about 390,00
and 330,000, that is about 3.8 or 3.2 times (mean 3.5) the number of pups. These
factors could be used to obtain an estimate of the total size of unexploited
populations for which pup counts are available.
Biomass estimates for the herd. At the time I did this work there were very few records
of the weights of elephant seals and this is a property that is subject to large seasonal
fluctuations during the annual cycle. I estimated a weight/length relationship from
the limited elephant seal data, which suggested a similar slope and intercept to the
linear regressions obtained for two Arctic species. Comparison with the elephant seal
data suggested that a bull elephant seal of maximum length (7.9 m) in average
condition would weigh about 5000kg. An average full-grown male (length 4.8 m)
would weight about of 2032 kg. (Original calculations in ft. and lbs. pounds are here
converted to m. and kg). Because of the paucity of data, to calculate the biomass of
the herd I assumed that such a linear relationship represented the weight-length
relationship of both sexes, but this may have given a slightly higher estimated female
biomass than real. The biomass of each age group for each sex was calculated by
converting length at age data to weights and multiplying by the numbers in the life
tables age classes for the two sexes.
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