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

with the exception of division II, many of the large beaches still held very sizeable
colonies in 1951.
Other evidence supported this. The average harem size at the Dartmouth Point
reserve in 1951, where sealing was prohibited, was 1 male to 24 cows and the overall
mature sex ratio there 1:13. In contrast, at Hestesletten and Maiviken, Cumberland
Bay, exploited beaches, the average harem sizes were 46 and 52 respectively and the
mature sex ratios were respectively 1:28 and 1:34 – although seals had not been
removed from these beaches thus far that season. The interpretation of the figures
was complicated by the fact that sealing operations were continually removing a
proportion of males from these exploited beaches, replaced by other males, fresh
from the sea. [Also the average age of the bulls was much lower than expected in an
unexploited herd, including sexually mature animals only [41/2] yrs old. In some
areas it was very much below the usual age at which harems are acquired in an
unexploited herd.
I made an estimate of the size of the stock in the various divisions. In November
1951 I counted all the pups on the beaches visited in areas I and II. They leave the
beaches when they are [a few] [about six] weeks old and the figures were adjusted to
allow for this; regular control counts were made on Cumberland Bay beaches. On the
beaches visited in divisions I and II the proportions of cows and pups on a given date
approximated to the proportions of the control counts. It was therefore assumed that
divisions I, II and III (Cumberland Bay) were more or less synchronous as regards
the pupping season. The actual counts were therefore corrected (to allow for pups
which had departed), by reference to the control counts.
The corrected total was 31,075 and a further allowance was made for the few
beaches not visited, which brought the total to about 35,000 for these two divisions. I
concluded that about 20,000 of these pups were born in division I, and 15,000 in
division II. Because of my previous experience the basic counts were unlikely to be
much in error, though the totals were extrapolations. This could not be said for the
estimates on the other two divisions, which weren’t based on counts. After talks with
the sealers, in which their opinions of the relative status of the herds in divisions III
and IV was sought, an arbitrary figure of 32,500 live pups was assumed for each of
these divisions, making a grand total of approximately 100,000 pups which survived
to weaning. This was a very round figure, but the best I could do, given my
resources! Pup mortality from birth to weaning was assessed at 2% (from more
detailed observations). This gave a total estimated annual recruitment to the
population of about 102,000 pups. Of these I estimated that 20% were in division I,
15% in division II, and 32.5% in each of divisions III and IV. Although these
estimates could only be approximate figures they were the first ever to be made and
served as a basis for further calculations.
Other parameters used in assessing the size of the population included: the sex
ratio at birth (estimated from detailed sexed samples); average ages of the sexes at
sexual maturity (estimated from tooth-aged samples); the pregnancy rate of mature
females, from post-mortems; information on longevity of females (from the tooth
collections). Also data on the size of the commercial catches in previous years, and on
the age composition of the commercial kill of males in 1951, were available. For the
purpose of the calculations it was assumed that recruitment and natural mortality
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