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

catching to the breeding season. Fortunately when my recommendations turned out
to be succeeding, a full time sealing inspector was appointed in 1956, the additional
safeguard of sample counts was introduced and marking experiments begun. Now I
should go into more detail.
Population Studies
How did I come to conclude that a progressive decline of the stock had set in?
What were the results of my preliminary assessment of the overall population and its
composition and dynamics? My conclusions as to the optimum target age
distribution of the catch? The application of the results of my population studies to
management?
Evidence of progressive deterioration up to 1952. I extracted, from the CAP records and
the Magistrates’ reports, details of the annual catches of seals at South Georgia over
the period 1910 to 1951. (Up to 1927 some 752 leopard seals and, up to 1916, 97
Weddell seals had been taken, averaging about 50 a year, but this had little effect on
the results). The total number taken up to 1951 was 193,328 mainly adult male
elephant seals, which produced 299,000 barrels of oil or 1.6 barrels per seal. This was
several times the return from an uncontrolled slaughter of the whole herd, as had
taken place in the nineteenth century. It demonstrated, if indeed that is necessary, the
value of rational exploitation in producing a sustained yield. But there was evidence
to suggest that from the late 1930s onwards the condition of the herd had been
progressively worsening in spite of the conservation measures then in place. The
first hint was given in the Magistrate’s annual report for 1936, which stated that
although there was no sign of diminution of numbers, the animals were vacating the
large open beaches for smaller beaches often inaccessible to sealers. The reports for
1939 and 1945 also remarked on the increase in the size of the colonies on these
smaller beaches, and it was evident to me in 1951 that many of the large open
beaches were under-populated.
As a measure of the relative size of an exploited population over time we can
define a unit of fishing or hunting effort and then calculate the changes in the size of
the catch per unit of effort (CPUE) which occur from year to year. This is a well
known practice in fishery research where it gives a measure of the density of fish
stocks in the sea. So far as I know, it had not then been deployed to study mammals
In the case of the elephant seal, the most convenient unit of effort – probably the only
one - was the catcher’s day’s work (CDW). This was not open to the same objection
as the CDW as a unit of effort in whaling research, where the factor of gear saturation
(days of ‘stop catch’) was significant. Nor had there been much change in the type of
ship used or in the method of working the beaches. Reliable data for calculating
CDW were available to me only from 1931 onwards (in the Magistrates’ reports) and
in the period 1931 to 1951 there had been no change in the ships engaged, and very
little if any change in the nature of the operations. However, the spring sealing
operations had shifted to a later date, so variation in the length and timing of the
season could perhaps affect the figures for CPUE s calculated for the whole period of
spring sealing. In order to minimise this possible source of error, effort data were
calculated only for October sealing which had usually accounted for well over half of
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