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

the total annual catch. But a decline was also shown for calculations based on the full
season.
There was a large variation in the calculated CPUE s, due to the system of
rotation practised - by which the stock in each division was rested every fourth year.
The CPUE should vary according to which three divisions were worked. Factors
involved were types of coastline, varying exposures and difficulties in landing,
varying sea ice conditions, and the distance of beaches from the station so that more
CDW were spent travelling to and from division IV than division II for example. To
eliminate that source of variation, I therefore lumped the data in 4-year periods and a
more consistent picture emerged. It showed that there had been a progressive decline
in the CPUE from 38.3 seals/CDW at the beginning of the 1930 s to 27.6 in 1949-51,
that is a decline by 28% over two decades. This doesn’t reflect a similar proportional
decline in the male stock in all divisions and some (for example Division II – closer
and much more accessible) had certainly been more reduced than the rest. This
change had begun well before the higher catches in 1948-51, which followed the
raising of annual quotas in that period.
Almost all the quantitative data on abundance came from the adult male
component of the herd, because few if any systematic studies or counts of females,
pups or young males had been made. Also, in an exploited polygynous species like
the elephant seal (in which one alpha male can impregnate dozens of females – often
more) the ratio of adult male and female seals can vary quite appreciably without
affecting recruitment. Up to a certain point a reduced number of males can serve the
same number of females as formerly achieved by a much larger stock of males. Also
young sexually mature males, which formerly had little prospect of breeding, now
played an important role in maintaining recruitment to the population. The
estimated pregnancy rate of about 82% was quite high. But if the male stock
continued to decrease, at some point this trend would affect conceptions, recruitment
and in due course the adult stock. I believed that danger level had been reached in
Division II and Cumberland Bay (Division III) by 1951. [The determination of the
optimum harem size is one of the more difficult aspects of management]
Estimated Size of the stocks in 1951. The detailed observations and counts that I made at
South Georgia in 1951, mainly in Divisions I, II and parts of III, indicated that the
condition of the herd, particularly in the first two, was deteriorating. I was able to
make only one visit to division IV (a special voyage was arranged for me at the
beginning of the breeding season because it was closed to sealing). Supplementary
information obtained from the sealers indicated that seals were much more abundant
in divisions III and IV than in the two northern divisions. In the latter I noticed that
some of the large open bays with extensive beaches were depopulated (by
comparison with photographs taken in earlier years) and that the seals were much
thicker on the ground in the smaller inaccessible bays and rocky stretches of the
coast. This in itself didn’t mean that the stock was deteriorating; perhaps only that
the sealing activities had induced seals to move to the less accessible (to sealers), and
so less visited parts, of the coast but it was a disturbing trend. It may also perhaps
partly explain the decline in CPUE since 1940 or earlier. But I considered that this
fall in the CPUE was too large to be attributed solely to behavioural changes and
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