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

were stable and natural mortality rates were assumed which fitted the other
parameters. (They now appear to have been fairly close to the actual figures). My
calculations resulted in rough estimates of the size of the population in each division
and of the annual accession, by sex, to the different (yearly) age classes. In view of
the low average age of the bulls that the calculations showed, I concluded that
measures were needed to raise the stock of breeding bulls in each division to equal
one twelfth of the estimated stock of adult cows, to promote an average harem size of
12. The estimated surplus annual accessions of bulls calculated by divisions on this
basis, and tabulated, and I calculated from my life tables the sum of surplus bulls at
5,842 in a stabilised population under natural mortality only (see below). It therefore
seemed reasonable to return to a catch quota of 6,000 adult bulls per year, leaving
one division ‘fallow’ (unworked) each year in rotation. But it was clear that this
overall quota should not be divided equally among the divisions, as before, because
the size of their stocks differed. Depending on what subsequent monitoring told us
about the validity/accuracy of these assumptions it should eventually be possible to
exploit all four divisions each year – leaving out the one in four years ‘fallow’ cycle,
originally introduced as a safety factor.
My calculated estimated surplus annual accessions were Division II, 876, division
I, 1168, divisions III and IV, 1899 each. Over the period 1940 -51 the average annual
catch per division had been 1447, 1450, 1675 and 1489 in divisions I, II, III and IV
respectively. The proposed management plan therefore required a substantial
reallocation of divisional quotas within an overall island quota of 6,000. This would
have to be phased in over a period. According I calculated and proposed quotas, by
division, from 1952 to 1956, when they should be reconsidered in the light of results
from the proposed scientific monitoring and commercial results. The proposed
proportional distributions of the sub-quotas between the various divisions appeared
to be reasonable, although division II might require further protection.
This revision of the divisional quotas to bring the contribution into line with the
estimated size of the available surpluses was the most important practical result of
my work in 1951. The setting up of a monitoring programme based on tooth
collections to follow the age distributions was also a key requirement. Considering
the approximations necessary to the calculations I felt remarkably confident that I
had it right! Was I over-confident? Time would tell, but first I had to convince the
Governor to implement my proposals and set up the monitoring programme,
including the appointment of a sealing inspector.
Provisional life tables and population dynamics
I considered that in principle rational management of the herd should aim at
allowing the bulls to reach an optimum size (that is the time when the year class is of
maximum weight) and then catching them before natural mortality has reduced their
numbers too much. Account needed to be taken of the age at sexual and breeding
maturity and the optimum sex ratio of mature seals. The relationship between yield
and effort would also be important.
As a first step I attempted to draw up provisional life table models from the ages
of the seals I had collected. Then from the growth curve (body length) I had
established, converted from length/weight data on elephant and other seals, I could
estimate the biomass – the weight of living matter - of each age class in the
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