Biology_of_Mustelids_Vol_1.pdf

165.
Even in the Pechora-Ilych game reserve the ratio of adult to young is 1:5,
i.e. there are 10 first year animals to each pair of adult animals:
in
the north o't' Tyumen l
province there are 18 first year animals per pair of
adults.
known.
It is difficult to explain this age ratio from the fertility data
My explanation was that it was due to increased mortality in the
old age groups (Kopein 1965):
data obtained in other regions have confimred
my conclusions.
It may be suggested that the high proportion of first year animals
in winter reflects selectivity in trapping.
According to Smirnov 0964) and
Grakov (1963), young animals are trapped more often than old ones.
The age
structure of trapped samples may therefore be biased.
It is highly
probable that trappihg is a selective operation, particularly during the first
half of the trapping season.
But this factor must apply equally in all
seasons and regions, since traps are used everywhere for catching ermine, yet
the age ratios are still different.
It follows therefore that there actually
are seasonal and regional variations in the age structures of ermine populations,
,which are accurately reflected in these samples.
In the north of Tyumen l
province, the age ratios vary between years,
but the proportion of young is always high, in spite of abrupt population
fluctuations.
According to trapping results, over a six year period the
numbers of ermine varied by almost 14:1 (40.500 pelts were treated in the
1960-61 season, 3 000 in 1962-63). Even a small reduction in the percentage
of first year animals trapped (e.g. to 75.9%) therefore reduces the number of
ermine by several times.
Obviously, unfavourable conditions one year will
mean that the breeding stock next year will be lower, and productivity will
drop.
Then, the number of first year animals in the coming commercial
trapping season will be sharply reduced.
Since the mean life of ermine
in Northern latitudes is about 12-14 months, many of the adults i.e. those
born the previous year, will die in the autumn, so the number of adults
trapped will also be low.
Although the ratio of first year to adult