
with the best information available on length of life, age, mortality 
rates, and pregnancy rates to calculate the approximate total size of the 
Pribilof seal herd and of its sex and age classes. Sufficient data from 
one year have never been available to permit a calculetion of the seal 
population for any specific year. The present summary gives the approxi- 
mate average size of the herd and its components in the period 1947-51. 
Sealing statistics and harem-bull counts indicate that during the early 
1930's herd growth was leveling off rapidly. Before 1940 the point was 
reached at which the factors governing life and death in the seal herd 
approached equilibrium. Although the population naturally fluctuates, it 
has passed the stage of rapid active growth. It is important to note that 
a 1947-51 appraisal of the Pribilof herd is valid for this period only, 
for the herd will not have arrived at complete equilibrium until the late 
sixties, by which time its oldest member (perhaps 30 years of age) will 
have been born after the leveling-off period of the late thirties. For 
example, many seals living in 1951 were born when the pup mortality rate 
was certainly lower and when the pregnancy rate was probably higher than 
today. 
Life tables 
‘The life table (14) for male seals is based on four values that 
determine its shape and magnitude. Between these values, or check 
points, it has been necessary to interpolate rather freely. The values 
are-- 
1. 265,000 male pups born in 1947; from 530,000 total 
pups born (tag recovery method) and a sex ratio of 1:1. 
2. 61,365 bachelors, combined ages 2, 3, and 4, killed 
annually during the study period 1950-52 (table 7). 
3. 5,709 estimated average escapement of 4-year males 
through and after the kill (appendix £). 
4. 16,017 bulls counted annually during the postwar period 
1945-51 (table 2); mean of harem bulls 12,008, of idle 
bulls 4,009. 
The essential procedure has been to work forward and backward 
from the 4-year age group. The estimated escapement of 4's is consis- 
tent with other observed factors, in particular the total bull count. 
Only by assuming that the mortality in the 4-to-l0-year range is very 
much larger than that postulated here can it be asserted that the 
escapement from the kill exceeds 6,000. An escapement below 1,000 is 
likewise unreasonable. Consequently, it may be assumed that the number 
of returning 3-year~olds (approximately 70,000) postulated in the life 
table is correct to within 3,000 to 4,000 in either direction. 
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