
Sources and methods 
An exact enumeration of the mumber of living seals at any 
particular time during the year is impossible. The number of seals alive 
changes constantly during the year. Mortality through natural and arti- 
ficial causes combined is high. For example, it is about 74 percent for 
both sexes up to age 3 from natural causes alone, and nearly 99 percent 
for males up to age 5 from natural and artificial causes combined. From 
sealing records, counts, and calculations, the sizes of all age and sex 
classes in the herd have been estimated. Data gathered from seals taken 
at sea and on the breeding islands during the recorded history of the 
Pribilofs have been used, although the most useful and important data are 
those accumulated since 1947. 
Studies of the fur-seal population are still in progress and 
are planned for the future. Since new data are accumulating annually, 
and because of the volume of statistical work necessary in analyzing these 
new data, the writers heave set deadlines for inclusion of source material 
in the current report: (1) Tag recoveries, series A, through the summers 
of 1950 and 1951. (2) Age composition of the kill from tooth-ridge counts 
in 1950, 1951, and 1952. (3) Pregnancy rate; samples of females taken 
1944-1952 inclusive from St. Paul Island, Sitka, and Northwest Coast are 
combined. (4) Commercial kil] and annual count of bulls, through 1951. 
Certain terms unfamiliar to the reader, or familiar terms with 
an unusual meaning may be found in the Jordan report (1898-99). 
Counts and measurements of seals 
Sealing statistics include the numbers and length-measurements 
of all seals taken in commercial sealing operations on the Pribilofs 
since 1918 and, collectively, are one of the best sources of population 
data. 
Bull counts conducted annually since 1905 by the general manager 
of the Pribilofs are also of great value. Pups were counted on the Pribi- 
lof rookeries between 1912 and 1924: Counting was discontinued when, as 
the herd grew, the numbers rose above 200,000. It was resumed in 1948 
for the purposes of the present study. Sample counts of living pups and 
complete counts of dead pups have been used in conjunction with breeding- 
ground area measurements based on aerial photographs. Pup counts have 
also been studied in relation to bull counts and to counts of bachelors 
killed. Evidence on the number of pups born on the Pribilofs in recent 
years has been derived from 19,183 seals tagged as pups in 1947, of which 
2,413 were recovered at ages 3 and 4 during commercial killing operations. 
In addition to the tagged animals used in this study, annual tagging pro- 
grams were conducted in 1948, 1949, 1951, 1952, and 1953. For every 
tegged seal recovered, the time and place of tagging and recovery and the 
body length at time of recovery are recorded. 
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