
bear a fairly constant relation to the size of the pup class. A smooth 
curve has been fitted to the increase in harem bulls (table 5). The 
mean number of bulis in the reference period 1947-51 is 1.4757 times 
the number in 1928-32. Therefore, the size of the pup recruitment today 
is estimated to be 492,961 (that is, mean count of pups 1928-32, or 
334,052, times 1.4757). Applying the same correction for increased 
mortality, the figure becomes approximately 550,000. 
Cows 
According to custom, any female seal 2 years old or older is 
called a cow. For example, a female pup born in the summer of 1950 would 
become a yearling on 1 January 1951 and a <~year cow on 1 January 1952. 
Since the cows are the largest and most transient component of the herd 
on the breeding grounds (they number over half a million and shift 
constantly between the land and sea), no direct method of counting them 
has been developed. They are here calculated from (a) the estimated 
number of pups born, which is equal to the total successfully breeding 
cows, and (b) the percentage of nonpregnant cows as determined from a 
sample of 894 cows of breeding age. 
The pregnancy rate is based on specimens taken at sea and on 
the Pribilof Islands. The female fur seal may become pregnant for the 
first time at the end of her third year. A traditional belief (Lucas, 
in Jordan 1899, p. 48) that impregnation occurs at the end of the 
second year has never been substantiated (Enders, Pearson, and Pearson, 
1946, p. 214; also specimens collected in 1952 and 1953). Females 
sampled at sea along the northwest coast of America and on the Pribilofs 
indicated that relatively few (about 7 percent) become pregnant at the 
end of their third year, an increased number (about 57 percent) at the 
end of their fourth year, and the greatest number (about 79 percent) in 
the age classes above the fifth year (International Fur Seal Investigations, 
1954, MS). Of the 894 females sampled in their fourth year or older the 
mean rate of pregnancy was 69 percent (the percentages given by age 
groups are listed in table 15). Since a cow bears one pup, the number 
of pups produced each summer on the Pribilofs is the best clue to the 
number of breeding females. The age when a female ceases to reproduce 
is unknown. The oldest cows seen on the breeding grounds were <l years 
of age (U. S. Bureau of Fisheries, 1923, p. 136). 
Among four groups of seals for which data are available, the 
pregnancy rate of Pribilof fur seals is the lowest. 
Percent pregnant 
Pribilof fur seals, Callorhinus ursinus, 
Soy specimensi a seks wore, eee where ares Lela Tega (OG 
Asian fur seals (including an estimated 27% 
migrants from the Pribilofs) Callorhinus ursinus, 
1,052 specimens from Japanese waters (International 
Fur Seal Investigation, 1954, MS) . . 2... +++ -+-+-s 80 
South African fur seals, Arctocephalus pusillus, 
about 300 specimens (Rand, 1952). . 2. ....e+--+- 7 
North Atlantic harp seals, Phoca groenlandica, '70 
specimens (Fisher, 1952) .. ie ee hae ee «8 
-3)- 
