
immediate effect would be greater survival of pups to killable ages of 3 
and 4 years. An accurate prediction of the results of harvesting females 
is difficult because of two opposing factors which would come into operation-- 
increase in pregnancy rate and decrease in actual number of breeding females. 
(3) An increase in the harvest of seal skins. 
The observed mortality among cows under present conditions is 
low. Commercial sealing on the Pribilofs annually accounts for about 
200 cows killed by mistake. A few, probably less than 500, die in 
parturition each season. Mortality rates as presented in table 15 are 
based to a large extent on the rates for males, which are based on tag 
recoveries on the killing fields. 
Yearlings 
The yearlings are the last group to arrive on the Pribilofs 
each summer, being preceded about < months by the 2-year-olds. Both 
classes are present on the breeding islands in greatest numbers during 
September and October. Field observations lead to the belief that many 
yearlings and <-year-olds remain at sea throughout the year. Because of 
the largely unknown migratory habits of the yearlings and 2-year-olds, 
any direct count of them is impossible. Their numbers are estimated very 
roughly from the recoveries of tagged 3-year-olds (tables 14 and 15). 
Mortality at sea is believed to strike the yearling class 
harder than any other. The transition from a milk diet on the Pribilofs 
to a diet of fish and squid in the stormy waters of the North Pacific in 
the fall is abrupt, and about half of the yearlings are believed to | 
succumb in making the adjustment. Starvation during prolonged storms is 
a direct cause of death. An estimated 700 yearlings washed ashore along 
the Washington and Oregon coasts in the severe winter of 1949-50 (Scheffer, 
1950d). Evidence from tag recoveries and dead-pup counts indicates that, 
between the time of tagging, when the seals are about 3 months old, and 
the start of the commercial kill 3 years later, about 60 percent are lost 
(tables 11 and 14). Seals with missing and badly scarred flippers seen 
on the Pribilofs suggest losses to sharks and killer whales. The only 
authentic observations of killer whales attacking seals were made during 
the late 1800's and early 1900's near the Pribilofs (Preble and McAtee, 
1923, p.- 117). In recent years, killer whales have rarely been seen 
near the breeding islands. 
PRESENT SIZE OF THE HERD 
The foregoing sections have dealt with methods and results of 
computing the size of component groups of the seal herd. The estimates 
believed to be most reliable for each group are now used in conjunction 
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