166 U. S. BUREAU OF FISHERIES 
The number of dead pups did not appear to be proportionately any 
greater this year than in the past, but the evidence available is not 
conclusive. To anyone observing the great activity on closely crowded 
harem areas it would seem highly probable that the death rate of pups 
is somewhat greater than indicated. The pups in the front portions 
of the rookeries have in many places a long way to travel to reach the 
rear of the areas occupied by active harem bulls. 
It was quite surprising to note the number of old cows in heat at 
the end of the season. Many of them had pups recently born. 
Theoretically the older cows should be bred the first part of the season. 
If not bred at the proper time they would no doubt come in heat a 
second time. This would result in the pup being born the following 
year at a later date than would be normal for the age of the cow. 
MORTALITY OF SEALS AT SEA 
Considering the number of killable seals at the Pribilof Islands in 
the last few years, as well as general observations regarding the num- 
ber of older bachelors and surplus bulls on the hauling grounds, it is 
advisable to increase the mortality rates of pups at sea. Accordingly, 
there have again been applied the mortality rates for pups which 
were used in 1925 and which were continued until 1929, when, as a 
result of additional reserves made in 1923 and 1924, the number of 
3-year-old seals returning to the islands was so great that it was 
necessary to reduce these mortality rates. If present conditions do 
not improve, it will be necessary to increase mortality rates for several 
other ages of male seals and to reduce the percentage increase of pups 
and cows. There is a strong possibility that the mortality rates in- 
crease with the growth of the herd. However, past experience would 
indicate that the decrease in the rate of growth of the herd is more 
likely to be directly comparable to the percentage of animals killed. 
Observations indicated it was advisable to apply the larger mortality 
rate for pups to all ages up to and including the 5-year-old male class. 
Mortality rates for female pups were increased from 40 percent to 
45 percent and for males from 40 percent to 50 percent. Experience 
has shown that for all animals there is a greater death rate among 
young males than young females. It will be necessary to change 
mortality rates as new information regarding the life-history of seals 
is secured. 
COMPLETE COMPUTATION 
The following summary shows the methods used in computing the 
number of animals in the Pribilof Islands seal herd in 1938. It will 
be noted that the mortality rate for female pups was increased from 
40 percent to 45 percent and for male pups from 40 percent to 50 
percent. In addition, figures carried over from 1937 for yearling 
females and for all males up to 6 years of age were altered to provide 
as the newly applied mortality rates from the year the animals were 
orn. 
