118 U. S. BUREAU OF FISHERIES. 
41.20. Recently acquired information enabled the computation with 
reasonable accuracy of the number of adults in existence in addi- 
tion to the harem bulls and idle bulls. And since we know the 
number of births, it now becomes possible for the first time to 
compute the average natural loss from the third year to maturity. 
It gives the first concrete information ever available upon the size 
of a reserve which should be made in the third year to supply the 
herd with stock at maturity. 
In order to present the figures intelligibly, the following table 
has been prepared. The figures have been taken from the various 
census reports since 1914. Nothing has been deducted for natural 
mortality from the third year on. Only animals killed have been 
removed from the various classes. 
ASSUMED MALE STRENGTH OF HeErRD IN 1920 EXCLUSIVE oF NATURAL Loss. 
Killed in census year.1 
Remain 
Class. Number der less 
in 1916. allie 
1916 | 1917 | 1918 | 1919 | 1920 oe 
: 
S-Vear-Oldsin iOU2 ssa. eee eed are 2, OOS shoe eral. SESE ee Roe ess Sas Se ee eee 2, 005 
3-yeaT-Oldsimal Ol 3) anseen sees eee emer 11, 271 LOS) Sis. Jee S| quelosns| dose snaeleeeee eee 11, 167 
Sayear-oldsaml Olan Soa ee eee ee 15, 848 354 97 G474).Ce see 14, 750 
SaVCAT-OLGSUNGL OT Descente eee eee nee 18,282 | 2,855 614) 1,058 | 4,302 |.......: 9, 453 
SayOntool Suri G1 Gee eee csae eee neta i 19,402} (2) 2,771 | 4,690| 2)950| 2,001 6, 990 
Potala qecneetsc oe Sean ee 66, 808 | Total bulls remaining in 1920 if no nat- 
urallLossioccurred--e = seen eee 44,365 
1 Animals killed in the fall of one year have been added to the killings of the following summer. 
2 Killings already deducted. 
This table shows that out of a stock of 66,808 animals which 
reached the age of 3 years from 1912 to 1916, inclusive, there would 
remain in 1920, 44,365 if there had been no natural loss. These 
animals would all be 7 years old or over in 1920. The younger ages 
do not enter into consideration here because they could not have 
been counted among rookery bulls that season. 
It now remains to properly divide this total among the various 
years and classes and derive the percentage therefrom. The problem 
is difficult because so many factors enter therein to modify the result, 
and it is necessary to take these into consideration. 
As an illustration, we may consider the 3-year-old class of 1914. 
The animals composing it were not subjected to any unusual condi- 
tions until the seventh year. Only the usual rate of natural mortality 
would therefor apply during these years. But when they were 
approaching maturity in numbers vastly exceeding the requirements 
of the cows, fighting for elimination began. The younger animals 
would naturally be vanquished. Thus the loss during the seventh 
year would be excessive and still more so during the eighth and ninth. 
In later years the percentage of loss would doubtless increase at a 
regular rate and 100 per cent would be reached at maximum old 
age. This, unfortunately, is not known and a figure must be assumed 
therefor. At the present time we seem not to have any good grounds 
to believe many bulls exceed 15 years. This, of course, is less than the 
