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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 8teexgth of Herd in 1920 Exclusive of Natural Loss. 



1 Animals killed in the fall of one year have been added to the killings of the following summer. 

 » 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 necessaiy to take these into consideration. 



As an illustration, we maj' 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 tlie cows, fighting for elimination began. The younger animals 

 Avould naturally be vanquished. Thus the loss during the seventh 

 3'ear Avould be excessive and still more so during the eighth and ninth. 

 In later years the percentage of loss would doubtless increase at a 

 I'egular 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 Ave seem not to haA^e any good grounds 

 to belieA^e many bulls exceed 15 years. This, of course, is less than the 



