68 



U. S. BUREAU OF FISHERIES 



Computation of breeding coivs, based on annual increase of 8 percent, and of average 



harem, in 1935 



PUPS AND COWS 



Since 1923 it has not been practicable to make an actual count of 

 the number of pups on all rookeries. Pups could be counted on the 

 smaller rookeries but, inasmuch as the larger rookeries normally show 

 a greater yearly increase than the smaller rookeries, the results 

 secured would be unsatisfactory and would not show the true growth 

 of the herd. It appears advisable, therefore, to continue using the 

 average rate of increase in the number of pups as determined from 

 actual counts for tlie period from 1917 to 1922, inclusive. This 

 average rate of increase of 8 percent will answer for all practical 

 purposes. 



The number of dead pups, also, can be estimated only, and the 

 percentage found in 1922 has therefore been applied to each rookery. 



The increase in size of rookery areas and the increase in the number 

 of animals in a given area will cause a slight increase in the per- 

 centage of dead pups. It is desirable, therefore, to make as close a 

 count as possible of the dead pups on each rookery in order to deter- 

 mine a new death rate. 



Inasmuch as a female gives birth to only one pup a year, the num- 

 ber of cows on a given rookery is the same as the number of pups. 



