PUR-SEAL CENSUS, PRIBILOF ISLANDS, 1917. 109 



The Average Harem in the Years 1912-1917, Inclusive — Continued. 



a Derived from estimates. 



Some rookeries, which in the earUer years had averages which ap- 

 proximated the whole, later made considerable deviations there- 

 from. And the converse is true. No area can be said to constantly 

 contain the average development of the herd in a single phase. But 

 no other method is known whereby as accurate an estimate can be 

 obtained as by the average harem method. Small rookeries, how- 

 ever, should not be considered in any deductions of this sort because 

 they are erratic in development and growth and are subject to much 

 greater variations than the larger ones. 



Average Harem and Percentage of Idle Bulls to Harem Bulls, 1916 and 1917. 



• Based on estimated number of pups. 



