FUR-SEAL CENSUS, PEIBILOF ISLANDS, 1917. 101 



Percentage of Increase or Decrease in the Number of Pups in 1917 from 1916. 



a Estimated. b Based on estimated number of pups in 1917. 



Percentage op Annual Increase op Pups, 1912-1917. 



It will be noted that the percentage of increase of pups for the herd 

 is 9.44. The difference in the increase on the two islands may appear 

 strange, but is entirely in accord with the facts gathered tlu-ough 

 six years of counting, and conforms to the general law of fur seals 

 that their instincts cause them to flock to the centers of gi'eater num- 

 bers, and the smallest rookeries grow most slowly. 



This increase of 9.44 per cent is below what is generally believed 

 to be the normal of 11 per cent. We see a reason for this in the lean 

 year of 1914, Female pups born then gave birth to their first pups 

 in 1917. Therefore the small increase in 1914 should be felt in 1917. 

 Another factor which goes to make a small increase may be somewhat 

 noticeable this year. This is the death rate of cows on land. In 

 1913, 1914, and 1915 the number of dead cows noted during the count 

 on the rookeries was considered negligible. But in 1916 there was a 



