92 



U. S. BUREAU OF FISHERIES 



Computation of breeding coics, baaed on annual increase of 8 per cent, and of average 



harems, in 1931 



PUPS AND COWS 



The number of cows and pups present was determined by applying 

 an increase of 8 per cent to the number of cows computed for August 

 10, 1930. This average rate of increase, as determined by actual pup 

 counts of 1917 and 1922, has been found to work out approximately 

 correct and has therefore been used continuously since 1923. 



Owing to the continual variance from year to year of the number 

 of animals present on the several rookeries, the figures computed for 

 each individual rookery will not be correct, but the total number, as 

 <5omputed for the two islands as a whole, will be very close to the 

 actual number of animals on hand. Actual rates of increase of the 

 total number present on both islands may also vary considerably 

 from year to year. It is necessary, therefore, to use a conservative 

 average rate of increase over a number of years. 



The providing of a sufficient breeding reserve during the last nine 

 years has, no doubt, resulted in a greater average annual increase for 

 this period, but with no definite data on hand as a basis for increasing 

 this rate, it is essential to continue to use the rate of increase estab- 

 lished by definite facts. If the yearly rate of increase of killable 

 3-year-old males continues, however, it will shortly be necessary to 

 raise the rate of increase of pups and cows. These increases in rates 

 of growth are undoubtedly due to provision of a larger breeding 

 jeserve than was provided previous to 1923, making it possible 



