FUR SEAL3 AND OTHER LIFE, PRIBILOF ISLANDS, I914. 43 



can be laid at its door. But its indirect ramifying effects can not cease until an entire 

 regeneration of the herd has taken place. In a normal herd the cows would consist of 

 evenly graded proportions of young and old, and approximately one-tenth would drop 

 out each year through natural termination of life. The indiscriminate slaughter of 

 pelagic sealing probably destroyed these proportions, causing the death rate at present 

 to be abnormal. Although it is known that pelagic sealers secured large numbers of 

 old cows, it is also to be remembered that the young cows spend more time at sea than 

 the old ones and therefore must have been more exposed to the sealer. It naturally 

 follows that the cessation of pelagic sealing may have left the herd in 191 1 with a pre- 

 ponderance of aged cows. If this be the case, the number of deaths from old age in the 

 succeeding years would be abnormal and disproportionate, perhaps very few in 191 2 and 

 very many in 1913. An increase in young male life is plainly evident in 1914, making 

 it reasonable to infer a similar increase of young females, and thus the supposition is 

 favored that the small total of females is due to losses among those advanced in years. 

 Therefore it is not improbable that the number of old cows dying in 1913-14 was almost 

 equal to the number of young ones bearing pups for the first time, and if so the total 

 number of cows and pups in 191 4 is explained. The result would have been accom- 

 plished by a death rate among old cows only 3^ per cent above the normal. This is 

 shown by the following statement : 



Demonstration of probable abnormal mortality of old cows, lQlj-14. 



Old cows in 1913 92, 269 



Normal death rate of old cows, 10 per cent « 9, 226 



Normal expectation of old cows in 1914 83, 043 



Virgin cows in 1913 less 10 per cent probable mortality 13, 500 



Total normal expectation of cows in 1914 96, 543 



Actual number of cows in 1914 93, 250 



Deficiency of cows in 1914 3, 293 



Normal deaths of old cows,i9i3-i4 9, 226 



Possible total deaths of old cows, 1913-14 12, 519 



Percentage of possible deaths of old cows, 1913-1914 135 



Normal death rate of old cows 100 



Supposed percentage of excessive mortality, 1913-1914 035 



That pelagic sealing may have disturbed the death rate to the extent of 3>^ per 

 cent is not an unreasonable assumption, and in the absence of any other sufficient 

 explanation this may be accepted as the principal reason for the lack of a substantial 

 increase of cows and pups in 1914. 



Comparison of the number of pups found on the various rookeries in the three 

 successive censuses shows certain points of interest, but conclusions drawn from them 



« This is the rate due solely to old age. based on the knowledge that the average breeding life of the cow is about 10 years. 

 Some mortality of adults from other causes shou'd be added for absolute accuracy, but it can not be ascertained, and is doubt- 

 less too small to affect a calculation of this kind. 



