148 



U. S. BUREAU OF FISHERIES 



MORTALITY OF SEALS AT SEA 



In 1925, as a result of information secured from branding opera- 

 tions, it was found necessary to increase the mortality rates of seals 

 for the first 3 years of their existence. These mortality rates were 

 used until they were proved to be incorrect; in 1929 the great increase 

 in the number of 3-year-olds available for killing made it necessary to 

 reduce the mortality rates to those used prior to 1925. 



The following statement appeared in the report of the computa- 

 tion of fur seals at the Pribilof Islands in 1932: 



It is believed that the large increase in the number of 3-year-olds available for 

 killing during recent years was primarily the result of leaving a larger breeding 

 reserve, rather than to any marked decrease in the mortality rate. 



In 1937, 50,800 3-year-old males were killed. This was less than 

 in 1933, when 52,747 were taken. This clearly indicates that for the 

 past 4 years there has been a decrease rather than an increase in the 

 3-year-old males returning to the islands during the sealing season. 

 It would appear that some abnormal condition must have caused an 

 increase in mortality during this period. If there has been no ab- 

 normal condition, the mortality rates should be greater than at 

 present. It is very possible that as the herd increases in size the 

 mortality of pups and young animals also increases. Unless there is 

 a very much greater increase in killable males within the next year or 

 two, mortality rates must be materially increased and necessary ad- 

 justments made in all calculations. 



At this time all evidence seems to point to the conclusion that the 

 decrease in number of available 3-year-olds is due to closer killing 

 since 1930. In 1929 killings were discontinued on July 24. From 

 1923 to 1929 proportionately larger breeding reserves were left than 

 during any year since that period. The abnormally large increase of 

 killings in 1929 was, according to the general belief, due to the leav- 

 ing of larger breeding reserves on and after 1923. 



COMPLETE COMPUTATION 



The following summary shows the methods used in computing the 

 number of animals in the fur-seal herd of the Pribilof Islands in 1937. 

 The total number of seals of all classes on August 10, 1937, was 1,839,- 

 119, or 149,376 more than in 1936. This is an increase of 8.84 per- 

 cent. 



Complete computation of fur seals, Pribilof Islands, as of Aug. 10, 1937 



