268 



NORTH AMERICAN FAUNA 68 



SO that 



.085A 

 .766 



=.111A 



or 



.•.(l-T)^=:Jo=-.lllA 



A Y 

 (1-Ti=2(.lll)^ =.222/4 = - 



.222 



whence 



1-T=.53 

 and 

 T=A7 



Another way of considering the same problem is as follows: 

 Each 25 female young must survive to replace the 8.5 mortality 

 occurring among the mature females 



.-.(25) (1-T) .766=8.5 



or (1-T) (19.2) =8.5 



or 1-T=.44 and T=.56 



Although these are slightly different, they suggest that the tran- 

 sitional mortality from dependent to juvenile stage is about 50 

 percent. Some of this may occur during the dependent period. 



Also, if the mortality among juveniles is higher than 8.5 per- 

 cent this would mean the transitional mortality is slightly lower. 

 This could also mean that the estimated length of the independent 

 juvenile period is somewhat less than 3 years. 



It should be noted that these estimates of 8.5 percent as an 

 annual mortality, 50 percent mortality at the end of the dependent 

 young stage, and a juvenile period of 3 years apply to females. 

 The data do not provide comparable estimates for males. 



MORTALITY ON THE SHUMAGIN ISLANDS 



The outer Shumagin Islands provide interesting material for 

 comparison with observations from Amchitka. The outer Shu- 

 magins, including Nagai and all islands lying east and west of 

 it, are chosen because (1) A population of about 3,000 sea otters 

 is found there, (2) it appears to offer ideal sea otter habitat, and 

 (3) it was accessible for aerial and surface observations. 



The outer Shumagins are surrounded by nearly 700 square miles 

 of water less than 30 fathoms deep. The islands are near enough 

 to each other to allow sea otters to move easily from one to an- 

 other and near enough to other areas along the southeast coast 

 of the Alaska Peninsula to allow population dispersion. 



