THE SEA OTTER IN THE EASTERN PACIFIC OCEAN 267 



otters dying along the shores of the eastern half of Amchitka in the 

 period of high mortality were not found. A rough estimate of the 

 annual mortality in 1959 and 1962 on Amchitka is presented below : 



Number of dead otters 

 1959 1962 



Carcasses found in winter-spring period 84 106 



Estimate of carcasses not found (30%) 25 32 



Estimated total mortality this period 109 138 



Estimated mortality in remainder of 



the year (25%) 27 35 



Estimated annual mortality on eastern 



half of Amchitka 136 173 



Estimated annual mortality on western 



half of Amchitka (otter population 30% 



of eastern half, data from aerial 



surveys) 41 52 



Total estimated annual mortality 177 225 



The estimated Amchitka otter population in 1959 was about 

 2,080 (see Distribution and Numbers). Thus, the annual mortality 

 there in at least two seasons was about 9 to 11 percent of the total 

 estimated population. If, instead, the direct counts of living and 

 dead otters are used for the eastern half of Amchitka, then the 



84 



mortality in 1959 was about 8 percent i^r^^) 100 and in 1962 

 106 



about 10 percent (yQ^g) 100 of the population as observed in 

 1959. 



Mortality estimates derived from reproductive tracts 



The data available from other studies (see Length of stages of 

 reproductive cycle of female sea otter, by D. G. Chapman, and 

 Field counts of sea otters) makes possible an estimate of the 

 mortality that occurred in the transition from the dependent stage 

 to the juvenile stage. Since there are 50 young/ 100 adult females 

 per year and since the sex ratio among young is 50:50, this 

 implies 25 female young per 100 adult females. 



Let the mortality that occurs at the end of the dependent stage 

 be T; then ^ 

 Y 



(1-T) -2~=Jo if females alone are considered 

 Also 



Jo(.915)° from equation (2) (see "Length of stages of re- 

 ~ .085 productive cycles of female sea otter," by D. G. 

 Chapman, p. 243) 



