THE SEA OTTER IN THE EASTERN PACIFIC OCEAN 253 



of different seasons during the six field study periods. Two seasons, 

 1959 and 1962, included the entire period of greatest mortality. 



Data gathered in midsummer presented fragmentary evidence 

 of the magnitude of mortality that occurred in the preceding 

 winter-spring period. Natural decomposition of dead otters, storm 

 waves that sweep high onto beaches, and scavengers scatter, bury, 

 and break up skeletal remains. By midsummer of 1955, little 

 skeletal material remained on beaches to indicate the extent of 

 mortality that I observed briefly in late March and early April of 

 that year. 



Available data indicate that maximum mortality among sea 

 otters at Amchitka occurs annually during the winter and early 

 spring (fig. 96). At this time of year the Aleutians are subject 

 to frequent and often violent atmospheric storms, and also pro- 

 longed periods of high seas. In 1962, moderate weather conditions 

 prevailed from mid-January to mid-February and mortality up 

 to that time was insignificant. From this time on, weather condi- 

 tions were stormy. Mortality increased rapidly after 20 February 

 and remained high until 25 March. A search of all study areas 

 during the last week of March revealed only three freshly dead 

 otters (fig. 97). 



In 1959, late winter and early spring were less stormy than in 

 1962. Appreciable mortality did not occur until early March, but 

 it continued at a high rate until mid-May. During the intensely 

 stormy 1962 season, 106 dead otters were found in only 2 months 

 on the same beaches where 84 dead otters were found during the 

 comparatively mild 1959 season (fig. 96). Thus, it appeared that 

 the magnitude and the time of greatest mortality varied in accord- 

 ance with the stresses exerted on the animals by the environment. 



AGE-SPECIFIC MORTALITY 



No technique is known by which the chronological age of a sea 

 otter may be determined. On the basis of body size, suture closure, 

 and dentition, however, certain age classifications are roughly 

 indicated. In this discussion I will distinguish only between im- 

 mature and adult animals-. Table 55 summarizes available data, 

 and indicates that immature otters (fig. 98) constitute about 70 

 percent of the average annual mortality. Among adults, most of 

 those found dead or dying show signs of aging (fig. 99), such as 

 grizzled pelage and severe dental attrition. 



The juveniles weigh from about 10 to 30 lb. and are still in the 

 company of their mothers when the winter period of storms be- 

 gins. Large juveniles, although capable of obtaining food by 



