LIMITING FACTORS 



The sea otter is highly specialized to occupy a limited aquatic 

 environment. The feeding habitat lies between the beach and the 

 30-fathom curve. Its specialized characteristics which confine it 

 to this inshore area offer both advantages and disadvantages to 

 its survival. The advantages are that it may usually find shelter 

 from violent storms and that its prey species, unlike the wandering 

 populations of the pelagic prey organisms of other marine mam- 

 mals, are localized. A disadvantage, though, is that population 

 growth and expansion of range may be hindered and a local popu- 

 lation, isolated by broad expanses of deep water, may overutilize 

 food resources. In times of stress, a large isolated population may 

 suffer heavy mortality caused primarily by starvation. 



The sea otter's natural enemies are few. Depredation by man 

 and natural attrition resulting from overutilization of food re- 

 sources are the mortality factors that limit population size. 



Mortality among sea otters was studied during the winter-spring 

 period of environmental stress only at Amchitka Island. In other 

 seasons, different areas were visited and an effort was made to 

 find beach remains that would give an indication of the magnitude 

 of winter-spring mortality. 



Amchitka differs from most areas inhabited today by sea otters. 

 It and the other densely populated Rat Islands are isolated from 

 other areas of sea otter habitat by broad, deep passes and the 

 whole island group is densely populated. Emigration of otters 

 from Amchitka appears to be insignificant. Emigration from a 

 crowded population at Kanaga Island was observed (see Distri- 

 bution and Numbers). Unfortunately, this island is difficult for 

 biologists to reach and no observations of mortality there are 

 available. 



The Shumagin and Sanak Islands groups, which I visited, have 

 relatively small otter populations in proportion to available feed- 

 ing habitat when compared to Amchitka. The observations ob- 

 tained from these three areas are presented. 



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