27.0 



NORTH AMERICAN FAUNA 68 



Amchitka was 8 to 11 percent of the population (perhaps higher 

 in certain seasons). In the areas of uncrowded population, the 

 annual mortality was much lower. It was not calculated because 

 too few dead otters were found. 



The natural forces which control the magnitude of the sea 

 otter population at Amchitka were a limited food supply and 

 climate. From his observations on Amchitka, Rausch (1953) sus- 

 pected that much of the annual mortality was the result of **A 

 population that exceeded the carrying capacity of the habitat." The 

 applicable generalization was aptly stated by Lack (1954) : 



climatic factors can exert an important modifying influence in the control of 

 animal numbers. Thus, although one may speak loosely of mortality due to 

 climate, the actual cause of death is often starvation, which can be density- 

 dependent. 



The data presented here and in the sections on Food and Feed- 

 ing Behavior and Distribution and Numbers, I believe, support 

 Lack's statement. 



The following points are the evidence that lead to the conclusion 

 stated above : 



1. The food habits studies of otters at Amchitka indicated that 

 healthy adults, having ample body fat in the late-winter, early- 

 spring period, fed to a large extent on fish and octopus. Sea otters 

 at Amchitka were limited in their search for food to a relatively 

 narrow belt of water surrounding the island where water depths 

 do not exceed 30 fathoms (see Diving). Observations at Amchitka 

 indicated that populations of bottom-inhabiting food species were 

 reduced (see Food and Feeding Behavior). 



2. Observations indicated that large juveniles when deserted 

 by mothers were unable to obtain a sufficient quantity of high- 

 calorie food species. Apparently they lacked both the skill and 

 strength to capture and hold most fish and octopus. This may be 

 true also of aging adults. 



3. Both of these age groups were forced to obtain what nourish- 

 ment they could from depleted populations of sedentary and often 

 hard-shelled and low-calorie invertebrates. Because of the hard 

 shells of these organisms the teeth of adult otters in the Amchitka 

 population showed greater attrition than did those of animals 

 from less crowded populations. The teeth of old adults were often 

 worn to the gumline. Evidence of bone infection, which often 

 resulted when tooth attrition was severe, was found in 72 per- 

 cent of a sample of dead adults. Juveniles in the process of replac- 

 ing deciduous teeth were unable to eat a sufficient quantity of the 

 available hard-shelled organisms. 



