156 



NORTH AMERICAN FAUNA 68 



per year in early stages of growth (fig. 74) is based on the 

 observed growth rate in a part of the Andreanof Islands (Seguam 

 Island) where conditions seemed similar to the Amchitka area. 

 The field counts of otters by Loy and Friden (1937) and Loy 

 (1940) are projected to estimated totals on the basis of Lensink's 

 (1958) data indicating that about 60 percent of the otters present 

 may not be seen on a surface survey. These estimated totals assume 

 logical positions (Points 2 and 3) on the curve between the esti- 

 mated totals of Swicegood (1936), Point 1, and Beals (1943), 

 Point 4. The observations of mortality which occurred after the 

 Amchitka population reached a density of at least 42 otters per 

 square mile are discussed under Limiting Factors. 



Lensink (1960) estimated the total population of sea otters at 

 Amchitka on the basis of a field count of otters obtained during a 

 period of weeks in the summer of 1956. His count was 2,568 otters 

 on the eastern half of the island and his total estimate was 6,000 

 to 8,000 animals. 



Our 1959 and 1965 aerial surveys for all of Amchitka were 

 made under excellent observation conditions and our total counts 

 were 1,560 and 1,144 otters, respectively. Why were our total 

 counts much less than Lensink's partial count? Two reasons are 

 suggested: (1) The number of otters at Amchitka may have been 

 reduced between 1956 and 1959. General field observations and 

 observed high winter-spring mortality indicated that population 

 reduction may have occurred. Emigration may also have occurred. 

 (2) Lensink made many trips to different contiguous counting 

 areas during a period of weeks. It is possible that otters moved 

 in the time between counts and the same animals were included 

 in more than one count. If Lensink's (1960) estimate of 6,000 to 

 8,000 otters at Amchitka is accepted, it would mean that the popu- 

 lation there was 55 to 73 animals per square mile of feeding habitat. 

 All other observations and studies made at Amchitka and in other 

 areas indicate that this estimate is very high — nearly twice as 

 large as the indicated maximum population observed by Beals 

 (1943). 



In summary, our studies conducted since 1955 and intermittent 

 surveys made since 1935 have indicated that the sea otter popula- 

 tion on Amchitka Island grew to a large size, causing overutiliza- 

 tion of food resources during the mid-1940's. Population reduction 

 through starvation was then observed and a fluctuating population 

 in balance with the habitat resulted. 



It was not surprising, therefore, that our 1965 observation of 

 1,144 otters at Amchitka was less than our 1959 tally of 1,560, a 



