128 



NORTH AMERICAN FAUNA 68 



nutritional source. Because of the general depletion of inverte- 

 brates and the apparent inability of juvenile otters to obtain an 

 adequate number of fish and mollusks, these young animals are 

 compelled to eat the abundant and easily obtained immature sea 

 urchins. An otter would have to consume nearly 6,500 of these im- 

 mature urchins daily to supply the 3,000+ calories which appear 

 to be required. 



Depletion of Food Resources 



The requirement for large amounts of food by sea otters has 

 been discussed. Feeding grounds are limited by depth to relatively 

 shallow waters and tag returns indicate that individual sea otters 

 do not range widely along the coast (see Home Range). Because 

 of these circumstances which concentrate feeding activities in 

 rather limited areas, it appears probable that a large population 

 of sea otters could seriously deplete food resources within their 

 home range. Evidence is available that this does in fact occur. 



SEA URCHIN DEPLETION 



McLean (1962) presents convincing evidence that the sea urchin 

 Strongylocentrotiis franciscanus has been nearly exterminated in 

 a particular area on the California coast which is occupied by a 

 considerable number of sea otters. Of the area he studied he says 

 (p. 101) "the large sea urchin was totally absent, although spines 

 and test fragments were present in gravel samples." 



Indirect evidence from Amchitka Island, where a large sea otter 

 population exists, indicates that sea otter predation has drastically 

 reduced certain food species there. Small green sea urchins are 

 abundant. It is not possible, however, to find large individuals in 

 the intertidal zone and I seldom saw an otter eating an urchin 

 that approached in size the large individuals which are abundant 

 in other Aleutian areas where the sea otter is scarce or absent. 

 Bottom samples obtained by R. D. Jones, while diving with 

 SCUBA equipment, both at Amchitka and in comparable areas 

 at Adak (where at the time few sea otters occurred), showed that 

 sea urchins at Amchitka are relatively scarce and small. 



In a recent letter (16 December 1966), Jones reported on a 

 subsequent visit to the Adak area he had explored prior to the 

 time that it was occupied by a large number of sea otters : 



In 1957 the green sea urchin was numerous and obvious in this area, and this 

 time I saw none. I have no quantitative data because I had neither the equip- 



