Food of Sea Otters and Harbor Seals at Amchitka Island

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Studies of the food of sea otters (Enhydra lutris) have, because of the limited populations and stringent prohibitions against killing the animals, been necessarily restricted to scat examinations. Scat analyses published by Murie (1940), Williams (1938), and Jones (1951) were alike in showing sea urchin, ( Strongylocentrotus drobachiensis), mussel (Mytilidae), chiton (Polyplacophora), crab (Anomura and Brachyura), and limpet (Acmaeidae) as the dominant food items. Because of the method of feeding and the relative proportion of hard and soft parts, both scat and stomach examinations often do not accurately represent the relative importance of certain food species. For example, the shell of the rock oyster, Pododesmus macroschisma, is usually discarded, and the soft parts of limpets are also extracted from their shells. By contrast, small sea urchins and mussels are simply crunched, shell and all, and swallowed. Large sea urchins may be cracked by beating on a hard surface, but much of the rather fragile shell is swallowed. Miss Fisher (1939) stated that the internal structures of a sea urchin are sucked or licked out after a hole is bitten in the test. Such behavior is not usual when the otters are feeding on the small sea urchins that predominate on Amchitka Island. In captivity, sea otters crack clams by beating them on any available hard surface to make the soft parts accessible, and some fragments of the shell may be swallowed when the body is eaten. Clams do not appear to enter the diet of sea otters in the wild to any appreciable extent. It is not known whether or not sea otters have the ability or inclination to dig clams from a mud or gravel bottom. It appears that their feeding is confined to species that lie on the bottom or are attached to the surface of rocks and marine vegetation. In Table 1, giving the stomach contents of five otters, the same food species appear as in the scat analyses. Sea urchins and mussels are most important. The extent to which sea otters feed on fish is not fully known. A fringed greenling appeared in one of the five stomachs, and occasionally a collection of scats may be made up largely of fish bones. There is some evidence that fish are eaten more frequently in the winter and early spring, when feeding conditions are less favorable. In Alaska, the harbor seal (Phoca vitulina richardi) is the subject of more intensive control measures than any other predator. These measures are mostly confined to the mouths of important salmon rivers where the seals feed primarily on eulachon (Thaleichthys pacificus) in the spring months but take salmon as the season advances (Heintzlemann, et al., 1954). No stomach-examination results are TABLE 1. STOMACH CONTENTS OF FIVE SEA OTTERS FROM AMCHITKA ISLAND, MARCH 1954