Activity and Prey Election in the Sea Otter: Influence of Population Status on Community Structure

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The population of sea otters (Enhydra lutris) at Amchitka Island has been at or near equilibrium density for at least 3 decades. At Amchitka, sea otters spent 55% of daylight hours foraging; there were distinct early morning and late afternoon peaks in foraging activity; and fish made up over 60% of their overall diet by volume. In contrast, the more recently established populations at Attu Island and off the coast of Oregon are far below equilibrium density. At both Attu and in Oregon, sea otters spent 17% of daylight hours foraging; there was no peak in foraging activity; and herbivorous macroinvertebrates made up their entire diet. We interpret these observations in the following way. Subsequent to the over-exploitation of sea otters, uncontrolled populations of herbivorous sea urchins greatly reduced or eliminated sublittoral kelp assemblages. In areas only recently reoccupied by otters, sea urchins are abundant and easily accessible as prey. Otters in these areas invest relatively little effort in foraging, and distribute that effort uniformly throughout the day. Where otters are abundant sea urchins are scarce, in turn promoting the growth of kelp beds and an associated fish fauna. Consequently otters at Amchitka invest more time in pursuit of more elusive prey (fish), and distribute that effort disproportionally toward morning and evening to correspond with availability of kelp bed fishes. These observations demonstrate that foraging tactics of a consumer are closely linked to its influence on the structure of the community.