Mysterious Disappearance of Sea Otters in Alaska

Back to Map Page February, 1999 Sea otters, with their round faces, big eyes, and the endearing habit of floating about on their backs, are one of the most charming animals. Ecologists point to them as a classic "keystone" species, one whose position in the food web is key to keeping a whole ecosystem in balance. Now sea otters are in the news as the center of an ecological mystery. In parts of their range along the Alaskan coast, their numbers have dropped by 90%, and ecologists are trying to puzzle out why and how this has happened. Orcas appear to have developed a taste for sea otters in parts of Alaska. Hunger may be the cause. The short answer, as reported in Februrary, is that orcas (also known as killer whales) are eating the sea otters, something that has never been observed before, and a surprise since the two species have coexisted in the same waters for millenia. The longer answer to why the otters are disappearing, or why the whales have suddenly started eating them, may be as complex as the coastal ecosystem itself. The more complete explanation of the orcas' sudden interest in otters appears to be overfishing by humans. Orcas normally subsist on seals and sea lions, whose rich body fat is an excellent energy source for the whales. Seals, in turn, need oil-rich fish such as salmon and herring to survive in the cold northern waters, but these fish are also prized by humans, and their populations have dropped as a result of improving fishing technology and strong world markets for fish. Fish catches both near shore and off-shore have climbed steadily in recent decades, even as fish numbers have fallen. With the depletion of oil-rich fish stocks, less fatty fish, chiefly pollock, appear to have moved into the coastal waters of Alaska, but pollock apparently provide a meagre food source for seals and sea lions. Their numbers have dropped precipitously in recent years. In the absence of enough seals and sea lions, ecologists believe, orcas have found that sea otters, which can be about 4 feet long and weigh as much as 50 pounds, make good prey. But sea otters play an important role in their ecological community. They subsist principally on sea urchins, which live on the sea floor and graze on kelp. Kelp is a large vine-like algae that grows to great heights, in kelp "forests" that shelter a complex ecosystem of fish, sea lions, sea birds, and of course the otters themselves. When otter numbers fall, sea urchin populations explode, and their grazing can destroy the kelp forest on which so many species depend. Marine biologists worry that orcas' new appetite for otters will lead to the dissolution of the complex ecological system that supports a wide range of marine mammals, fish, and birds. Will the loss of sea otters continue, and will it lead to a "decline spiral" in other coastal marine populations? Only time will tell, but watch the news for further developments as research continues. To read more, see Environmental Science, A Global Concern, Cunningham and Saigo, 5th ed. Sea otters: decline and recovery: p. 69-70 Keystone species: p. 76-78 Species interactions and community dynamics: p. 75-76 Environmental Science, A Study of Interrelationships, Enger and Smith, 7th ed. Predation, competition, symbiotic relationships: p. 53-54 Food chains and food webs: p. 58 For further information, see these related web sites: Information on sea otters National Marine Mammal Laboratory, information on marine mammals Back to Map Page feedback form | permissions | international | locate your campus rep | request a review copy digital solutions | publish with us | customer service | mhhe home Copyright ©2001 The McGraw-Hill Companies. Any use is subject to the Terms of Use and Privacy Policy. McGraw-Hill Higher Education is one of the many fine businesses of the The McGraw-Hill Companies.