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June 15, 2000
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Where Have All the Sea Otters Gone?
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Until the 1700s, sea otters thrived in the cool waters of the North Pacific Ocean. An estimated 150,000 to 300,000 were found in an arc stretching from northern Japan, across southern Alaska, and down the North American coast to Baja, California. Prized for their furs, sea otters were hunted to the brink of extinction by the beginning of the twentieth century. By the time the International Fur Seal Treaty of 1911 put an end to the mass slaughter, only about 1,000 to 2,000 sea otters remained worldwide. Sea otters made a remarkable comeback in the twentieth century. By the mid–1970s, the sea otter population off the southern coast of Alaska — where the vast majority of sea otters are now found — was estimated at between 110,000 and 160,000 animals. But a recent population study shows that these creatures are in trouble again. A team of marine scientists led by Dr. James Estes of the Western Ecological Research Center has studied sea otters in the coastal ecosystem of Alaska's Aleutian archipelago since the 1970s. Their research led to the disturbing conclusion that between 40,000 and 45,000 sea otters have been killed in Alaskan waters since 1990. Along a 500–mile stretch of the Aleutian Islands alone, only 6,000 of the 1970s population of more than 53,000 sea otters remain.
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A Murder Mystery |
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Estes's evidence pointed to a different culprit: orcas, or killer whales. The orcas' nickname is well earned. They are voracious hunters with a strong appetite for meat. But orcas had never been known to hunt sea otters regularly. What evidence linked orcas to Alaskan sea otters? Since the 1970s, the researchers often observed orcas swimming close to the shoreline and the sea otters. Beginning in 1991, they witnessed a dozen direct attacks. Over a six–year period, the sea otter population plunged between 70% and 90% in waters that orcas could reach. But in areas that were so shallow that they were inaccessible to orcas, the number of sea otters remained the same. These observations led Estes to the conclusion that "we're virtually certain it's the killer whales that have done it."
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Eating Machines |
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Orcas are extremely efficient eating machines. Researchers have found that, in theory at least, a small number of orcas could wipe out a very large number of sea otters. For this activity, assume that one orca could eat up to five sea otters each day.
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A Complex Web |
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On the surface, the relationship between orcas and sea otters seems to be a textbook example of a predator pursuing its prey. A closer look reveals a complex web of changing food relationships affecting the entire coastal ecosystem along the Aleutian archipelago. Until recently, sea otters were not the orcas' meal of choice. In fact, orcas much preferred to hunt in deep ocean waters rather than along the coast, where the sea otters live. Orcas' usual diet includes large numbers of two other marine mammals—sea lions and harbor seals. Did they suddenly develop a taste for sea otters instead? Evidence suggests the orcas' change of diet was by necessity and not by choice. North Pacific sea lion and harbor seal populations have declined steeply in recent years. For example, the number of Stellar sea lions in Alaskan waters has dropped by an estimated 100,000 animals in the past 20 years; only about 50,000 are left.
Many scientists speculate that overfishing by humans is the most important cause of the sea otter's decline. The lack of food has likely taken a toll on North Pacific sea lion and seal populations. With fewer sea lions and seals available, orcas have been forced to change their hunting and eating habits. In any ecosystem, even a small change in the tangled web of interactions can have far–reaching ripples. Sea otters normally are the top predator in the Aleutian coastal ecosystem. Their prey of choice is sea urchins, spiny–skinned echinoderms related to sea stars and sea cucumbers. Sea urchins in turn feed on the huge forests of kelp that are the main producers of the ecosystem. The kelp forests provide habitat and protection for a rich variety of marine life.
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The density of the kelp forest has decreased 12–fold in the decade that the sea otter population declined. Estes' team believes this is no coincidence. The scientists now worry that the sea floor of more than 3,300 kilometers (2,000 miles) of Alaska's coast is at risk of turning barren. That in turn would trigger a staggering ecological disaster wiping out fish, mollusks, crustaceans, birds, and other marine life. |
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The Uncertain Future |
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What can be done to save Alaska's sea otters and restore the Aleutian kelp forests? Unfortunately, there are no simple answers. As in numerous ecological crises around the globe, humans play a central role. At the core of the problem is humans' hearty appetite for fresh fish. This puts us in head–to–head competition with the sea lions and seals that orcas prefer to eat. The sea lions and seals are losing the competition, and sea otters and other inhabitants of the Aleutian ecosystem are paying a heavy price. The new century offers hope of new solutions. People will need to make some hard choices. Will we find a way to curb our appetite for fish enough to stop the overfishing that has stressed ocean ecosystems around the world? The sea otters' fate may hang in the balance.
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Check out these activities to explore the dynamics of population interactions in greater depth:
Find out more about sea otters with these links:
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Few animals can captivate aquarium visitors like sea otters. Their playful antics and impressive dives are guaranteed to delight the crowds. What makes the future of these fun-loving mammals so uncertain?
Who or what has been killing the otters? The researchers tested several possibilities. Did they die from some unknown disease? Did toxins kill them? Did they starve? All of these hypotheses seemed reasonable. All proved wrong.
The decline of Alaska's sea otters has devastated the Aleutian coastal ecosystem. With fewer sea otters to eat the sea urchins, the sea urchin population grew unchecked. With more hungry sea urchins munching away, the kelp forest has been virtually wiped out in some areas.
