SANTA CRUZ, CA--With seals and sea lions in short supply in the North Pacific, killer whales are now feeding on sea otters, causing an abrupt decline in sea otter populations in western Alaska, according to researchers studying the area's marine ecosystems. The decline in sea otters has allowed their primary prey, sea urchins, to increase in number and strip coastal kelp forests over large areas, said James Estes, a marine ecologist with the U.S. Geological Survey and the University of California, Santa Cruz.
The new phenomenon of killer whales preying on sea otters appears to be one link in a chain of interactions extending from the open sea to the coastal zone and involving a wide range of species at different levels of the food chain. Overexploitation of certain North Pacific and Bering Sea fisheries may have initiated this cascade of ecological effects, Estes said.
Estes and his coworkers reported their findings in the October 16 issue of the scientific journal Science.
The researchers have been studying the role of sea otters in the coastal ecosystem of Alaska's Aleutian archipelago since the early 1970s. During their field studies they often saw killer whales swimming near sea otters, but never saw one attack a sea otter until 1991. Since then, about a dozen such attacks have been reported.
Sea otter populations, meanwhile, have declined by about 25 percent each year during the 1990s throughout large areas of western Alaska. In waters inaccessible to killer whales, however, sea otter numbers have remained stable. Otters maintain the coastal kelp forests by controlling populations of sea urchins and other animals that graze on kelp. The kelp forests in turn provide food and habitat for a broad range of species.
Where sea otter populations have dropped, the kelp forest ecosystem is collapsing, said Estes, who has spent years documenting the central role of otters in the ecology of the kelp forests. Exploding sea urchin populations have decreased kelp densities by a factor of 12 since the sea otters began to disappear, he said.
"A wide array of species will be affected by these changes: coastal fish, filter-feeders like mussels and barnacles, marine birds, and other predators in the system could all be impacted," Estes said.
Estes attributed the change in killer whale feeding behavior to a collapse in populations of the killer whales' usual prey, seals and sea lions. Reduced abundance of certain fish species in the open ocean, possibly due to overfishing, has caused the populations of Steller sea lions and harbor seals in Alaska to decline precipitously since the late 1970s, he said.
"This chain of events has occurred over an immense area and involves a large number of linkages connecting the open ocean and the nearshore environment," Estes said.
"There were some big surprises in terms of the scale and the unlikely nature of this situation, but it makes a lot of sense that the whales have had to look elsewhere for food when seals and sea lions declined," he added.
Estes acknowledged that certain links in this chain of ecological consequences are less clearly documented than others. He said he feels least sure about what happened in the open ocean system to initiate the observed changes. The declines in populations of seals and sea lions have been extensively documented, but it is hard to establish a link between those declines and the region's burgeoning fisheries, he said.
"It is pretty well established that harbor seals and Steller sea lions have declined due to changes in fish stocks, and overfishing probably played a significant role in that, but it's very difficult to chronicle and document in a definitive way," Estes said.
Other factors, such as increased ocean temperatures, may also have contributed to changes in the kinds and amounts of fish available to sustain populations of seals and sea lions.
The role of killer whales in the sea otter decline is supported by several lines of evidence, Estes said. First is the sudden increase in observed attacks. A careful statistical analysis found it extremely unlikely that the cluster of recent observations was due to chance alone.
The researchers also compared sea otter populations at two sites on Adak Island, one an open bay and the other an area inaccessible to killer whales. From 1993 through 1997, sea otter numbers were stable in the protected Clam Lagoon, while in the adjacent Kuluk Bay they declined by 76 percent.
According to calculations based on the team's observations in one section of the Aleutian Islands, killer whales must have killed 6,788 sea otters per year between 1991 and 1997 to account for the observed decline in the population. Taking into account the number of hours of field observations by the research team during this period, the number of those attacks they could expect to have observed is 5.05, which is very close to the actual number of six observed attacks.
The researchers ruled out other possible causes of the sea otter decline, such as disease, toxins, and starvation, Estes said.
"It took about two years for me to become convinced that killer whales were responsible, as we dismissed one possibility after another and gradually gathered direct evidence pointing to killer whales," Estes said.
Terrie Williams, an associate professor of biology at UCSC and coauthor of the paper, analyzed the caloric value of sea otters, which are a less nutritious food source than killer whales' usual prey because they have very little blubber. Williams estimated that a single killer whale would need to eat 1,825 otters per year to meet its energy requirements. These figures suggest that as few as four killer whales feeding exclusively on sea otters could have driven the population decline observed over a large area of the Aleutian archipelago.
Sea otters were nearly extinct by the early 20th century due to overhunting for the fur trade. Protection from hunting allowed a steady recovery, and many areas of their historic range had thriving populations by the early 1970s. Estes and other researchers studying the species' recovery documented its importance in the coastal ecosystem by comparing areas with and without healthy sea otter populations.
One of the important implications of the current study, as far as ecologists are concerned, is the linkage between the coastal ecosystem and events and species seemingly far removed from it. The transient presence of killer whales, driven to prey on sea otters by changes in the open ocean, has affected the entire structure of the coastal ecosystem.
If overfishing is indeed at the root of these changes, the implications for fisheries management are profound, Estes said.
"This case provides a potential example of how far-reaching the effects can be from overexploitation of fisheries, extending into areas where you would never expect to see impacts," he said.
In addition to Estes and Williams, the authors of the Science paper include Tim Tinker, who worked on the project as a biologist with Glenside Ecological Services in British Columbia and is now a graduate student in biology at UCSC, and Daniel Doak, Pepper-Giberson Associate Professor of Environmental Studies at UCSC.
Materials provided by University Of California, Santa Cruz. Note: Content may be edited for style and length.
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