Who researched:
J. A. Estes, M. T. Tinker, T. M. Williams, D. F. Doak
Where researched:
Western Alaska
When researched:
Received for publication May 27, 1998
Accepted for publication July 20, 1998
Research methods used:
-population surveys at Adak Island in the central Aleutian archipelago
-population surveys at Little Kiska
-population surveys at Amchitka
-population surveys at the Kagalaska Islands
-Aerial surveys of the Aleutian archipelago conducted by the U.S. Fish and Wildlife Service in 1965 and 1992
-Studies of radio-tagged sea otters at Amchitka Island from 1992 to 1994
-Studies of radio-tagged sea otters at Adak Island from 1995 to 1996
-Contrasting otter population and trends in Adak Island with population and trends in Clam Lagoon
-Compared actual number of observed orca attacks on sea otters with expected number of witnessed attacks (based on estimations of how many orca attacks must drive the sea otter population decline rates)
-Kelp forests were surveyed at 28 randomly selected sites at Adak Island in 1987 and compared to surveys done in 1997
What they found:
Due to the maritime fur trade of the late 1800s to early 1900s, the sea otter (Enhydra Lutris) population was in great decline. Almost extinct, the North Pacific felt the need to act and created the International Fur Seal Treaty in 1911 in hopes of re-growing the sea otter population. By the 1970s, the sea otter population had somewhat bounced back, particularly in the North Pacific. However, in the late 1980s marine biologists once again noticed a decline in the population in Western Alaska. Through eyewitness reports, studies of radio-tagged sea otters, sea otter population surveys and random kelp forest surveys, scientists noticed a decline of about 25% of the sea otter population in Western Alaska from 1987 to 1997. This decline in population was evidenced by the decline of kelp forests and a sharp increase in sea urchin biomass in this region. This drastic decline in a sea otter population can be explained by three phenomena: redistribution, increased mortality or reduced fertility. Studies of radio-tagged sea otters showed that fertility rates were similar to those of stable populations and the sea otters of Western Alaska had not redistributed. Therefore, the sea otters must have been declining due to an increased mortality rate.
This increased mortality rate is most likely due to killer whale (Orcinus Orca) predation. This is evidenced by 10 eyewitness reports of killer whales attacking sea otters, which had never been observed before 1991. Weighing in the number of eyewitness reports, the chances of witnessing this phenomenon and the number of days this scientific team spent observing sea otters; J. A. Estes, M. T. Tinker, T. M. Williams, and D. F. Doak concluded that the sharp decline of the sea otter population in Western Alaska was most likely due to killer whale predation.
What this means/Why it’s important:
Killer Whales had never been known to hunt sea otters before the 1990s. However, outsourcing to a new prey is not an unexpected act. Due to drastic overfishing in the North Pacific, pinniped populations have declined forcing killer whales to find new food sources. Knowing the intelligence levels of these marine mammals, it is not surprising that they would know to turn to a new, abundant food source readily available to them. However, what is surprising, and important, about this research is how a species from one ecosystem can have such a large effect on a completely separate ecosystem. Killer whales (from the oceanic ecosystem) are completely changing the coastal ecosystem of Western Alaska by killing off the keystone species (sea otters). This decline in sea otters is causing kelp forests to disappear and sea urchins to become the most abundant species. This shift from a three-trophic-level system to a four-trophic-level system has completely shifted the dynamic of the coastal ecosystem and shows just how big an influence a top predator can have on an entire food chain.
In addition to the findings of the research, we can learn from the research itself. These findings could not have been uncovered if it weren’t for the steady, slow process of a 10 year study, showing the need for large-scale approaches in some research (particularly ecological research).