Transient Killer Whales – Culprits in the Decline of Sea Otters in Western Alaska?
By Nadine Pinnell, Cara Lachmuth, and Doug Sandilands
Historically, killer whales have been looked upon with fear, known as ‘the devils of the sea’, and in fact their Latin name, Orcinus orca, means ‘Demon from the Underworld’. As people have become more familiar with the social structure and life histories of three different assemblages that inhabit the British Columbia coast (resident, transient and offshore killer whales), attitudes have changed. Educational research facilities such as the Vancouver Aquarium have also exposed the public to these intelligent mammals. However, recent findings may remind us that killer whales are efficient hunters, as their name suggests. In western Alaska, populations of pinnipeds (seals and sea lions) have been decreasing since the 1960s, and sea otters in the Aleutian archipelago have diminished dramatically over the last ten years, leading some scientists to suggest that killer whales are to blame.
In 1998, Dr. Jim Estes and several colleagues published an article entitled “Killer Whale Predation on Sea Otters Linking Oceanic and Nearshore Ecosystems” in the journal Science. Their study examined causes for the recent decline of sea otters in the North Pacific. By the early 20th century, sea otters were nearly extinct due to the rampant fur trade. This resulted in the decline of local kelp forests, as herbivorous sea urchins that feed on kelp were no longer kept in check by the otters. In 1911, sea otters were granted protection under the International Fur Seal Treaty, and their population began to recover, as did the kelp forests. However, sea otter populations in western Alaska have declined again in the last ten years, causing scientists to search for possible explanations.
Estes and his colleagues determined that neither malnutrition, toxins, nor disease could adequately account for the plummeting sea otter population. Therefore, predation was examined as a possible cause. Their calculations suggest that only four transient marine mammal eating killer whales would have needed to switch to an exclusive sea otter diet to account for the decline. In support of this theory, the first well-documented attack of a sea otter by a transient killer whale was observed in 1991, with 9 other attacks observed since then. Estes’ suggestion led to other scientists theorizing that perhaps killer whale predation could explain the population crashes seen in other small marine mammals.
In 2003, Dr. Alan Springer and co-authors published “Sequential megafaunal collapse in the North Pacific Ocean: An ongoing legacy of industrial whaling?” in the Proceedings of the National Academy of Sciences. Prior to the late 1970′s, nearly every species of great whale in the North Pacific Ocean was hunted to the brink of extinction. As these whale species were a traditional food source for transient killer whales, they would have been forced to find other sources of nourishment, such as smaller marine mammals, like harbour seals. Springer and his colleagues hypothesized that this lead to killer whales over-fishing small marine mammals, causing a chain reaction in the food web, which eventually affected near-shore kelp forests.
Springer and colleagues suggested that this new killer whale diet had dramatic consequences for harbour seals in Western Alaska, causing their population crash in the early 1970′s. As harbour seals became scarce, killer whales could have switched to fur seals as their meal of choice. This predation impact could have resulted in the fur seal population decline observed in the mid 1970′s. As numbers of both types of seals diminished, killer whales may have then turned to Steller sea lions for food, as the sea lion population also began to decline in the late 1970′s. According to Springer and colleagues, sea otters were the last small marine mammal killer whales turned to as prey. Sea otters use fur rather than large amounts of blubber to keep warm; hence they would be lower quality prey than other marine mammals.
Beginning in the early 1990′s, sea otter populations in the North Pacific began to plummet, and they continue to do so today. Springer’s theory is based on the idea that populations of smaller marine mammals could not withstand the impact of increased killer whale predation. As their biomass is much smaller than that of a large whale, killer whales must eat many more individuals to satisfy their energetic requirements. Springer estimated that there are 3,900 killer whales (resident, transient and offshore ecotypes) in the North Pacific; according to his model, a 1% shift in their diet could result in the population crashes of small marine mammals that have occurred.
Both Estes’ and Springer’s studies have proven quite controversial, as many scientists feel that predation alone cannot explain the extraordinary population collapses of pinnipeds and sea otters. Many feel that over-fishing, global warming, climate change, reduced fertility due to smaller population numbers, and the effects of pollution have played a role in these declines as well. There is also little direct evidence of increased predation on small marine mammals by killer whales, or of transient killer whales preferentially eating large whales in the past. Other dissenters of the theory point to the fact that minke whales were never severely depleted by the whaling industry in the North, so transient killer whales could have continued to feed on them. Grey whale populations have also made a tremendous comeback from the brink of extinction since being protected from whaling in 1947 by the International Whaling Commission. Since this population growth occurred over the past 50 years, while populations of Steller sea lions and sea otters were declining, why would killer whales continue to selectively hunt these small marine mammals when there were large whales available?
With so many questions left unanswered, more research is necessary to clarify the causes of the various declines of pinniped and sea otter populations in the North Pacific. Dr. Lance Barrett-Lennard of the Vancouver Aquarium has been conducting research on transient killer whales in the Aleutian Islands with the North Gulf Oceanic Society, and perhaps his findings will shed more light on the situation. Most likely, a myriad of different factors play a role in each decline and the cause cannot be solely attributed to killer whales. However, these studies do highlight the tremendous impact human offshore activities, such as whaling, can have on coastal ecosystems, as well as increasing our awareness of the interconnectedness of marine populations. Commercial whaling may have begun a catastrophic chain of crashing populations by removing a major component of the North Pacific marine ecosystem, the great whales. As we are seeing in so many marine ecosystems, over-fishing can have unforeseen long-range consequences.
Estes, J.A., M.T. Tinker, T.M. Williams and D.F. Doak. 1998. Killer whale predation on sea otters linking oceanic and nearshore ecosystems. Science 282: 473-476.
Springer, A.M., J.A. Estes, G.B. van Vliet, T.M. Williams, D.F. Doak, E.M. Danner, K.A. Forney and B. Pfister. 2003. Sequential megafaunal collapse in the North Pacific Ocean: An ongoing legacy of industrial whaling? Proceedings of the National Academy of Sciences 100(21): 12223-12228.
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Springer, A.M., J.A. Estes, G.B. vanVliet, T.M. Williams, D.F. Doak, E.M. Danner and B. Pfister. 2008. Mammal-eating killer whales, industrial whaling, and the sequential megafaunal collapse in the North Pacific Ocean: A reply to critics of Springer et al. 2003. Marine Mammal Science 24(2): 414-442.