Sequential megafaunal collapse in the North Pacific Ocean: An ongoing legacy of industrial whaling?

doi:10.1073/pnas.1635156100

Sequential megafaunal collapse in the North Pacific Ocean: An ongoing legacy of industrial whaling?

^aInstitute of Marine Science, University of Alaska, Fairbanks, AK 99775; ^cUnited States Geological Survey, Center for Ocean Health, Santa Cruz, CA 95060; ^dP.O. Box 210442, Auke Bay, AK 99821; ^eDepartment of Ecology and Evolutionary Biology, University of California, Santa Cruz, CA 95064; ^fNational Marine Fisheries Service, Santa Cruz, CA 95060; and ^gNational Marine Fisheries Service, Seattle, WA 98115

Communicated by Robert T. Paine, University of Washington, Seattle, WA, August 11, 2003 (received for review June 1, 2003)

Abstract

Populations of seals, sea lions, and sea otters have sequentially collapsed over large areas of the northern North Pacific Ocean and southern Bering Sea during the last several decades. A bottom-up nutritional limitation mechanism induced by physical oceanographic change or competition with fisheries was long thought to be largely responsible for these declines. The current weight of evidence is more consistent with top-down forcing. Increased predation by killer whales probably drove the sea otter collapse and may have been responsible for the earlier pinniped declines as well. We propose that decimation of the great whales by post-World War II industrial whaling caused the great whales' foremost natural predators, killer whales, to begin feeding more intensively on the smaller marine mammals, thus “fishing-down” this element of the marine food web. The timing of these events, information on the abundance, diet, and foraging behavior of both predators and prey, and feasibility analyses based on demographic and energetic modeling are all consistent with this hypothesis.

Footnotes

↵ b To whom correspondence should be addressed. E-mail: ams{at}ims.uaf.edu.
↵ h Steller sea lions range across the North Pacific Ocean from California to Japan. The western stock of this species, which ranges westward from Cape Suckling (longitude 144° W), was listed as Endangered under the U.S. Endangered Species Act in 1997.
↵ i The absence of beach-stranded carcasses is one of the most intriguing and perplexing features of these declines. Sea otter mortality from nutritional limitation, disease, and pollution typically results in large numbers of stranded carcasses. Pinnipeds often sink when killed at sea, although many such individuals float to the surface and wash ashore later. Malnourished or diseased pinnipeds commonly haul out to die. The near absence of stranded carcasses and a lack of reports of distressed animals on beaches or of emaciated animals taken by subsistence hunters thus are most consistent with losses to predators.
↵ j Initial declines of fur seals on the Pribilof Islands (in the Bering Sea) and harbor seals on Tugidak Island (in the Kodiak archipelago, depicted in Fig. 2) prior to the 1970s were caused in substantial part by excessive human killing, of fur seals during an experimental harvest in 1956–1968 and of harbor seals by a commercial harvest in 1964–1972. After cessation of these harvests, numbers of both species continued to decline because of elevated mortality of juveniles and adults. It is particularly noteworthy that these “unexplained” declines began well in advance of the climate regime shift of 1977, which has been blamed for altering many facets of marine ecosystems of the North Pacific (3–5).
↵ k Although these data are from a single location, Tugidak Island, they are representative of the timing and magnitude of harbor seal declines that occurred elsewhere. For instance, the harbor seal population at Otter Island (in the Pribilof Islands), which numbered ≈1,200 when first counted in 1974, declined 40% from 1974 to 1978 and an additional 70% from 1978 to 1995 (L. Jemison, personal communication). Similar harbor seal declines, although not quantified, have occurred throughout the Aleutian archipelago (J.A.E., unpublished observations).
↵ l A corollary is that the increase in abundance of killer whales in the late 1980s on the continental shelf of the eastern Bering Sea, in the region of the Pribilof Islands and in Bristol Bay (40, 41), also resulted from the collapse of pinnipeds in the Aleutian Islands. The increase of killer whales on the shelf was accompanied by the resumption of the overall decline of fur seals at the Pribilofs after a brief interval of stability at St. Paul Island (Fig. 2) and by numerous observations of attacks on a variety of marine mammal species in Bristol Bay.
↵ m Dahlheim, M. E. (1994) Abundance and Distribution of Killer Whales in Alaska (Unpublished Report, National Oceanic and Atmospheric Agency, National Marine Mammal Laboratory, 7600 Sand Point Way N.E., Seattle).
↵ n Maldini, D., Maniscalco, J. & Burdin, A. (2002) Fourth International Orca Symposium and Workshop (Centre National de la Recherche Scientifique, Villiers en Bois, France), pp. 92–94 (www.cebc.cnrs.fr/Fr_collo/ORCA.pdf).
↵ o Survival estimates were calculated from two-sex matrix models by using demographic rates from ref. 52.