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, 100 (21), 12223-8

Sequential Megafaunal Collapse in the North Pacific Ocean: An Ongoing Legacy of Industrial Whaling?

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Sequential Megafaunal Collapse in the North Pacific Ocean: An Ongoing Legacy of Industrial Whaling?

A M Springer et al. Proc Natl Acad Sci U S A.

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.

Figures

Fig. 1.
Fig. 1.
Geography of reported whale harvests (all species) in the North Pacific Ocean and Bering Sea from 1946 through 1976. Data (latitude/longitude and number of individuals) are from International Whaling Commission records and are binned by sequential 7- to 8-year intervals to show temporal trends.
Fig. 2.
Fig. 2.
The sequential collapse of marine mammals in the North Pacific Ocean and southern Bering Sea, all shown as proportions of annual maxima. Great whales: International Whaling Commission reported landings (in biomass) within 370 km of the Aleutian archipelago and coast of the western Gulf of Alaska. Harbor seals: counts and modeled estimate (1972) of Tugidak Island (36). Fur seals: average pup production on St. Paul and St. George islands, Pribilof Islands (from ref. and A. E. York, personal communication). Steller sea lions: estimated abundance of the Alaska western stock (from ref. 38). Sea otters: counts of Aleutian Islands (from ref. 39). For fur seals and harbor seals, 100% represents population sizes at the time effects of excessive harvesting ended and “unexplained” declines began.
Fig. 3.
Fig. 3.
Biomass estimates for great whales and pinnipeds before and after recent declines in the Aleutian Islands and Bering Sea region. These estimates were derived from current and historical estimates of abundance and per capita biomass, in some cases adjusted for estimates of sex and size composition (from B.P., unpublished work). Historic and current abundance estimates are species-specific. “Historical” is defined as the period before largescale commercial exploitation, ranging from the mid-1800s to the mid-1900s. “Current” is defined as the most recent available estimate of abundance, reflecting population levels during the mid-1990s to early 2000s.

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