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KILLER WHALES AND MARINE MAMMAL TRENDS IN THE NORTH PACIFIC—A RE-EXAMINATION OF EVIDENCE FOR SEQUENTIAL MEGAFAUNA COLLAPSE AND THE PREY-SWITCHING HYPOTHESIS |
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| Authors: | Paul R Wade Vladimir N Burkanov Marilyn E Dahlheim Nancy A Friday Lowell W Fritz Thomas R Loughlin Sally A Mizroch Marcia M Muto Dale W Rice Lance G Barrett‐Lennard Nancy A Black Alexander M Burdin John Calambokidis Sal Cerchio John K B Ford Jeff K Jacobsen Craig O Matkin Dena R Matkin Amee V Mehta Robert J Small Janice M Straley Shannon M McCluskey Glenn R VanBlaricom Phillip J Clapham |
| Institution: | National Marine Mammal Laboratory, NOAA Fisheries, 7600 Sand Point Way NE, Seattle, Washington 98115, U.S.A. E-mail:; Vancouver Aquarium Marine Science Center, Vancouver, British Columbia V6B 3X8, Canada; Monterey Bay Cetacean Project, P.O. Box 52001, Pacific Grove, California 93950, U.S.A.; Alaska Sealife Center, 301 Railway Avenue, P.O. Box 1329, Seward, Alaska 99664, U.S.A.; Cascadia Research Collective, 218 1/2 W Fourth Avenue, Olympia, Washington 98501, U.S.A.; Molecular Systematics Lab, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024, U.S.A.; Pacific Biological Station, Fisheries and Oceans Canada, 3190 Hammond Bay Road, Nanaimo, British Columbia V9T 6N7, Canada; Department of Biological Sciences, Humboldt State University, Arcata, California 95521, U.S.A.; North Gulf Oceanic Society, 3430 Main Street, Suite B1, Homer, Alaska 99603, U.S.A.; North Gulf Oceanic Society, Gustavus, Alaska 99826, U.S.A.; Boston University Marine Program, Woods Hole, Massachusetts 02543, U.S.A.; Alaska Department of Fish and Game, 1255 West 8th Street, P.O. Box 25526, Juneau, Alaska 99802, U.S.A.; University of Alaska Southeast, Sitka, Alaska 99835, U.S.A.; Washington Cooperative Fish and Wildlife Research Unit, School of Aquatic and Fishery Sciences, University of Washington, Seattle, Washington 98195, U.S.A.; Washington Cooperative Fish and Wildlife Research Unit, School of Aquatic and Fishery Sciences, University of Washington, Seattle, Washington 98195, U.S.A. and Biological Discipline, U.S. Geological Survey, Seattle, Washington 98195, U.S.A.; National Marine Mammal Laboratory NOAA Fisheries, 7600 Sand Point Way NE, Seattle, Washington 98115, U.S.A. |
| Abstract: | Springer et al . (2003) contend that sequential declines occurred in North Pacific populations of harbor and fur seals, Steller sea lions, and sea otters. They hypothesize that these were due to increased predation by killer whales, when industrial whaling's removal of large whales as a supposed primary food source precipitated a prey switch. Using a regional approach, we reexamined whale catch data, killer whale predation observations, and the current biomass and trends of potential prey, and found little support for the prey-switching hypothesis. Large whale biomass in the Bering Sea did not decline as much as suggested by Springer et al ., and much of the reduction occurred 50–100 yr ago, well before the declines of pinnipeds and sea otters began; thus, the need to switch prey starting in the 1970s is doubtful. With the sole exception that the sea otter decline followed the decline of pinnipeds, the reported declines were not in fact sequential. Given this, it is unlikely that a sequential megafaunal collapse from whales to sea otters occurred. The spatial and temporal patterns of pinniped and sea otter population trends are more complex than Springer et al . suggest, and are often inconsistent with their hypothesis. Populations remained stable or increased in many areas, despite extensive historical whaling and high killer whale abundance. Furthermore, observed killer whale predation has largely involved pinnipeds and small cetaceans; there is little evidence that large whales were ever a major prey item in high latitudes. Small cetaceans (ignored by Springer et al .) were likely abundant throughout the period. Overall, we suggest that the Springer et al . hypothesis represents a misleading and simplistic view of events and trophic relationships within this complex marine ecosystem. |
| Keywords: | North Pacific killer whale Steller sea lion sea otter harbor seal fur seal ecosystem predation whaling population dynamics |