Mehta Amee V.

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Amee V.

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  • Article
    Baleen whales are not important as prey for killer whales Orcinus orca in high-latitude regions
    (Inter-Research, 2007-10-25) Mehta, Amee V. ; Allen, Judith M. ; Constantine, Rochelle ; Garrigue, Claire ; Jann, Beatrice ; Jenner, Curt ; Marx, Marilyn K. ; Matkin, Craig O. ; Mattila, David K. ; Minton, Gianna ; Mizroch, Sally A. ; Olavarría, Carlos ; Robbins, Jooke ; Russell, Kirsty G. ; Seton, Rosemary E. ; Steiger, Gretchen H. ; Víkingsson, Gísli A. ; Wade, Paul R. ; Witteveen, Briana H. ; Clapham, Phillip J.
    Certain populations of killer whales Orcinus orca feed primarily or exclusively on marine mammals. However, whether or not baleen whales represent an important prey source for killer whales is debatable. A hypothesis by Springer et al. (2003) suggested that overexploitation of large whales by industrial whaling forced killer whales to prey-switch from baleen whales to pinnipeds and sea otters, resulting in population declines for these smaller marine mammals in the North Pacific and southern Bering Sea. This prey-switching hypothesis is in part contingent upon the idea that killer whales commonly attack mysticetes while they are in these high-latitude areas. In this study, we used photographic and sighting data from long-term studies of baleen whales in 24 regions worldwide to determine the proportion of whales that bear scars (rake marks) from killer whale attacks, and to examine the timing of scar acquisition. The results of this study show that there is considerable geographic variation in the proportion of whales with rake marks, ranging from 0% to >40% in different regions. In every region, the great majority of the scars seen were present on the whales’ bodies when the animals were first sighted. Less than 7% (9 of 132) of scarred humpback whales with multi-year sighting histories acquired new scars after the first sighting. This suggests that most killer whale attacks on baleen whales target young animals, probably calves on their first migration from low-latitude breeding and calving areas to high-latitude feeding grounds. Overall, our results imply that adult baleen whales are not an important prey source for killer whales in high latitudes, and therefore that one of the primary assumptions underlying the Springer et al. (2003) prey-switching hypothesis (and its purported link to industrial whaling) is invalid.
  • Article
    Killer whales and marine mammal trends in the North Pacific : a re-examination of evidence for sequential megafauna collapse and the prey-switching hypothesis
    (Blackwell, 2007-10-26) Wade, Paul R. ; Burkanov, Vladimir N. ; Dahlheim, Marilyn E. ; Friday, Nancy A. ; Fritz, Lowell W. ; Loughlin, Thomas R. ; Mizroch, Sally A. ; Muto, Marcia M. ; Rice, Dale W. ; Barrett-Lennard, Lance G. ; Black, Nancy A. ; Burdin, Alexander M. ; Calambokidis, John ; Cerchio, Salvatore ; Ford, John K. B. ; Jacobsen, Jeff K. ; Matkin, Craig O. ; Matkin, Dena R. ; Mehta, Amee V. ; Small, Robert J. ; Straley, Janice M. ; McCluskey, Shannon M. ; VanBlaricom, Glenn R.
    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.