Are killer whales responsible for harbor seal, Steller sea lion, and sea otter declines in the North Pacific Ocean and Bering Sea?

Harbor seal, Steller sea lion, and sea otter populations have sequentially collapsed over large areas of the northern North Pacific Ocean and southern Bering Sea during the last several decades. In a paper published in the Proceedings of the National Academy of Sciences (U.S.A.) on 14 October 2003 (Vol. 100, no. 27, pages 12223-12228), Dr. A. M. Springer and a group of other authors suggested that a increased predation by killer whales drove the sea otter collapse and may have contributed to the pinniped declines.

The publication of this article started a hot debate among marine mammal scientists, who will be meeting at the XV Biennial Conference on the Biology of Marine Mammals, being held 14-19 December at the Sheraton Greensboro at Four Seasons and the Joseph S. Koury Convention Center, NC. Another large, and prominent group of marine mammalogists immediately began to prepare a rebuttal. Dr. Aleta A. Hohn, the Chair of the Scientific Program Committee for the conference, approached both groups and offered them a prime opportunity to present their views in the opening plenary session of the conference on Monday morning, 15 December 2003.
The conference also includes over 200 other spoken presentations, over 500 poster presentations, a student affairs workshop, a video night. On Wednesday evening, December 17, 2003, at 7:00 p.m., there will be a public information session about the problem of noise and marine mammals. This session will examine all sides of the current controversy from a scientific point of view.

   The Society for Marine Mammalogy is an international scientific organization whose members conduct scientific research on marine mammals. The Society's membership currently includes approximately 2,000 marine science professionals and students living and working around the world. Every two years the Society sponsors a conference to bring members together in order to present the latest research results and to share their experiences. Over 1500 professional marine mammalogists are expected to attend the conference in Greensboro, coming from all over the world.
   In the article in PNAS, Springer, from the Institute of Marine Sciences at the University of Alaska, and his coauthors suggested that the decimation of the great whales by post-World War II industrial whaling greatly reduced the sustainable biomass of marine mammals in the North Pacific ecosystem, thus causing the foremost natural predator of marine mammals, transient killer whales, to begin feeding more intensively on the smaller and less sustainable pinnipeds and sea otters. They presented a variety of evidence, including the timing of these events, and information on the abundance, diet, and foraging behavior of both predators and prey, which they claimed were consistent with this hypothesis. Daily food requirements of killer whales, the energy content of the prey, and demographic changes in marine mammal populations were used to assess the potential impact of killer whales on sea otter and Steller sea lion populations in the Aleutian Islands. Their results suggested that fewer than 40 killer whales could have caused the recent Steller sea lion decline in the Aleutian archipelago, whereas a pod of 5 individuals could account for the decline in sea otters and the continued suppression of sea lions.

In rebuttal, Dr. Paul Wade, with NOAA Fisheries in Seattle, WA, and his coauthors re-examined trends in abundance and biomass of potential marine mammal prey of killer whales in 3 regions (Aleutians/Bering Sea, Gulf of Alaska, and S.E. Alaska to California). They suggest that predation by killer whales is an unlikely explanation for the reported declines, for several reasons. Many pinniped and sea otter populations are stable or increasing in areas where extensive whaling occurred and large numbers of transient killer whales exist. In addition, gray whales have increased steadily since the 1940s, populations of humpback and fin whales have increased substantially, and minke whales have likely always been abundant. Thus, Wade et. al. rebut that to suggest that baleen whales have been unavailable as potential prey during much of the period concerned is not correct. The hypothesis also ignores small cetaceans that are known to be prey of killer whales but have remained abundant in much of the eastern North Pacific (notably Dalls porpoise). Finally, they question the assumption that adult baleen whales were ever a significant prey item for killer whales in the high﷓latitude habitats in which the purported declines have occurred. Evidence from field observations, stomach contents, and from scarring on baleen whales strongly suggests that when killer whales in these regions attack whales, they prey primarily upon minkes and on calves of species such as gray and humpback whales. Regardless of whether or not killer whales have caused or contributed to the decline of species such as sea otters, they conclude there is little evidence to suggest this would have occurred due to a lack of available cetacean prey.

Both groups will be provided time to present their evidence at the SMM Biennial Conference in Greensboro, and time to then rebut the other side. Dr. Hohn hopes that the airing of this controversy will provide a provocative and stimulating start to what promises to be an exciting and informative conference.