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Environmental Change in the Bering Sea and Gulf of Alaska

The Problem

  • For approximately the past twenty-five years, researchers have documented an array of unusual changes and dramatic events affecting the marine wildlife and ecosystems of the Bering Sea and Gulf of Alaska.
    • Several taxa of piscivorous birds have undergone declines of as much as 50% in Prince William Sound, although populations of benthic-feeding avifauna in the same region have remained stable or increased during that time.
    • During the first six months of 1993, an estimated 120 000 guillemots died in a mass mortality event along the northern coast of the Gulf of Alaska. In the summer of 1997, hundreds of thousands of short-tailed shearwaters died in the south-eastern Bering Sea. In 1998, another mass mortality affected tens of thousands of common murres in the Gulf of Alaska and southern Bering Sea, from Cook Inlet west to the Aleutian Islands.
    • Populations of several taxa of waterfowl, among them Steller’s eider, spectacled eider, and various species of loon, have undergone precipitous declines, sometimes in excess of 50%, on their breeding grounds in the Yukon-Kuskokwim Delta; seabirds such as common murres, thick-billed murres, and red-legged and black-legged kittiwakes, have also declined significantly in the western Gulf of Alaska and Aleutian Islands.
    • There have been several significant changes in absolute and relative biomass of marine fish and invertebrates. From 1977 on, the region experienced a significant increase in populations of groundfish such as pollock and flounder, although another flatfish, turbot, declined. From the mid-1980s, capelin, which had been abundant during the 1970s, virtually disappeared from the western Gulf of Alaska and Bering Sea. Populations of several crab species collapsed at around the same time. Since the early 1990s, there has been a dramatic increase in the biomass of jellyfish in the eastern Bering Sea.
    • Steller sea lion populations in the Bering Sea and western Gulf of Alaska have declined by approximately 80%. In parts of the Gulf of Alaska, harbor seal numbers have dropped by as much as 90%. Sea otter numbers in the Aleutians have fallen by 70% since 1992 and, in some areas of the archipelago, at least 95% since the 1980s.

The Causes

  • Few if any researchers suggest that all these changes and anomalous events are directly attributable to any one cause. However, many scientists believe that much of the answer may lie in large-scale shifts in climatic and oceanic conditions in the Bering Sea and eastern North Pacific Ocean. Such ‘regime shifts’ have been recorded as taking place in 1925, 1947, 1977, 1989, and possibly 1998. According to a 1996 report by the National Research Council (NRC) of the National Academy of Sciences (NAS), the 1977 shift in particular acted in concert with human influences to bring about the profound changes documented in the region.
  • This ‘cascade hypothesis’ postulated that large reductions in whales and some fish as a result of overexploitation increased the amount of food available for other fish and invertebrates, so that by the 1960s and early 1970s, the Bering Sea ecosystem changed from one dominated by capelin to one dominated by pollock. This change was intensified by the 1977 regime shift, which brought warmer water to the region, to the benefit of groundfish (except for cold-water species such as turbot) and the detriment of species such as capelin and sandlance.
  • According to the NRC thesis, this regime shift forced Steller sea lions, which previously fed on capelin and herring, to subsist on the less nutritionally-valuable pollock, initiating that species’ decline. This is known as the ‘junk-food hypothesis.’ Various environmentalists and some researchers thus argue that the sea lions’ decline is exacerbated, or their recovery impeded, by intense fishing for pollock.
  • A 1998 paper in the journal Science considered that the decline of sea lion populations was forcing orcas, which normally fed on the pinnipeds, to switch their attentions to sea otters, and that it was this increased predation that was largely responsible for the sea otters’ decline in the region. The decline of sea otters, irrespective of the cause(s), allowed for the continuing cascade of impacts: sea urchins, on which sea otters prey, increased which then resulted in the deforestation by urchins of the region’s kelp beds.
  • Added to the natural climatic shifts, some researchers note that the region appears to be undergoing a progressive, underlying warming. According to the Alaska Climate Research Center, mean annual temperatures in Alaska increased by 2.57oF between 1971 and 2000.
  • Not all changes can be attributed solely to the cascade hypothesis. Harbor seals, orcas, sea otters and many seabirds in Prince William Sound were affected by the grounding of the Exxon Valdezin 1989, which spilled at least 11 million gallons of oil. Sea ducks in the Yukon-Kuskokwim Delta have displayed high blood lead concentrations, suggesting their decline in that region may be at least partly due to exposure to lead shot. In addition, bald eagles and sea otters in the Aleutians, and orcas in Prince William Sound, have been found with potentially harmful levels of organochlorine chemicals.

The Context

  • Concern has mounted during the last two decades over what appears to be an increase in the number and scale of disturbances affecting marine species and populations. These include: massive declines in abundances of once common species; the apparent emergence of previously unknown or rare diseases across a range of taxa; disease-related mass mortality events involving, notably, dolphins and seals; and the apparent ‘epidemic’ of harmful algal blooms and subsequent wildlife poisoning events.
  • Such wildlife disturbances are now considered by scientists as being multifaceted events and, often, determination of causes and effects has proven to be elusive. There is, however, increasing recognition that human activities are now inextricably bound with global processes, and that the past and present impacts of fisheries, nutrient and chemical pollution, introduced species, habitat destruction and disturbance, and climate-related changes can interact in complex and unexpected ways. This has prompted calls for management initiatives that take a ‘no regrets’ or ‘precautionary’ perspective and the application of regulatory action in advance of scientific proof of suspected human-caused deleterious effects.

Further Reading

Benson, A.J., and Trites, A.W. 2002. Ecological effects of regime shifts in the Bering Sea and eastern North Pacific Ocean. Fish and Fisheries 3(2): 95-113.

Estes, J.A. et al. 1998. Killer whale predation on sea otters linking oceanic and nearshore ecosystems. Science 282(5388): 473-476.

National Research Council. 1996. The Bering Sea Ecosystem. National Academy Press, Washington, D.C. 307pp.

Rosen, D.A.S. and Trites, A.W. 2000. Pollock and the decline of Steller sea lions: testing the junk-food hypothesis. Canadian Journal of Zoology 78(7): 1243-1250.

Trites, A.W. and Donnelly, C.P. 2003. The decline of Steller sea lions Eumetopias jubatus in Alaska: a review of the nutritional stress hypothesis. Mammal Review 33(1): 3-28.