Bering Sea Changes Baffle Scientists
BBC News - Sci/Tech
By Alex Kirby
Environment Correspondent
Marine biologists are puzzled by a series of abnormal conditions which have affected the Bering Sea, between Alaska and Siberia, during the last two summers.
The changes observed include extreme die-offs of seabirds, rare algal blooms, abnormally warm water temperatures, and very low numbers of salmon.
Almost half the USA's fish comes from the Bering Sea, and the commercial value of the catch is more than a billion dollars a year.
The US National Oceanic and Atmospheric Administration (NOAA) has published the conclusions of an international workshop on the problems of the Bering Sea.
James Balsiger, head of NOAA's Alaska Fisheries Science Center, said: "People whose livelihood depends on the Bering Sea need to know if these are fleeting anomalies or persistent large-scale changes.
"Scientists want to determine the cause of these unusual conditions and their portent for the future of this national resource."
An important migration route
Most of the fish and shellfish caught in the Bering Sea come from the continental shelf, a broad, shallow area that runs down Alaska's west coast.
The shelf is also used by large numbers of resident and migratory seabirds and marine mammals.
The sea has the largest international aggregation of seabirds in the world, and represents 43% of all breeding seabirds in the USA.
NOAA says that some of the changes observed in the 1997 and 1998 summers - warmer than usual ocean temperatures, and altered currents and atmospheric conditions - might have been caused by El Nino.
But the area has been undergoing change on a much longer timescale, going back several decades.
Over that period one species of sea lion, for example, has declined by between 50% and 80%.
And northern fur seal pups on the Pribilof islands - the major Bering Sea breeding grounds - have declined by half between the 1950s and the 1980s.
In parts of the Gulf of Alaska harbour seal numbers are as much as 90% below what they were in the 1970s.
Growth as well as decline
There have been significant declines in the populations of some seabird species, including common murres, thick-billed murres, and red- and black-legged kittiwakes.
There have also been big variations in the abundance of some fish and shellfish species over the past 30 years. Some have registered large increases.
One of the most striking changes observed involved the appearance in 1997 of large areas of milky, aquamarine water over most of the continental shelf.
This was caused by changes in water temperature and atmospheric pressure, which led to a massive bloom of coccolithophores, a type of non-toxic, microscopic marine plant.
The coccolithophores replaced the normal summer plankton community, and had what NOAA calls "profound, but not well-understood effects on the rest of the food chain".
Blooms of this sort have never been seen in the Bering Sea for extended periods.
Despite different atmospheric conditions in 1998, the bloom returned.
Altered migration patterns
Other changes recorded included unprecedented mortality in one seabird species, the short-tailed shearwater, and unsuccessful reproduction rates for another, the kittiwake.
The number of salmon was far below expected levels, the fish were smaller than average, and their traditional migratory patterns seemed to have been altered.
There was also an unusual sighting of Pacific white-sided dolphins in one area, and large numbers of baleen whales (species without teeth) appeared on the shelf.
NOAA says all the changes observed, "taken together, show how responsive the ecosystem is to climate and suggest that climate change would have a strong effect on the ecosystem".