Current State of the Bering Sea
|Fig 2. (a) Air temperature at St. Paul Island and (b) sea ice concentration in the southern Bering Sea|
The Bering Sea is one of the major Large Marine Ecosystems in the world (fig 1); fisheries from Alaska represent half of the marine harvest in United States waters. The Bering Sea is also home to large bird and marine mammal populations. The climate and ecosystem of the Bering Sea has changed over the previous fifty years. There are two shifts in climate associated with increased warm temperatures and other factors, the first in the late 1970s and again around 2000. There was a major ecosystem reorganization following the late-1970s shift. These changes represent a transition from primarily cold Arctic ecosystems earlier in the 20th century, dominated by sea ice, to sub-Arctic conditions.
The record of annual surface air temperature on St. Paul Island in the southeastern Bering Sea beginning in 1916 shows the warm temperatures after 1976, with a short period of warm temperatures in the late 1930. Figure 2 (a) shows warm temperature anomalies at St. Paul Island for the previous 15 years. Summer temperatures at Nome inferred from vegetation records indicate generally cold temperatures from 1800 to the 1970s. Ocean temperatures for the previous decade (Fig 3) from an oceanographic mooring (M2, Fig.4) show a shift toward warmer temperature of 2 deg C around 2000. Of particular importance is that recent winter temperatures are above the freezing point, indicating no or little sea ice in the southeastern Bering Sea for the previous four years (Fig 2 (b)).
Fig 3. Vertically averaged ocean temperature at the Mooring 2 (M2) location.
From NOAA FOCI program.
|Fig 4. Oceanographic mooring in the Bering Sea at the M2 location during the 1990's.|
|The biomass of pollock, a major commercial fishery, increase by 400 % following the late-1970s climate shift (fig.5) and has generally remained high at 10 million metric tons, an enormous amount compared with many other fisheries. Other species have decreased in the last 25 years; Greenland turbot which favors cold, near bottom waters dropped in biomass by over 80 %. Lack of sea ice should favor biological productivity in the upper ocean compared to near bottom species. The reduction in snow crab in the last 5 years is in part thought to be due to a combination of warmer temperatures and decreased bottom productivity (fig.6). Marine mammals may also be affected by climate. There was a major decrease in fur seal pups born on St. Paul Island after the late 1970s and again in the last four years (fig. 7). There is some indication of increases in fin and humpback whales in the late 1990s compared to earlier decades.|
|Fig 5. Bering Sea Pollock. Diamonds indicate biomass, and vertical bars indicate recruits to the population each year. From the NOAA/NMFS SAFE report.|
|Fig 6. Biomass of eastern. Bering Sea crab. From the NOAA/NMFS SAFE report.||Snowcrab. Photo from Coal Point Seafood, Homer, Alaska.|
|Fig 7. Northern fur seal Callorhinus ursinus pups born on St. Paul. Error bars are approximate 95% confidence intervals. From NOAA NMFS AFSC||Northern fur seal Callorhinus ursinus. From the NOAA Photo Library.|
Changes are also seen in the northern Bering Sea. Native peoples noted a shift away from Arctic conditions after 1976 with more variable timing of seasonal changes and a reduction in the quality of sea ice for hunting activities. Productivity and biomass of bottom species have decreased since 1998 and the dominant species of clam has shifted. Gray whales which were found south of Bering Strait in the 1980s were found north of the Strait in 2002.
Will such changes away from Arctic conditions continue? In the past, the Bering Sea was known for large differences in weather conditions from year-to-year. Change in the last five years is characterized by the persistence in warm ocean temperatures and lack of sea ice in the southern Bering Sea. Without sea ice, the ocean can absorb more solar energy and provides resistance to ice formation in the following winter. There may be additional warm ocean temperatures brought by currents from the Gulf of Alaska. The changes in the Bering Sea are part of a large regional climate change from Siberia eastward to northern Canada. Thus, while it is impossible to predict future climate, the balance of the evidence suggests a continuation of current conditions.
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