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Fisheries in the Bering Sea J. Overland NOAA, Pacific Marine Environmental Laboratory, Seattle, WA Click to see Biology Summary During 2000 to 2005 the Bering Sea was showing indications that Arctic species that require the presence of sea ice were being replaced by sub-Arctic species that don't require sea ice. This is shown schematically in the Figure F1 as a shift of the biological energy pathway that favors bottom animals (Benthic) to one favoring species that live closer to the ocean surface (Pelagic).
An indicator of this shift is the observed multi-year persistence of change. Summer bottom water temperatures of the southeast Bering Sea were warm from 1996–2005 (except for 1999) (Figure F2), due to sea ice leaving the region earlier in the spring. Pollock expanded their range northward during these years. Since 2005, the Bering Sea has been relatively cold, with more sea ice in the winter. In 2008, the winter sea ice extent in the Bering Sea was at a near record maximum and bottom water temperatures were at a near record minimum. Cold water species such as Arctic cod have returned toward the south. At present Bering Sea climate change and ecosystem response are more characterized by natural variability in terms of large multi-year warm and cold periods than by an emerging global warming trend.
Pollock are currently a major economic resource for the Bering Sea (Figure F3). Warm temperatures and lack of sea ice tend to favor pollock, which has had one of highest biomass for any large marine ecosystem at the expense of bottom-living fish and crab and walrus, which feed on bottom dwelling clams. The biomass for pollock in the Bering Sea and the number of new fish added each year (called recruitment) are shown in Figure F4. These data imply that the status of pollock stocks may be more complicated than a simple temperature relationship. Winters of 2006, 2007 and 2008 were of cold or average temperature, which typically contributes to less favorable conditions. Pollock recruitment, however, began to decrease before the return of these cold temperatures. It is suggested that after almost a decade of warm temperatures, the food supply for pollock shifted to less favorable species and that pollock predators, such as arrowtooth flounder, became well established. Although there was a large year class in 2006, given the present low biomass, the recent shift in water temperatures, low abundance of food, high abundance of predator species, and the unknown timing of future climate events, the future status of pollock is uncertain.
Marine mammals are major consumers of fish and other species in the Bering Sea ecosystem and may have been impacted by climate shifts. The number of new fur seal pups (Figure F5) added to the population on St. Paul Island (in the Pribilof Islands) (Figure F6), shows a decrease after the 1970s and another more recent decrease after 1998. Although it is difficult to assign direct causes to the declines, there was a climate shift in the North Pacific Ocean in 1977, and, since then, a general sustained warming. Bering Sea temperatures respond both to global warming and large natural variability. While the Bering is cold at present we anticipate a swing back to extreme warm temperatures in future years, which will continue the negative impact on Arctic and bottom species, while favoring sub-Arctic species such as salmon.
References Stock Assessment and Fishery Evaluation Report (SAFE Report) from NOAA / AFSC. North Pacific Groundfish Stock Assessment and Fishery Evaluation Reports (from NOAA / AFSC). Printable Handout :: Full Arctic Report Card (PDF) |
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DOC | NOAA | NOAA Arctic Research Program |
