Fisheries in the Bering Sea

J. Overland

NOAA, Pacific Marine Environmental Laboratory, Seattle, WA

The Bering Sea is showing indications that Arctic species which require the presence of sea ice are being replaced by sub-Arctic species that don't require sea ice. This is shown schematically in the Figure 1 as a shift of the biological energy pathway that favors bottom animals to one favoring species that live closer to the ocean surface:

Figure 1. Bering Sea Ice shifting from Benthic to Pelagic Pathway

An indicator of this shift is that summer bottom water temperatures of the southeast Bering Sea were warm from 1996-2005 (except for 1999) (Figure 2). Warmer temperatures were due to the sea ice leaving earlier in the spring.

Figure 2. Southeast Bering Sea summer ocean bottom temperatures. From NOAA Alaska Fisheries Science Center.

Pollock are currently a major economic resource for the Bering Sea (Figure 3). 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 millions of tons (diamonds) for Bering Sea, and the number of new fish added each year (called recruitment) are shown in Figure 4. The status of pollock stocks may be more complicated than a simple temperature relationship. Winters of 2006, 2007 and 2008 were cold or average temperature which 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 warn 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 abundance of food and predator species, and the unknown timing of future climate events, the future status of pollock is uncertain.

Figure 3. Walleye Pollock. Photo from the NOAA AFSC		Figure 4. Bering Sea Pollock. Diamonds indicate biomass, and vertical bars indicate recruits to the population each year. From the NOAA/NMFS SAFE report

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 5) added to the population on St. Paul Island (in the Pribilof Islands) (Figure 6), 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 there has been a general sustained warming since then, with a continued negative impact.

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, which will continue the negative impact on Arctic and bottom species, while favoring sub-Arctic species such as salmon.

Figure 5. Northern fur seal Callorhinus ursinus. From the NOAA Photo Library.		Figure 6. Northern fur seal Callorhinus ursinus pups born on St. Paul, 1975-2006. Error bars are approximate 95% confidence intervals. From NOAA NMFS AFSC

References

Stock Assessment and Fishery Evaluation Report (SAFE Report) from NOAA / AFSC

March 4, 2008

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