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Physical forcing of ecosystem dynamics on the Bering Sea Shelf

P. J. Stabeno,1 G. L. Hunt, Jr.,2 J. M. Napp,3 and J. D. Schumacher 4

1NOAA, Pacific Marine Environmental Laboratory, Seattle, Washington, 98115
2University of California
3Alaska Fisheries Science Center
4Two Crow Environmental Consultants, Silver City, NM, 88061

Chapter 30 in The Sea, Vol. 14, A.R. Robinson and K. Brink (eds.) ISBN 0-674-01527-4.
Copyright ©2005 by the President and Fellows of Harvard College. Further electronic distribution is not allowed.

6. Processes of the Western Shelf of the Bering Sea

Recent changes have been observed in the ecosystem of the western Bering Sea (Radchenko et al., 2001). Although the Bering Sea responds to climate shifts, a significant portion of the variability occurs on year-to-year time scales, which can make the identification of regime shifts difficult. This year-to-year variability is clearly evident in recent years. For instance, 1999 was an unusually cold year, characterized by negative anomalies in atmosphere and sea surface temperatures, and extensive and prolonged sea ice cover. In contrast, positive sea surface temperature anomalies were predominant throughout the northwestern shelf of the Bering Sea in 1997 (spring and summer) and 1998 (summer). In response to the physical conditions in 1999, some biological processes were less active and/or delayed. During 1999, the return of Pacific salmon to streams in northeastern Kamchatka was approximately two weeks later than typical.

Over the continental shelf of the western Bering Sea, the importance of transport to distribution of gonatid squid (Berryteuthis magister) has recently been investigated (Arkhipkin et al., 1998). This squid is one of the most abundant in the North Pacific Ocean/Bering Sea and is exploited commercially by both Russian and Japanese bottom trawl fisheries. Distinct patterns of size composition, age structure and growth occur over the western shelf, and given the life history of this squid, time-dependent transport of planktonic stages provides the most likely explanation (Arkhipkin et al., 1998). While much of the counter-clockwise current over the Bering Sea basin is related to inflow of the Alaskan Stream through passes in the Aleutian Island Chain (Overland et al., 1994; Stabeno et al., 1999; Stabeno et al., 2004), seasonal changes in wind stress can alter transport in the gyre by ~50% (Bond et al., 1994). As a result of this strong seasonal signal, regional shelf edge current patterns in the western Bering Sea apparently have two modes: a strong along slope mode and an eddy mode that exists as the current relaxes (Verkhunov and Tkachenko, 1992). As noted in Section 3.4, this is also the case for the eastern BSC. Associated with these modes are two primary transport routes for the juvenile squid: winter and spring hatched juveniles are first transported into the eastern region of the shelf and then westward along the shelf; spring/summer hatched juveniles are transported directly across the basin into the western region of the shelf (Verkhunov and Tkachenko, 1992).

In an analysis of long-term fluctuations in several species of pelagic and benthic fishes of the western Bering Sea, Naumenko (1996) notes the correspondence between changes in fish biomass and conditions in the ecosystem. Using catch data, he discerned that the fish community could be divided into four periods: 1958–1964, when herring dominated, 1965–1974, a transitional period when no single species dominated and stocks were highly variable, 1975–1987, when pollock dominated in both biomass and abundance, and 1987–1993 (last data set presented), when pollock declined and groundfish populations increased. While some of the fluctuations could be accounted for by fishing mortality, it appeared that environmental conditions have played an important role in the fluctuations. The first connection Naumenko (1996) made was between bottom water temperatures in fall and surface temperatures in spring and summer, with the four fish community periods. The first period was moderately warm, the second abnormally cold, the third abnormally warm. The fourth period appeared to be one of transition. He notes that changes in the biomass of zooplankton closely correlated with the temperature changes, with zooplankton biomass being greatest during the warm period and lowest in the cold period. Note the correspondence of these observations to the OCH (discussed in section 7): cold regime implies low zooplankton biomass and warm regime high biomass. Changes in recruitment of pollock in the western Bering Sea have also been attributed to changes in the climate/oceanic regime (Balykin, 1996), although the processes that might link the climate/oceanic system to fish survival were not discussed.

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