Change in water circulation and its implication for distribution and abundance of salmons in the western Bering Sea in the early 21 century
Water circulation in the western Bering Sea is considered for summer and fall seasons on the data of oceanographic surveys conducted in 2002-2012 by Pacific Fisheries Research Center (TINRO) under BASIS program. In general, cyclonic circulation was observed over the deepwater basin, without prominent changes form summer to fall. However, the cyclonic gyre was wider until 2006 than later when it was limited by the Commander Basin in both seasons, while the northward flow from the Near Strait became stronger and the westward flow from the Aleutian Basin along Koryak shore and the Bering Slope Current became weaker. The former inflow formed the hydrodynamic front along the border of Russian EEZ that prevented eastward spreading of the cold subsurface water. This change was connected with reconstruction of atmosphere circulation: change of the Aleutian Low orientation in spring. It extended latitudinally from Asia to North America till 2006 that favored to eastern and northeastern winds over the Bering Sea, then moved to southwest-northeast position with shift to south of its western trough, so northern and northeastern winds prevailed. After reconstruction of air and water circulation, feeding migration of immature chum, sockeye and chinook salmons to the western Bering Sea became less intensive and their abundance decreased in 2, 5, and 9 times, respectively, with corresponding changes of their distribution patterns. About 70 % of immature salmons fed in the waters of the Aleutian Basin in 2002-2006 but < 50 % in 2007-2010; their concentration density in this area decreased in 2-3 times though it did not changed significantly in the Kamchatka Basin. After five years of abnormal orientation, the Aleutian Low returned to latitudinal position in 2012, just as the water circulation changed back: the northward flow from the Near Strait weakened, the eastward stream appeared along Aleutian Islands, the front at the border of Russian EEZ relaxed and the cold subsurface water penetrated to the Aleutian Basin again. As the result, the feeding salmon abundance in the EEZ of Russia grew and reached the level of 2002-2006.