Abundance and Distribution of Arctic cod (Boreogadus saida) and other Pelagic Fishes over the U.S. Continental Shelf of the Northern Bering and Chukchi Seas

Two acoustic-trawl (AT) surveys were conducted in the northern Bering and Chukchi Seas during ice-free periods in 2012 and 2013 with a focus on Arctic cod (Boreogadus saida), a key species in Arctic ecosystems. The mixed species assemblages in the study area required a refinement of standard AT survey methods. Sensitivity analyses indicate that the AT abundance estimates, are relatively robust to the assumptions of the analysis. These surveys indicate that midwater fishes are dominated by age-0 Arctic and saffron cod (Eleginus gracilis), capelin (Mallotus villosus), and Pacific herring (Clupea pallasii). In both years, age-0 Arctic cod were distributed north of 70 °N, age-0 saffron cod were abundant in coastal areas between 66.5-69.5 °N, and Pacific herring were distributed south of 67 °N. These three species exhibited consistent environmental associations: e.g. age-0 Arctic cod were abundant at lower temperatures than saffron cod. In contrast, capelin were distributed throughout the study area and were not associated with water temperatures. There was a latitudinal trend in body length, with smaller Arctic cod, saffron cod and capelin in northern areas, but smaller Pacific herring in the south. Abundances of Arctic cod, saffron cod, herring, and capelin all increased by more than factor of 2 in 2013 compared to 2012. Sizeable populations of age-0 Arctic cod were observed in the northern Chukchi Sea, which suggests that this area is an important nursery ground. However, relatively few larger Arctic cod were observed in this and other surveys of the area, which suggests that either overwinter mortality of age-0 Arctic cod is high, and/or these fish are not retained on the Chukchi shelf.