Impacts of Extreme Warming in 2016–2019 on the Stock of Pacific Cod Gadus macrocephalus (Gadidae) in the Russian Waters of Northwestern Bering Sea

The heat wave in the Bering Sea in 2016–2019 and its consequences for oceanographic conditions and populations of Pacific cod Gadus macrocephalus are considered. The feeding stock of cod in the northwestern part of the sea (Russian EEZ) fluctuated in 1965–2012 from 25 000 to 654 000 t, but increased to 1 329 700 t in 2017. The features of the size–age structure for the cod that fed in this area in the years of heat wave showed a prevalence of migrants from the southeastern Bering Sea. Such active migrations of cod were due to extreme heating of water and destruction of the cold water pool at the shelf bottom caused by strengthening of warm currents that supported the traditional migration from the southeast to the northwest and opened a new route across the shelf of Alaska to the coast of Chukotka and even to the Bering Strait and the Chukchi Sea. Both routes coincided with the streams of the Bering Slope Current and Alaska Coastal Current, which encircled the cold water pool from two sides and destroyed this water mass. A hypothesis is proposed that the cod migrations are driven by northward transport of the krill, Thysanoessa inermis, by these currents from the area of their reproduction at the continental slope accompanied by feeding migrations of their main grazer, walleye pollock. These study results allow one to optimize the cod fishery in the Bering Sea for comprehensive utilization of this strongly fluctuating stock.