Current state of stocks for demersal fish on the continental shelf in the exclusive economic zone of Russia in the northwestern Bering Sea

Abstract

Stocks of demersal fishes on the depths of 20-200 m in the northwestern Bering Sea are considered, excluding walleye pollock Theragra chalcogramma , on the data of 10 bottom trawl surveys conducted in the summer-fall of 1985-2015. The mean total biomass of demersal fish in two parts of this area (Western Bering Sea zone and Chukchi zone) in this period was 747.78 . 10 3 and 119.83 . 10 3 t, with almost equal density distribution: 5757 and 5709 kg/km 2 , respectively. The biomass had some redistributions in recent times: the assessments were 832.97 . 10 3 and 69.88 . 10 3 t in 2012, and 946.31 . 10 3 and 141.40 . 10 3 t in 2015, respectively. Pacific cod Gadus macrocephalus is usually the dominant species in the Western Bering Sea zone (up to 57 % of the total biomass in 2015) followed by subdominant species as alaska plaice Pleuronectes quadrituberculatus , flathead sole Hippoglossoides spp., alaska skate Bathyraja parmifera , and great sculpin Myoxocephalus polyacanthocephalus with portions of each species between 5-13 %. Generally, the ichthyocenosis of both zones was monodominant; the polydominant structure was observed in the Western Bering Sea zone only in 2001 and 2005, in conditions of abnormally low cod abundance. The total stock of demersal fish is determined mostly by state of the cod stock. It was very high in the middle 1980s, then had a significant decline in the late 1990s - early 2000s, but rises again since the middle 2000s. A considerable part of the mass demersal species migrate in summer from the Western Bering Sea to the Chukchi zone for feeding that is the background for significant fluctuations of their biomass assessments, which depend on the environments variability. The densest aggregations of the mass demersal fish form mainly in the area between Cape Olyutorsky and Cape Navarin and in the Anadyr Bay in the waters of the Navarin Current.