Bathymetric distribution of Greenland halibut Reinhardtius hippoglossoides in conditions of deoxygenation in the intermediate layer of the Okhotsk Sea

Bathymetric distribution of greenland halibut Reinhardtius hippoglossoides at the continental slope of the Okhotsk Sea is considered on the data of trawl surveys collected in 1980–2010s. This period is distinguished by prominent tendency to winter warming, with the ice cover decreasing and weakening of slope convection that ventilates the intermediate layer, accompanied by depletion of dissolved oxygen at the depths of halibut habitat below of 300–400 m, on average from 3,24 mL/L in 1970s to 2,68 mL/L in 2010s at the 500 m depth (to 2,25 mL/L in 2020). The deoxygenation is potentially unfavourable for deep-water species, which could be forced to leave the layers with oxygen content below the values dangerous for them. Within annual cycle of bathymetric migration, the deepest aggregations of greenland halibut are formed by large-sized adult fish at the depth up to 1000 m at Kamchatka in their pre-spawning and spawning periods (fall–winter), whereas a larger part of the population concentrates in feeding aggregations at shallower depths. Redistribution of the spawning aggregations from the depth of 900–1000 m to upper isobaths is observed in the last decades that could be caused by depletion of oxygen in this layer to the values <1 mL/L. However, the densest feeding aggregations at the depth of 400–700 m did not shift up, in spite of lower oxygen, but even were found deeper in the 2000–2010s (below 600 m) than in the 1970–1990s (500–600 m). So deoxygenation in the core of the intermediate layer was not critical for greenland halibut, though the same process in the deepest portion of this layer caused its partial bathymetric redistribution. The process of deoxygenation coincided with period of weak reproduction of greenland halibut: any strong year-class did not appear in three last decades, and its population came into stable depression. However, this depression is unlikely caused by lack of oxygen, so far as redistribution of the spawners to the upper isobaths compensates completely the oxygen depletion