Feeding of walleye pollock and its feeding base in the Chukchi Sea in summer-autumn period

Abstract

Data on feeding of walleye pollock <i>Gadus chalcogrammus</i> (3–70 cm long) in the Chukchi Sea in summer-autumn of 2017–2020 are presented. The food base includes zooplankton, benthic invertebrates and fish. The juveniles in the northwestern part of the sea consumed 0.033 t/km2 of prey per day, mainly fish (30.9 %), mysids (20.1 %) and amphipods (13.9 %), whereas the portion of copepods and euphausiids was 5.0–7.6 % of the diet. In the southwestern part of the sea, the juveniles consumption was much higher, as 0.200 t/km2 of prey per day, and included mainly euphausiids and copepods (24.9–51.1%). The adults with length 40–70 cm long dominated in the southwestern Chukchi Sea and consumed on average 13.2 t/km2 of prey per day, shared between fish (42 %), euphausiids (20 %), copepods (6 %), and decapods (11 %). Composition and structure of plankton was different between the main water masses on the shelf occupied the northern and the southern parts of the sea. In the northwestern Chukchi Sea, zooplankton distributed with the mean spatial density of 56.3 t/km2, formed mostly by chaetognaths (35.3 t/km2) and copepods (12.6 t/km2). In the southwestern Chukchi Sea, the mean spatial density of zooplankton was 49.2 t/km2, mostly euphausiids (17.3 t/km2), chaetognaths (15.8 t/km2) and copepods (9.2 t/km2). The total density was close to the level observed in the 2000s, but slight decreasing in biomass of chaetognaths and copepods was observed, mainly due to the recent lack of cold-water species <i>Calanus glacialis</i>, whereas the biomass of euphausiids, in particularly <i>Thysanoessa inermis</i>, had almost doubled. Biomass of the main groups of zooplankton prevailing in the diet in the Chukchi Sea in summer-autumn, as copepods and euphausiids, exceeded daily consumption of these groups by walleye pollock in 6–11 times that may indicate a sufficient food supply for this species. Besides, walleye pollock dwelled at the sea bottom and fed on shrimps, gammarids, bottom polychaetes, echiurids, mollusks, and also bottom fish. Total fullness of the pollock stomachs was in the range of 218–228 ‱ that also suggested successful feeding.