This study assessed variations in demersal fish assemblages with respect to the study site and water depth. Seasonal samplings from May 2018 to March 2019 were conducted along the northern (Ayajin, Goseong) and southern (Hupo, Uljin) sites of the East Sea off the Korean coast, using commercial gill nets. Samples were collected at depths of ~50, ~80, ~150 m across the study sites, with concurrent monitoring of water column structures. A total of 73 species and 6250 specimens were collected. Distinctive fish species compositions were observed according to the study site and depth. Although Glyptocephalus stelleri was the most abundant fish species in both Ayajin and Hupo, Gadus macrocephalus, Icelus cataphractus, and Alcichthys elongatus were most predominant in Ayajin, whereas Cleisthenes pinetorum, Hippoglossoides dubius, and Gymnocanthus herzensteini were more prevalent in Hupo. In terms of depth layer, in Ayajin, G. stelleri dominated in both intermediate and deeper layers, with Hemilepidotus gilberti, A. elongatus, Enophrys diceraus common in shallower depths. Conversely, in Hupo, G. stelleri, C. pinetorum, and A. nadeshnyi dominated across all depth layers, whereas Dasycottus setiger and G. herzensteini dominated in deeper and shallower depths, respectively. Significant influences of the study site and water depth on fish assemblage structures were observed due to variations in water temperature at the seasonal thermocline boundary.

The influences of oceanographic changes on diet composition and trophic level for pollock (Gadus chalcogrammus) inhabiting the East Sea off the Korean coast were examined based on stomach content and stable isotope analyses during 2016 and 2017. The diets of pollock consisted mainly of benthic crustaceans (particularly carid shrimps and euphausiids) and cephalopods, with a predominance of teleosts in the diets of larger individuals in deeper habitats. In 2016, amphipods, carid shrimps and cephalopods featured strongly in pollock diets, and the contribution of amphipods decreased in the diets of larger individuals and deeper depths. In 2017, euphausiids dominated at shallower depths, whereas the contributions of carid shrimps and teleosts increased in deeper habitats. Body-size-related differences in carbon stable isotope (δ13C) values were present in both 2016 and 2017, but size-related differences in nitrogen stable isotope (δ15N) values were only observed in 2017. The increased contribution of euphausiids during 2017 resulted in a distinct decrease in the trophic level of pollock compared to co-occurring higher trophic level predators, which can be linked to changes in habitat water temperature. Combined stomach contents and isotopic analyses provide a more comprehensive understanding of how fish diets and trophic levels fluctuate with changes in the type and abundance of prey resources in response to environmental changes.