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Influences of Oceanographic Features on Spatial and Temporal Distributions of Size Spectrum of Walleye Pollock, Gadus chalcogrammus Inhabiting Middle Eastern Coast of Korea

This study investigated the seasonal and inter-annual changes in vertical distribution and size spectrum of walleye pollock, Gadus chalcogrammus inhabiting middle eastern coast of Korea (hear after pollock). Pollock was distributed between 50 m and 600 m depth range, and body size (total length) ranged from 16.6 cm to 81.5 cm. The trends of population body size were increased in autumn and winter and decreased in spring and summer. Vertical distribution of pollock showned depth-dependent patterns with distributing smaller individuals mainly in the upper layer (shallower depth), while larger fish in deeper habitats. Those patterns in vertical distribution of pollock population is probably due to be the results of energy-saving strategy, metabolic effects, and changes in prey selections according to pollock growth, derived from spatial and temporal changes in oceanic condition in habitat grounds. When water temperature in upper layer were increased and that of below thermocline depth became decreased in 2017, the ratio of smaller ( < 35 cm) and larger ( ≥ 35 cm) individuals was biased toward larger fishes, extending their distribution into shallow depth, and consequently main fishing ground was formed in far from coastal area. In addition, the ratio of smaller individual distributing between 100 ~ 300 m was increased with decreasing temperature gradient between below thermocline and bottom layer. Changes in spatial and temporal distributions of pollock population likely be related with vertical and horizontal changes in oceanic conditions and, consequently food supplies.