Combined stomach content and stable isotope analyses revealed variations in trophic ecology of pacific cod in the east sea

This study investigated changes in the dietary composition and trophic levels of Pacific cod (Gadus macrocephalus) using stomach content and stable isotope analyses. Pacific cod samples were collected seasonally from May 2018 to February 2022 using bottom gill nets at the northern (Ayajin) and southern (Hupo) sites of the East Sea along the Korean coast. The primary dietary components included teleosts, amphipods, carid shrimps, and cephalopods. Gravimetric dietary analysis revealed distinct latitudinal, seasonal, and size-related variations, with teleosts, cephalopods, carid shrimps, and euphausiids serving as the key prey items. During the cold season, teleosts constituted the major dietary components of the large-sized Pacific cod, whereas smaller individuals consumed more carid shrimps, amphipods, and euphausiids. In the warm season, large-sized individuals predominantly consumed cephalopods irrespective of location, whereas smaller individuals favored amphipods and euphausiids at Ayajin and carid shrimps at Hupo. Differences in stable isotope values (δ¹³C and δ¹⁵N) between Ayajin and Hupo, between size classes, indicated higher values in the Hupo and in large-sized individuals. Increased consumption of teleosts and cephalopods by larger Pacific cod led to enriched isotope values, whereas the contribution of carid shrimps elevated the trophic level at Hupo. This study provides comprehensive insights into the dietary variations and trophic levels of Pacific cod in response to changes in prey availability.