Annual and Spatial Variation in the Diet of Juvenile Pacific Cod in Mutsu Bay, Japan

To evaluate Mutsu Bay as a nursery habitat for Pacific cod (Gadus macrocephalus Tilesius, 1810), we analyzed settled age-0 juveniles collected with a small bottom otter trawl over 10 years. The three stations with the highest juvenile densities were targeted, and prey-specific feeding intensity (SCIi), its sum (SCI), and the relative condition factor (Kn) were quantified, followed by examining their relationships with juvenile attributes and environmental variables. Diets varied among stations and shifted ontogenetically from small-sized calanoid copepods to larger planktonic and benthic prey. SCI was highest at stations where juveniles consumed medium-sized plankton (0.1–1.0 mg ind⁻¹), including Calanus pacificus, Mesocalanus tenuicornis, Metridia pacifica, Anomura zoeae, and Euphausiacea furciliae, and lower where other prey dominated. High-SCI individuals were rarely observed, likely reflecting enhanced digestion at high temperatures near the upper habitat limit (~12 °C) and consistently low prey density independent of temperature. Kn increased with body size and SCI and tended to be higher in cooler water and closer to the bay mouth, suggesting coupled environmental and physiological constraints. These results suggest that after late May, juveniles may benefit from moving toward the bay mouth, where prey encounter rates are likely higher, including relatively larger prey, which may improve feeding opportunities and condition.