The diet of an arctic cod varies greatly as it grows, and at different times includes different components of plankton (mainly small crustaceans such as copepods, amphipods, euphausiids and arrow worms), shrimp and other arctic cod. Arctic cod are unlike other cod because they feed not at the bottom of the ocean but instead in the upper layers of the ocean.
There are several different stages in the diet of an arctic cod. When arctic cod are between 4 and 6 centimeters long they feed mostly on the eggs and larvae of copepods and adult amphipods. Amphipods are tiny shrimp-like organisms that grow from 5 mm to 20 mm long. Copepods are tiny crustaceans that are usually 1 to 2 millimeters long, although some polar copepods may reach a centimeter in length.
Later, when the cod grow to be 8 to 12 centimeters long they feed on adult copepods and amphipods. In addition they seek out euphausiids, small shrimp-like creatures, as part of their diet.
When they are fully grown arctic cod will still feed on copepods and amphipods, but they will also eat arrow worms, which are small predatory worms that also form part of plankton. In addition, they sometimes feed on smaller arctic cod.