Their first ocean year is spent feeding on sand lance, herring, insects, copepods, amphipods, crab larvae and euphausids. In the second year at sea, their growth rate increases due to heavy feeding on squid, herring, sand lance and large zooplankton, taking their weight from an average 1.3 kg in March to 5.4 kg in the fall and winter when they return to their home streams to spawn. As adults, coho have silvery sides and a metallic blue back with irregular black spots. Spawning males in fresh water may exhibit bright red on their sides, bright green on their back and head, with darker colouration on their belly. They also develop a marked hooked jaw with sharp teeth.
Females change colour and develop hooked snouts, but the alteration is less spectacular.