They follow the tides into shallow areas where they forage on scuds, sand fleas, shrimp, crab megalops and the occasional stickleback, sculpin, sand lance or small baitfish. They stay close inshore and avoid crossing bodies of deep, open water. While in salt water, juvenile cutthroat trout grow about 1 inch per month. Predation on juvenile sea-run cutthroat trout is believed to be their major cause of marine mortality. Pacific hakes, spiny dogfish, harbor seals, birds and adult salmon are the most likely marine predators of cutthroat trout.
Mortality is greatest for young cutthroat trout entering salt water for the first time.
Sixty to eighty percent of the smolts succumb to predators. Unlike salmon and steelhead which spend two or more winters in salt water, sea-run cutthroat trout return to fresh water only four to six months after they have migrated to the ocean. Adult sea-run cutthroat trout returning to fresh water from the ocean for the first time measure about 12 to 14 inches in length. Migration of adult cutthroat trout upstream into fresh water usually occurs in two or three groups. Those that have spawned before return to the streams first, usually in late July or early August.