greenling, common name for any of several species of the genus Hexagrammos, carnivorous, spiny-finned fishes of the family Hexagrammidae, common in the Pacific Ocean, especially in the waters N of Monterey, Calif. Greenlings have fleshy flaps on the top of the head. They are found in kelp beds and among rocks and are also called rock trout. Several species grow to a length of 20 in. (50 cm) and are valued as game fishes. Most important of the greenlings is the Atka mackerel, found near the Aleutian Islands, a handsome food fish with striking vertical stripes. The kelp greenling is unusual in that the male and female exhibit different coloration and markings, a rare phenomenon among fish. Greenlings are classified in the phylum Chordata, subphylum Vertebrata, class Osteichthyes, order Perciformes, family Hexagrammidae.