Trevally is a species of Yellowtail Jack fish that lives in the Atlantic, Pacific, and Mediterranean. It is popular in Australia and New Zealand, and popular for use in sushi.
When young, the fish has yellow stripe running the length of its body which fades as it matures. A Trevally fish can grow up to 30 inches (80 cm) long, and up to 10 pounds (4.5 kg) in weight. Common market sizes, however, are 14 to 23 inches (35 - 60 cm) long and 14 oz to 5 1/2 pounds (0.4 - 2.5 kg) in weight. For sushi use, they are best caught at a size of 1 1/2 to 2 feet (45 to 60 cm) long.
The flesh is mild-tasting, with a thin layer of red flesh under the skin. The flesh is slightly oily, but leaner than the flesh of Japanese Yellowtail.
'Shima aji' means literally 'striped mackerel', though it's not really a mackerel. The 'aji' word for Horse Mackerel in Japanese gets applied to a lot of fish. Sometimes this fish is just called a "Yellowtail Jack".