Although Gosline and Brock ("Handbook of Hawaiian Fishes") identify only two species of 'opelu common to Hawaii waters, there are actually four species of 'opelu that are common to Hawaii.
'Opelu (mackerel scad) is a group of slender, torpedo-shaped fish belonging to the jack or carangid family of fish. It is bluish on the upper third of the body and silver white below; however, some individuals are greenish yellow above and whitish below. There is a dark spot on the gill cover.
All four species are in the genus Decapturus. They are the inshore 'opelu, 'opelu mama, red-tail 'opelu, and pelagic 'opelu.
There are four species of ‘opelu found in Hawaii waters.
The most common and best-known 'opelu is the D. Pinnulatus (inshore 'opelu). The 'opelu sold at fish markets is this species. The other species are rarely caught and sold.
The 'opelu is a mid-water shoaling fish and often occurs in dense schools numbering thousands of individuals. The usual habitat of this fish is shallow coastal waters. Juveniles shoal separately from adults and have a tendency to inhabit depths of 10-40 fathoms, whereas mature adults appear to prefer deeper water of 50-100 fathoms. However, adults may be found in the pelagic sea as well.
In Hawaiian waters, spawning appears to occur from about March to the middle of August, with most of the spawning occurring during May through July. A mature 'opelu has about 80,000 eggs in its ovary. The planktonic eggs are spawned in the coastal inshore waters in depths ranging from 10-50 fathoms.
'Opelu are fast growers, and recruits of less than three inches move far out to sea in small schools numbering a few hundred fish in a couple of months. 'Opelu recruits do not seek shallow water for shelter when they change from larvae to the juvenile stage, unlike akule. Instead, the recruits remain in the pelagic sea until they are 6-7 inches in length. Around summer, these recruits, commonly called "cigar 'opelu," will enter the inshore shelf habitat.
The young 'opelu feeds night and day on plankton, with crustacean elements (such as sand fleas, larval crabs, shrimps), squid and fish larvae comprising the major portion of its food. They are frequently preyed upon at the surface by schools of aku and flocks of birds, which the tuna fishermen use to locate good fishing grounds.
'Opelu reach maturity in a year when it is about ten inches in length or about a third-of-a-pound in size. They reach a terminal growth size of about 12 inches, weighing two pounds, in two years and can live up to about three years, reaching a maximum size of 20 inches.
Known predators of adult 'opelu are tuna, ono, mahi mahi, rainbow runner and marlin. When used as bait, they are second to none to catch these fish.
'Opelu is an excellent food fish, both raw and fried. When fried, the meat is firm, moist and very tasty. It can be baked in shoyu sauce mixed with ginger. It is super when dried and makes great beer pupu; when served with hot rice, it is beyond compare.