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The jacks are a large family of strong-swimming, open-water, carnivorous fishes which are usually silvery in color. Although not reef fishes in the sense of residing on coral reefs, they often prey upon reef fishes or feed on zooplankton near reefs; they also may come to cleaning stations in reefs.

Jacks vary greatly in shape from fusiform, such as the species of Decapterus, to deep-bodied, as seen in the species of Alectis and Caranx. The caudal peduncle is slender and usually reinforced by a series of external overlapping bony plates called scutes; the caudal fin is strongly forked to lunate.

The eye is generally protected by the transparent so-called adipose eyelid (but immovable and not fatty). There are 2 separate dorsal fins, the first of 4 to 8 spines (first dorsal fin embedded in adults of a few species); the anal fin nearly always has 2 initial spines which are detached from the rest of the fin.

The family is well represented in Hawai'i with 24 species. The general Hawaiian name for the species of jacks and trevallies is ulua. The larger jacks and trevallies are highly prized gamefishes.

Island Jack

Silvery blue-green on back, shading below to silvery, usually with several small elliptical yellow spots with dusky centers on side of body. Largest, 27.5 inches (70 cm). Indo-Pacific and tropical eastern Pacific. Feeds on benthic crustaceans, worms, and small fishes, mainly on sand bottoms.

Leatherback

Scomberoides lysan (Forskall, 1775) Lai

Silvery with 2 longitudinal series of small blackish spots on body; outer half of anterior lobe of soft dorsal fin black; body moderately elongate and strongly compressed; scales slender and pointed; posterior dorsal and anal rays joined only basally by membrane.

Reaches 27 inches (68 cm). Indo-Pacific; may occur as solitary individuals or in small groups. Chorinemus sanctipetre Cuvier is a synonym.

Mackerel Scad

Decapterus macarellus (Cuvier, 1833) Opelu

Silvery blue-green on back, silvery on side and ventrally, with a small black spot at edge of operculum at level of eye; caudal fin yellowish; body elongate and only slightly compressed; last dorsal and anal rays as a separate finlet. Reported to 12.5 inches (32 cm).

Circumtropical; a schooling species known from near the surface to over 650 feet (200 m); feeds on zooplankton. Most common of 4 species in Hawai'i.

Black Trevally

Caranx lugubris (Poey, 1860) Ulua la'uli

Dark olive-gray to almost black dorsally, shading to bluish gray ventrally, the scutes black; dorsal profile of snout to above eye straight to slightly concave; dorsal soft rays 20-22. Attains 32 inches (81 cm). Circumtropical; usually seen on outer reef slopes at depths greater than 100 feet (30.5 m).

Bluefin Trevally

Caranx melampygus (Cuvier, 1833) Omilu

Brassy dorsally, shading to silvery with iridescence below, finely blotched with bright blue and dotted with small black spots; fins blue; dorsal soft rays 21-24. Reported to 39 inches (100 cm); world angling record, 96 lbs (43.5 kg). Indo-Pacific and tropical eastern Pacific; the most common of the larger jacks in Hawai'i. Feeds primarily on fishes. The young may be found in small schools, sometimes in estuaries.

Giant Trevally

Caranx ignobilis (Forskall, 1775) Ulua aukea

Silvery gray, shading ventrally to silvery white, with numerous very small black spots; centers of scutes blackish; mature males may be nearly black; dorsal soft rays 18-21. Reaches at least 65 inches (165 cm); world angling record, 145 lbs (66 kg), from Maui. Indo-Pacific; common in the Northwestern Hawaiian Islands. Often occurs in small schools; feeds mainly on fishes.

All information and pictures in this section are from John E. Randall's Shore Fishes of Hawai'i by permission of the author.

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Jacks (Carangidae)

Island Jack

Leatherback

Mackerel Scad

Black Trevally

Bluefin Trevally

Giant Trevally

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