tow 81; 78 dph; 28 mm; C. ferdau

Jacks, pompanos, and trevallies (family Carangidae) are typically strong swimming, carnivorous, silvery, pelagic fishes with a deeply forked tail. The family comprises about 32 genera and 140 species, many of which are valuable food and game fish.

Mackerel Scad – Decapterus macarellus  (February of 2012)

The Decapterus, commonly known as mackerel scads, is a genus of the Carangidae made up of 12 recognized species. Mackerel scads are schooling, coastal pelagic, slender bodied fishes that feed on zooplankton and are typically preyed on by larger fishes. They are important to commercial fisheries but are not aquacultured. Several species have been cultured for research. Four Decapterus species inhabit Hawaiian waters.

The Mackerel Scad (Decapterus macarellus) is a popular food and baitfish that grows to about 12 inches. It is the most common mackerel scad species in Hawaii waters. Mackerel scads are locally known as opelu.

Mackerel Scad (Decapterus macarellus) in Hawaii.
Adult Mackerel Scad (Decapterus macarellus) in Hawaii.

D. macarellus was reared many times from 2012 to 2014 from wild eggs. The eggs were collected from December through September and found to be most abundant in the winter and spring. D. macarellus eggs are covered with bright white pigment spots; contain a single, clear oil globule; and measure about 0.8 mm in diameter. The newly hatched larvae measure about 2.2 mm TL; lack developed eyes, a mouth, and fins; and have a distinct bright white pigment pattern across the mid-body. After one day the pigment moves out into the edge of the fin-folds.

D. macarellus larvae grow quickly and are voracious hunters. They have a large mouth and can prey on larger copepod nauplii (70-80 um) at first feeding (3 dph, 3.7 mm TL). By 7 dph, the pigment on the fin-fold disappears and the gut begins to coil. By 10 dph (6 mm TL), the larvae start to undergo flexion and develop dark pigment on the brain, the tail and along the upper abdomen. At this stage they already prey on adult Parvocalanus copepods. Flexion is complete by about 14 dph (8 mm TL). Schooling behavior along the tank sides is observed a few days later. Weak head spination appears to be the only specialization for pelagic life of the larvae. The larval stage is complete at 23-25 dph (about 15 mm) when the body acquires a silvery appearance.

Mackerel Scad (Decapterus macarellus) larvae reared in the laboratory.
Mackerel Scad (Decapterus macarellus) larvae reared in the laboratory.

The pelagic juveniles were grown to 40-50 dph (35-43 mm) on newly hatched artemia nauplii and then released into the ocean. D. macarellus eggs are easy to rear to the juvenile stage and have no obvious culture bottlenecks. These fish seem to have good culture potential based on the rearing characteristics of the larvae.

Blue Trevally – Carangoides ferdau  (May 2013)

The Blue Trevally (Carangoides ferdau) is a common, moderately large (up to 27″/70 cm) jack species that is widespread throughout the Indo-Pacific from South Africa to Hawaii. It is an excellent food fish, unlike most other members in this genus. In Hawaii, adults have been seen following foraging bonefish or goatfish, consuming leftovers uncovered by the foragers. Juveniles sometimes seek shelter among the tentacles of large pelagic jellyfish. Within the Carangid family, the Almaco Jack (Seriola rivoliana), the Greater Amberjack (Seriola dumerili), the Yellowtail Amberjack (Seriola lalandi), the Japanese Yellowtail Jack (Seriola quinqueradiata), and the Florida Pompano (Trachinotus carolinus) are aquacultured for food. Little information about the reproduction and growth of Carangoides species is available.

School of Blue Trevally (Carangoides ferdau) on a reef in PNG.
School of Blue Trevally (Carangoides ferdau) on a reef in PNG.

C. ferdau juveniles were cultured in the spring of 2013 and 2014 from eggs collected in coastal waters off Oahu, Hawaii. This species has only appeared in egg tows during the spring and summer, suggesting that spawning occurs with increasing daylight and water temperatures. C. ferdau eggs are clear, spherical and measure close to 0.75 mm in diameter, with a single oil globule. Several brown spots cover the embryo. The larvae grow fast. They measure 2.1 mm TL at hatching, start to feed on 3 dph (3.1 mm TL), complete flexion near 12 dph (6.4 mm TL) and start to settle (associate with the tank sides) near 20 dph (12.4 mm TL). The larvae spend most of the time feeding on copepods at the surface and were prone to hyper-inflating their swim bladder. Frequent removal of surface scum and reducing light levels solved this problem. Settlement starts shortly after caudal fin formation is complete. The fish were transferred to grow-out at this time.

Blue Trevally (Carangoides ferdau) larvae reared in the laboratory.
Blue Trevally (Carangoides ferdau) larvae reared in the laboratory.

Caranx and Carangoides species develop dark bars on the body during juvenile transformation. Barring on the C. ferdau juveniles was not observed until 40 dph (17 mm). More frequent feedings during grow-out would likely shorten the late larval period significantly. The juveniles were released on a shallow reef together with cultured C. ovalis. While most of the C. ovalis juveniles were eaten by surrounding reef fish (Parupeneus insularis, Iniistius baldwini), all of the C. ferdau juveniles escaped the predators, racing off into the blue. This is the first documented larval rearing of the Blue Trevally (C. ferdau).