Jacks are fish belonging to the Carangidae family. The Carangidae family also includes the pompano, lookdown and trevally fish. With more than 150 identified species of jacks, these fish are found primarily in marine waters but occasionally inhabit both brackish and freshwater. They live in both temperate and tropical portions of the Atlantic, Pacific and Indian oceans. Many adult fish live within the pelagic zone and deeper waters, but other habitats of jacks include estuaries, bays, reefs, seagrass beds and sandy flats. Jacks will also not stray too far from the coastlines. Their bodies are laterally compressed with small scales that provide a smoother appearance. A row of enlarged scales known as scutes, however, can be found along their side near their tailfin. The tailfin itself is deeply forked and their heads are broad. The colors of a jack range from a bluish green, silvery or yellowish sheen on the body.
These fish are carnivorous with a diet of small fish, squid and crustaceans. The size of their prey increases as they grow in size. Sizes differ greatly depending on the species of jack. For example, the blacktip trevally could reach a max size of 1 m in length and 12.5 kg in weight while the crevalle jack could grow up to roughly the same length but weigh almost triple the amount of a blacktip trevally. Jacks may be found together in groups called schools and tend to perform seasonal migrations that differ by age. They are spawning fish that have females releasing eggs into the water column for the males to fertilize. A single female may lay up to 1 million eggs, but after the fertilization process both parents will not remain to tend to the offspring. The eggs will float in the water column until they hatch and then during the juvenile phase, the babies will move to the shore and sheltered habitats.
Some of the more popular jack species such as the crevalle jack are a favorite sport fish because of their strength and difficulty being reeled in. However, their meat is not usually consumed by humans because of its poor quality. Instead, jacks are utilized for fish oil, fishmeal and similar products. While the demand for jack fishing may be high, the International Union for Conservation of Nature (IUCN) lists species such as the crevalle jack as of least concern. Fishing regulations and successful aquaculture raising projects of jacks may be the reason why the populations remain stable.