A fish commonly encountered during Caribbean reef diving is the bar jack. Other English names include blue-striped cavalla, red jack, neverbite, passing jack, skipjack, and pointnose, although I believe bar jack to be far more common.
Bar jacks have a horizontally symmetric shape, and bear a remarkable resemblance to the fish symbol . They have a forked caudal (tail) fin, which implies they are quick, continuously-moving fish. They have dorsal, anal, and pectoral fins, although these are generally hard to see until the fish is close. They are moderately-sized fish, usually a little over a foot long (40 cm).
The bar jack's most distinguishing characteristic is the black stripe running along the top of the fish, down to the bottom of the caudal fin. Often there will be a vibrant blue stripe immediately below the black, making the bar jack immediately recognizable. The rest of the fish is a silvery color. Juveniles are not as easily distinguished, and can have up to 6 dark bands.
Geography and Habitat
Bar jacks are found in the western Atlantic, along the east coast of the US down into the Caribbean and Gulf of Mexico. They extend from the coasts of New Jersey down to Venezuela, and have even been found as far south as Brazil. They are uncommon in the northern Gulf of Mexico.
The bar jack likes clear, shallow waters, making it a familiar sight near coral reefs and to depths of around 60ft / 20 m. Studies have found that the species moves quite a bit from reef patch to reef patch, even over large regions of sand. As a diver, this means you will often see bar jacks hovering above or away from the reef, rather than in the coral as preferred by many fish.
Bar jacks can be found swimming alone, with a few other fish, or in a school of many fish. That's not very helpful for identification, but fortunately the bar jack is easy enough to spot without relying on it's social behavior.