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Fish and Coral Information

Giant Trevally

Giant Trevally

The giant trevally, Caranx ignobilis, is a species of fish belonging to the Jack/Carangidae family. Also known as ulua in Hawaiian, mamulan in the Marianas, r┼Źnin-aji ("ronin jack") in Japanese, gerepoh in Malaysian and "GT" in many places, the giant trevally has many local names.

Giant trevally are robust and solid in appearance and can be recognized by a steep and blunt head profile. They have immense power which can be attributed to thick shoulders and midsections of muscle and large almost paddle-like pectoral and tail fins. Coloration can range from an almost white-silver to jet black. They may also exhibit a dusky golden hue all over the body, particularly on the fins. The giant trevally lacks a dark spot/coloration on the operculum (found behind the eye). In addition to the body colour of the giant trevally, striking striations and markings on the top section of the fish may also be present, particularly on the back. This is generally seen when the fish has a much darker back than the rest of the body, the contrasting markings showing up as light silvery lines. Black dots a few millimeters in diameter can also be found scattered all over the body; coverage can vary between none, sparse and widespread. Scutes (small sharp plates) exist along the posterior portion of the lateral line and proceed along to the tail. Caution is recommend when holding a giant trevally due to the sharp scutes which can inflict significant pain.

Distribution


The giant trevally has a wide distribution, existing in the warm tropical waters of the Pacific and Indian Oceans. They are usually found as far south as central New South Wales in Australia, east to the Hawaiian Islands, west to the eastern islands of Africa and north to Japan.Habitat ranges from estuaries to coral reef systems. Juveniles tend to be more prevalent in estuaries and river systems. Larger specimens move out to deeper water where there are usually structures such as coral reefs, bomboras or drop-off/channels. Strong currents are usually present where these deepwater environments exist.They will also venture onto flats, headlands and shallow water to hunt food. Giant trevally can also be found along the reef edge and points where large pounding swells crash on the reef and rock stirring up food and creating the ideal environment to hunt. Aside from exclusive saltwater populations, certain C. ignobilis populations have been known to enter freshwater locations. These include freshwater lakes such as Taal Lake in the Philippines and brackish-water lakes like Chilka Lake in India. C. ignobilis have also been recorded from rivers such as the Chiku River, Ruerhmen River, Shihtsao River, Shiliao River and Tsengwen River in Taiwan. Giant trevally have also been found in Sikao Creek in Thailand.

Feeding ecology


The giant trevally uses its superior swimming abilities and power to hunt and smash baitfish. If the giant trevally doesn’t engulf the prey within the first strike, the prey is usually stunned or dead from the strike impact. They will usually devour the prey quickly with one bite as competition can be fierce from other specimens in the pack. Just about any baitfish (small fish) existing in tropical waters is in the giant trevally diet. Species of fusiliers from the Lutjanidae family seem to be particular favourites of the giant trevally.There have also been reports of juvenile turtles & dolphins being found within the stomach contents of larger giant trevally. Large giant trevally can also be found in the presence of large reef sharks as they use the shark as a tool to ambush prey. Large giant trevally have also been known to eat other smaller giant trevally and reef fish when the smaller fish is hooked by an angle

Giant Trevally

Life history


Giant trevally mature at around the ages of 3 or 4 years; they are generally around 60 cm in length. This indicates that the giant trevally is a very fast growing fish. Large, usually solitary specimens can reach over 90 kg (200 lb) and be around 1.7 m (5½ ft) in length.It is not known whether there is a gender ratio or abundance when it comes to larger more dominant specimens.ConservationThe only natural threats to the giant trevally are species of tropical sharks and man.Although not a commercial species in most Western countries, they are commonly commercially caught in third world and island nations.An angling technique known as "surface popping" has already realised the value of the giant trevally to recreational fishing. Most of these anglers are pro-actively engaging in catch & release, and careful fish handling practices. Some commercial operators who offer recreational fishing for this species have started to record and tag giant trevally for scientific purposes.In Hawaii the GT (otherwise known as ulua) is often taken as a trophy – the carcass is usually eaten, but sometimes is sent to the taxidermists or disposed of as it may contain a toxin known as ciguatera which can make humans very ill (there are however, many people who opt to pay for a $10 kit to test for Ciguatera and will eat the fish if it tests negative). Recently surface popping and catch-and-release angling have become popular, resulting in fewer fish being killed.

Giant TrevallyGiant TrevallyGiant Trevally

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