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, rōnin-aji ("ronin jack") in Japanese, gerepoh in Malaysian and "GT" in many places, the giant trevally has many local names.
AnatomyGiant 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.
DistributionThe 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 ecologyThe 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 angler.
Life historyGiant 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.
Conservation
The 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.
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Koh Racha Yai:
Dive sites at Koh Racha Yai:
Bungalow Bay, Home Run, Lucy's reef, Eastern coast, Siam Bay etc.
Bungalow Bay:
Bungalow bay is situated on the west coast of Koh Racha Yai. Average depth is around 17 meters / maximum depth 30 meters. This site can be split; you can opt to follow either of the bay's shorelines.
The rocky formations visible above the surface continue underwater, with a good presence of reef fish. Around the rocks swim surgeonfish and fusiliers, while below you can see inquisitive paired goatfish, nudibranchs, urchins and plenty of blue spotted ribbontail rays.
Eastern Coast:
The first site begins at the northeast tip of Koh Racha Yai and heads south, the second follows the southeastern apex. Average depth is around 12 meters / maximum depth 22 meters. This is a very popular diving destination and is suitable for all levels of diver. There's also enjoyable snorkeling around the rocks in the shallow waters off the islands coastline.
The shallower sections of the reef feature table corals and smaller examples of leaf corals; the deeper waters have similar species, but larger. There are many featherstars, lionfish, puffers and a few parrotfish and wrasse.
The site's southern end has the bonus of encountering mating cuttlefish along with a greater display of reef creatures, including good sized titan triggerfish, moray eels and octopus; plus on a smaller scale, a better selection of colorful fish and invertebrates. Leopard sharks and hawksbill turtles are occasionally seen around the coastline.
Koh Racha Noi:
Dive sites at Koh Racha Noi:
The rock, North Pinnacle, Kanpela point. The beach, South pinnacle.
Northern Tip:
Average diving depth is around 17 meters / maximum diving depth 30 meters at Koh Racha Noi. A very reasonable multilevel dive site, which takes divers around pinnacles and large rocks. There is a fair display of hard corals and soft corals, and the usual reef fish are present in reasonable quantities.
If you are lucky you should be able to sight small reef sharks, but the main attraction is probably the presence of large bullet-head parrotfish, and the magical manta rays. However, diving here can be demanding because of the currents.
Southern Tip:
Average diving depth around 23 meters / maximum diving depth 40 meters +. This is a really great dive for the experienced diver. The underwater terrain is similar to that around the island's northern tip, but much deeper, the shallowest part of any consequence is at 18 meters.
Fine soft corals highlight the smooth sided boulders. Crinoids and featherstars are well represented. There are fairly plentiful, reasonably sized fish, including lionfish, parrotfish, wrasse and triggerfish, as well as large visiting pelagic's such as barracuda, rainbow runners and trevally.
Manta rays and marbled rays frequently seen here as are whale sharks, although with slightly less regularity. All in all, this site can be compared with some sites found around Ko Similan Marine National Park, Thailand.
Phi Phi Islands:
Dive sites at phi Phi Islands:
Koh Bida Nok, Koh Bida Nai, Maya Bay.
The spectacular Phi Phi islands lie in the mouth of the Phang Nga Bay and form the Haad Nopparattara Mo Koh Phi Phi National Park. White limestone crags splashed with greenery jut vertically up wards out of the clear turquoise waters to tower over magnificent sandy beaches.
You will see an amazing variety of fish life, beautiful soft corals, staghorn coral gardens and huge sea fans. On the sandy bottom, leopard sharks can be found resting. Diving in the Phi Phi islands is an experience not to be missed. From shallow sheltered bays to wall and drift dives, there's something for every level of diver.
Visibility ranges on average from 10-25 meters with weak to moderate currents. Water temperature remains a fairly constant 27 - 30C throughout the year, depths from 10 - 30 meters, and, there's plenty of marine life! For non divers the Phi Phi area offers superb snorkeling, with the opportunity to see not only beautiful coral gardens but also some of the larger fish at shallow depths.
Koh Doc Mai:
Koh Doc Mai is a small island off Phuket, In Thai language Doc Mai means "Tiny Flower".
When you scuba dive here you will see why this island has this name. Koh Doc Mai's underwater area is famous for its hard and soft corals covering the vertical outcrop. Also famous for its circumnavigational wall dropping down to 30 meters.
Koh Doc Mai also has a few caves were you will find some weird and wonderful reef species. Perfect for macro photographers who want to spot that little yellow seahorse. Lots of soft and hard corals, white eyed moray eels, schools of trevally, trigger fish and lots lots more.
For more detailed dive site information: Koh Doc Mai, Kata Beach, King Cruiser, Koh Poo, Koh Racha Yai, Koh Racha Yai Bays, Koh Racha Noi, Anemone Reef, Similan Islands,Anita's Reef, Beacon Reef, Boonsung Wreck, Boulder City, Breakfast Bend, Christmas Point, Deep Six, Donald Duck Bay, East Of Eden, Elephant Head Rock, Honeymoon Bay, Koh Bon, Richelieu Rock, Rocky Point, Shark Fin Reef, Tachai Pinnacle, Tachai Reef, Turtle Rock, Waterfall Bay, West Of Eden.
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Day Trip Diving