ONE species that has reached cult status in sportfishing circles in recent years is the giant trevally (Caranx ignobilis), better known as the “GT” by those who target them.
This species can be encountered throughout the tropical Indo-Pacific region, prowling offshore reefs from eastern Africa, the Red Sea and Indian Ocean, throughout Asia, northern Australia, east to the Hawaiian and Marquesan Island groups and north to southern Japan.
In Australia, GTs are most commonly encountered by anglers fishing on or adjacent to coral or rocky reefs from Perth in WA north around the top end to central NSW, with the best known locations for targeting large numbers of large fish being the outer drop offs of the Great Barrier Reef and Coral Sea.
Also sometimes known as the lowly trevally (or Ulua in Hawaii), the giant trevally is the largest member of the trevally family (Carangidae). The GT is a powerful apex predator of coral reefs, growing to at least 1.7 metres long and 180 pounds on the old scale (80 kg), but the current world record is “only” 160lb/72.8 kg for a fish taken in Japan.
It’s difficult to determine the true upper size limit for this species because the largest GTs (commonly called “Volkswagen sized” versions) are extremely difficult to catch on conventional sportfishing gear, due to a combination of their size and the rugged coral reef environments this species frequents as adults.
The fact that large GTs are known to be ciguatoxic in some parts of their range, together with the recent trend towards catch and release of large GTs, may also reduce the number of big fish weighed in the future.
Adult GTs can be distinguished from other trevallies of similar appearance (like the Papuan or brassy trevally (Caranx papuensis) by virtue of the GT’s larger size, its steep head profile, strong scutes, and the absence of yellow fins on large fish. The colouration of adult GTs can vary from silvery to grey with irregular spots or light and dark bands on the dorsal region of the flanks, to almost uniformly black reminiscent of the black trevally (Caranx lugubris).
There is evidence from studies done in the Hawaiian Islands and South Africa that the black coloured GTs are mature males, which most commonly take up this unusually dark colouration in spawning aggregations once they exceed around 70cm in length. Juvenile GTs can be harder to identify, however, as they often frequent estuaries and inshore waters which are home to populations of other trevallies of similar appearance like brassy trevally and juvenile bigeye trevally (Caranx sexfasciatus), which can also have the yellowish pelvic and anal fins (as well as yellowish lower lobe of the tail) possessed by juvenile GTs. In these instances, it’s worth looking at the “chest” (pectoral girdle) area, which is scaleless on GTs, but not in the other species.
While GTs look very different to the beautiful bluefin trevally (Caranx melampygus), studies in Hawaii have shown that GTs can interbreed with bluefin trevally to form hybrids. The existence of the bluefin/GT hybrids was discovered after investigation of a world record claim for a 43 kg “bluefin trevally” that had bluish fins but other body shape characteristics typical of GTs.
The ability of GTs to hybridise with other species might also explain the existence of some of the supersized “brassy trevally” sometimes observed in Australia’s Northern Territory and Kimberley regions, although to date I am not aware of any specific studies that have confirmed or ruled out this possibility.
Growth of GTs is reasonably rapid, with studies showing this species matures at around 60cm in length after 3 to 4 years. Growth remains rapid after maturity and fish around 1 metre long are thought to be around 8-10 years old, while large GTs over 1.5 metres long may be over 25 years old.
Electronic tagging of adult GTs in the Hawaiian Islands showed that they spent the majority of their time prowling within relatively small home ranges of around 5-9 km, interdispersed with occasional atoll wide (25-30 km) movements which were often linked to lunar cycles and which were probably associated with spawning behaviours. Spawning is thought to occur over certain areas of coral reefs, in reef channels or over offshore banks.
It is well known that GTs are a superlative sportfish, with the catch and release fisheries for large GTs on our Great Barrier Reef and in the Coral Sea now being considered worldwide as one of the pinnacles of international sportfishing. In these locations, a typical scenario is represented by anglers using poppers or jigs to target small schools of large (20kg+) GTs or even individual trophy sized (40kg+) fish which patrol reef edges and drop offs looking to opportunistically prey on virtually anything smaller than themselves.
Anglers today target large GTs use modern high tech heavy spin tackle, armed with state of the art poppers and jigs with strong single hooks (usually barbless) and the best terminal gear. Acoustic tagging studies of GTs in Hawaii found that survival rates of fish captured using this sort of gear approaches 100 per cent.
By their nature, due to the cost associated with the gear and travel to the remote locations where large GTs occur, these fisheries generate significant socio-economic activity. Large monetary investments by anglers, with minimal or no take of the target fish, means each GT is potentially worth large amounts of money to Australia, but only if they remain accessible to anglers. Because of this, the potential shutdown of these fisheries through proposed large scale fishing closures advocated for places like the Coral Sea by international environmental groups needs to be reviewed, and a full analysis of the socio-economic value and management of GT fishing in Australia should be undertaken.
