Giant trevally
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Diet and feeding
Frontal view of a giant trevally illustrating the compressed form of the species

The giant trevally is a powerful predatory fish, and is an apex predator in most environments; from the estuaries it inhabits as a juvenile to the outer reefs and atolls it patrols as an adult. Hunting appears to occur at different times of the day in different areas of its range; off South Africa it is most active during the day, especially at dawn and dusk, while off Zanzibar and Hong Kong it is said to be nocturnal in its habits. The species diet has been determined in several countries and habitats, with this research finding the diet generally varies slightly between locations and age. In all but one study (which was of juveniles), the giant trevally dominantly takes other fishes, with various crustaceans, cephalopods and occasionally molluscs making the remainder of the diet. In Hawaii the species has a predominantly fish based diet consisting of Scaridae and Labridae, with crustaceans, including lobsters, and cephalopods (squid and octopus) making up the remaining portion of the diet. The large number of reef fishes suggests that it spends much of its time foraging over shallow-water reef habitats, but the presence of squid and the schooling carangid Decapterus macarellus indicates exploitation of more open-water habitats as well. Off Africa the diet is similar, consisting mostly of fish including eels, with minor squid, octopus, mantis shrimps, lobsters and other crustaceans. Younger fish inside Kaneohe Bay, Hawaii showed the only instance when crustaceans were preferred over fish; stomatopods, shrimps and crabs were the most common prey taken at 89% of stomach content by volume, with fish, mostly of the family Blennidae, making up only 7% of the stomach contents. Estuarine fish in both Hawaii and Australia have mostly fish-based diets, with crustaceans such as prawns and amphipods also of importance, and are known to take more novel prey such as spiders and insects in these habitats. There have also been reports of juvenile turtles and dolphins being found within the stomach contents of larger giant trevally. Studies of different size classes of fish have found the diet does change with age in some locations, with the changes relating to an increased volume of fish taken.

Studies in controlled environments on the giant trevally’s feeding strategies have found that hunting in schools increases their capture efficiency, however is not necessary for an individual’s survival. When a school is formed during feeding, one individual will take a leading position, with others trailing behind it. Several individuals will attack the prey school, striking and stunning the prey, with the leading individual generally being more successful. Some individuals act individually and opportunistically within the school if one of the prey fish becomes isolated, with the main advantage of schooling appearing to be the ability to further break up and isolate prey schools. The only time hunting in schools is a disadvantage is when only isolated prey are present, such as close to a reef; here an individual hunter has a greater probability of capturing it than if a group is present. Another hunting strategy of the giant trevally is to ‘escort’ Monk Seals, a behavior which has been observed in the Hawaiian Islands. The trevally swim close by the seal, and when the seal stops to forage, the trevally positions its mouth inches away from the seal. If a prey item is disturbed, the trevally will attempt to steal the prey from the seal, which routinely does occur. The seal does not appear to gain any benefit from this relationship, and it is thought juvenile seals being followed in this way may be outcompeted by the larger fish.[45] A similar strategy has been employed by fish in the presence of large reef sharks, as they use the larger animal as a tool to ambush prey. The opportunistic nature of giant trevally has also been made evident by studies on the mortality rate of undersized or egg-bearing lobsters released from traps at the water’s surface in the Hawaiian Islands. The fish are efficient predators of these crustaceans, with individuals often seizing a lobster before it could sink to the seafloor after being released, or attacking before the lobster moves into a defensive position. Some bolder large individuals are even known to eat the lobster head first when its in a defensive stance.
source

seafood, Types of Seafood