Large, predatory reef fish, including the Giant Trevally (Caranx ignobilis), are frequently avoided as food because they carry a high risk of ciguatera poisoning, a toxin that accumulates in the flesh of large reef-associated fish. While small trevally are considered safe, larger specimens—often exceeding 6 kg or caught in specific tropical regions—are dangerous to consume. 

Here are big fishes that are generally not eaten or should be avoided:

1. Reef Predators (High Ciguatera Risk) 

. Giant Trevally (GT): While technically edible, they are often avoided in the Indo-Pacific due to ciguatera, particularly in Hawaii and parts of the Cook Islands.

. Red Bass (Bohar Snapper): Widely recognized as a major source of ciguatera, often recommended to be avoided entirely.

. Barracuda: Large specimens, especially in the Caribbean and tropical Pacific, are high-risk.

. Moray Eels: These are top reef predators with a very high likelihood of containing toxins.

. Large Groupers & Coral Trout: Like the GT, these fish live long enough to accumulate high levels of poison, particularly in degraded reef areas.

. Titan Triggerfish: Large specimens of this species are often restricted from consumption. 

2. Fish with High Mercury or Toxicity 

Shark: Many shark species, particularly larger ones, are not considered good eating due to high mercury levels.

Greenland Shark: This deep-water fish is considered toxic and inedible due to the high concentration of trimethylamine oxide in its flesh, which acts as a type of anti-freeze.

King Mackerel (Narrow-barred Spanish Mackerel): High levels of mercury make large specimens risky.

Escolar: Often called the "diarrhea fish," it contains high amounts of wax esters that are indigestible by humans, leading to digestive issues. 

3. Other Reasons (Endangered/Protection)

Giant Freshwater Stingray: Considered a "monster fish," they are generally not eaten simply due to their extreme scarcity and protected status in many regions. 

Key Tips on Avoiding Ciguatoxic Fish

Size Matters: The larger the fish, the higher the risk.

Avoid Internal Organs: Ciguatera toxin is most concentrated in the head, roe, liver, and other viscera.

Regional Differences: Avoid eating fish caught near disturbed or dead coral reef ecosystems. 

Note: In the Cook Islands and other areas, the Giant Trevally is often caught solely for sport and released because of its reputation for toxicity.