It is moderate, not high. The FDA and EPA sort fish into best choices, good choices and avoid, and halibut lands in the middle group at about one serving a week, while cod, haddock, pollock and tilapia sit in the group you can eat two to three times a week. The reason is that halibut grows very large and lives a long time, so it accumulates more methylmercury than a small, fast growing fish. Nothing on this list is about food sensitivity, but it is a genuine reason to rotate halibut with lower mercury white fish rather than eat it daily.

Halibut grades borderline on histamine, which is the freshness column rather than a species trait. It is not one of the scombroid fish, so it starts with far less of the histidine that turns into histamine. The practical advice for a histamine sensitive reader is to buy frozen at sea halibut, defrost it in the fridge, cook it the same day and skip leftovers, because histamine forms before you cook and survives cooking intact.

Usually the opposite. Halibut is caught in a seasonal fishery and is routinely frozen solid on the boat within hours of landing, which locks in the quality and stops amine formation dead. A fillet sitting on ice at a counter in the off season is almost certainly thawed anyway, but without a date you can trust. If you are tracking histamine, frozen at sea is the more controlled purchase.

Yes on both. Plain halibut grades clean on fructans, GOS, excess fructose, polyols and lactose, and it is graded compliant with paleo and AIP, with no gluten, dairy protein or nightshade flags. It is one of the simpler proteins to build an elimination phase around, provided you cook it in olive oil or a compliant fat rather than the usual butter and wine treatment. A true fish allergy is a separate immune issue and needs an allergist, not a food chemistry chart.