It is graded moderate, which sits between a clean fresh protein and the fish you should genuinely avoid. Histamine is not in the living fish, it forms after the catch when bacteria convert histidine in the flesh, so time and temperature decide the number. The severe scombroid cases are classically tied to dark-fleshed fish such as tuna, mackerel and mahi mahi, which start with far more histidine, but any fish held badly will accumulate histamine.

Generally no. Smoking is a preservation process that involves curing, warmth and days of storage, all of which favour histamine formation, and low histamine protocols routinely rule out smoked, cured and canned fish while allowing very fresh or fresh-frozen fillets. If you tolerate fresh whitefish and react to the smoked version, that contrast is the clue rather than a contradiction.

Yes. Plain whitefish is graded clean on FODMAPs and on every subgroup, since fish contains no carbohydrate to ferment. Batter, breadcrumbs, creamy sauces and onion or garlic seasoning are what turn a low FODMAP fillet into a high FODMAP plate.

Strictly, whitefish means the Coregonus species of freshwater lakes, related to salmon and trout and best known smoked or as golden caviar. Commercially the word is used far more loosely for any mild lean white-fleshed fish, which sweeps in cod, haddock, pollock, hake, sole and often tilapia. They grade similarly, but if you are trying to isolate a reaction, note the species you actually ate.