It is histamine poisoning from decomposed fish, and it is not an allergy. Bacteria on the fish convert its natural histidine into histamine, and eating enough of it produces flushing, headache, palpitations, hives and gut upset within minutes to an hour. It is named after the mackerel family (Scombridae) because those fish cause it most, and it typically settles in a day, often helped by antihistamines. Because it is dose driven rather than immune driven, one bad fillet does not mean you must avoid mackerel for life, unlike a true fish allergy.

No, and this is the distinction that matters for a trigger journal. Fish fingers add a wheat crumb and often egg. Surimi, sold as imitation crab or crab sticks, is pollock flesh washed, minced and rebuilt with starch, egg white, sugar, sorbitol and flavour enhancers. Log those as their own food, not as pollock, or you will blame a fish that graded clean.

Frequently, yes. Sole and flounder are both flatfish, and most fish sold as sole in North America is a flounder species: petrale sole, grey sole and Pacific Dover sole are all flounders. Lemon sole in the UK is a dab. True sole is the European Dover sole, Solea solea, and it is the expensive one. If you are comparing your sole and flounder diary entries, be aware you may be comparing the same fish to itself.

Yes. Anchovies are fermented into the base and are what gives the sauce its savoury depth, so it is not vegan and it is unsafe with a fish allergy. This catches people out constantly because the sauce is invisible once it is in a dish. Caesar dressing, bloody Marys, cottage and shepherd's pie, gravies and steak marinades are the usual hiding places, and a few anchovy-free versions are sold specifically for this reason.

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.