Yes, considerably higher than the fresh fish. Pickling herring involves salting and holding it for days or weeks, and that is exactly the environment in which bacteria decarboxylate the histidine in the flesh into histamine. Herring starts with a lot of histidine to begin with, since it belongs to the same histidine-rich family as sardines and mackerel. If you have histamine intolerance, rollmops, matjes, soused herring and herring in cream or mustard sauce are all foods to test very cautiously, or skip, even if you tolerate fresh oily fish.

It depends entirely on the form. Pickled and smoked herring are named on conventional tyramine restriction lists for MAOI antidepressants, along with cured and fermented meats and aged cheese, because ageing and curing generate tyramine. Fresh herring, cooked promptly, is a much lower-tyramine food and is graded only moderate here. Anyone on an MAOI should follow their prescriber guidance rather than a food grade, but the practical distinction is that the fish is not the problem, the curing is.

Generally no. A kipper is a herring that has been split, brined and cold smoked, so it has spent time in salt at temperatures that favour amine formation and has then been smoked rather than cooked through. Both stages add to the histamine and tyramine load. The same applies to bloaters and to smoked herring fillets sold in vacuum packs. If you want the oily fish nutrition without the amines, fresh or frozen-at-sea fish cooked from raw is the better route.

Fresh herring grades moderate (C), a step above the lean white fish, because it deserves more care than they do. Herring has dark, oily, histidine-rich muscle, which is the substrate histamine is made from, so it forms histamine quickly if it warms up or sits too long. If you can get herring genuinely fresh or frozen at sea and cook it the day you buy it, many people with histamine intolerance manage it. Buying it from an open counter, or keeping it as leftovers, is where it tends to go wrong.