The term soused herring usually refers to a cooked herring in a mild vinegar marinade. The herring can be baked in the marinade or fried and then soaked in it. It is served cold. As well as vinegar, the marinade might contain cider, wine or tea, sugar, herbs (usually bay leaf), spices (usually mace), chopped onion.
The word 'soused' usually means 'soaked in a mild preserving liquid', and can be used to refer to raw herring in a mild vinegar pickle or the famous Dutch brined herring.
The Dutch soused herring (maatjesharing in Dutch, or matjes in (German and Swedish) is an especially mild salt herring, which is ripened using the enzymes in a salty solution, or brine. In Britain and Canada these are sometimes marketed as 'roll mop herring'.
The brine used for Dutch soused herring has a much lower salt content and is much milder in taste than the German Loggermatjes. To protect against threadworms (nematodes), Dutch regulations mandate freezing to at least minus 45°C before salting. In the modern day, soused herrings can therefore be produced throughout the year.
As skin removal demands experience, fillets or double fillets should be attempted first. The soused herrings are silvery outside and pink inside when fresh, and should not be bought if they look grey and oily.
Whereas salt herrings have a salt content of 20% and have to be soaked in water before consumption, soused herrings do not have to be soaked.