German Brötchen are little white breads in french Baguette stile but much shorter, we get them in every bakers shop and cut them at home in two halves like a burger bread. At home we put butter and jam on the halves, cheese or slices of sausage, to carry them around we close them like a sandwitch or burger, usually cheese or sausage and butter inside.
That closed variation you can get along the German coast lines with conserved fish filets, Matjes or Bismarck Herring with slices of onions, or baked Fisch in a breaded version with white sauces in mayonnaise stile. That are the most usual Fischbrötchen.
But they exist with shrimps or salmon or other sorts of fish too. And often I got very unusual ones like conserved fish with onions and plum jam what I got in Flensburg in the first left hand side fish restaurant coming from the railway station entering the old town shopping area. That was incredible good. It's a reason to visit Flensburg, which is a nice middle sized town, by the way.
en.m.wikipedia.org
en.m.wikipedia.org