Fish stock, also called fish fumet, is normally prepared with bones and trimmings of lean flatfish such as sole and flounder. Oily varieties of fish, such as salmon and mackerel, should be avoided except when preparing red wine sauces. In areas where fresh fish is abundant (an increasingly rare occurrence), fish stock can be made with fresh baby whole fish rather than bones. Whether fish or fish bones are being used, it is essential they be impeccably fresh and that the fish stock be prepared within an hour or two from the time the fish is filleted. If the bones have a fishy or strong iodine smell, throw them out.

Fish skeletons should be thoroughly gutted (fishmongers do not bother gutting fish for fillets), and the gills should be removed. The bones should be soaked for a couple of hours in cold water to remove any traces of blood, which would discolor the stock. Never add the skin from fillets to a fish stock; the stock will turn gray. The vegetables used for the stock, as well as the bouquet garni, can be varied depending on the final use of the stock. Fennel branches add a lightness and freshness and are too often ignored in recipes for fish stock. In any case, since the cooking time is so short, the vegetables should be chopped fine.

If impeccably fresh ingredients are not available, it is often better to substitute court-bouillon or the cooking liquid from mussels or clams in sauces that call for fish stock. Remember that fish stock and fish sauces should smell and taste of the sea and should not be fishy.