Most fish broth can be made from inexpensive ingredients, by-products of the items going into the soup. For example, in the Brodetto, the broth is made from the shells of the shrimp and the frame of the fillets. Here are a few guidelines for making broth from seafood trimming: Frames from round fish like bass and cod hold up better during long simmering than those from flat fish like flounder or sole. Do not use oily fish like bluefish, salmon or mackerel. Remove the gills and the air sac (the reddish organ located in the body cavity toward the head end) before cooking. When buying fish for soup, ask for fish heads and "collars" (the meaty part just below the head). Few seasonings are needed for a fish broth destined for soup. A small handful of white or black peppercorns, several sprigs of Italian parsley, a pinch of dried thyme and a medium leek, will season about 6 cups of broth.
Fish trimmings, heads, collars, shells 1 teaspoon peppercorns Italian parsley sprigs Pinch dried thyme 1 medium leek, cleaned, cut lengthwise
Place fish trimmings shrimp shells, round fish frames, fish heads and collars into a pot large enough to hold them comfortably. Add the seasonings peppercorns, parsley, thyme and leek; add cold water to cover by 1 inch. Bring to a boil; reduce the heat and simmer, uncovered, 1 1/2 to 2 hours. Strain through a fine sieve and cool to room temperature before refrigerating. Either freeze in airtight containers or refrigerate and use within 3 days.