Melt butter in a large stainless steel pot.
Sauté vegetables until just soft.
Add white wine and bring to a boil.
Add whole fish carcasses and cover with water.
Then add vinegar and bring it to a boil once more.
Skim off any scum that rises to the top.
Add thyme and parsley.
Reduce heat, cover, and simmer for at least 4 hours or as long as 24 hours.
Store in labelled pint-sized jars or containers.
Store in fridge or freezer.
Use in place of water as a base for soup, to boil pasta (or rice, or other grains), in homemade sauces and condiments like ketchup, and to saute vegetables.
Pick meat away from the bones (it will fall off by this time).
Refrigerate or freeze to add to soup later, or use as you would canned tuna fish.
*According to Nourishing Traditions, fish stock is ideally made from the bones of sole or turbot. Unfortunately, in America, sole is usually pre-boned. Snapper, rock fish, and other non-oily fish work equally well. You may just have to ask your fishmonger to save the carcasses for you.
Oily fish like salmon shouldn't be used for making fish broth, because the highly unsaturated fish oils may become rancid during the long cooking process.
How To Make Fish Stock https://traditionalcookingschool.com/food-preparation/recipes/the-secret-of-fish-stock/