I make stock with salmon carcasses and heads. Fish stock is different from that made from meats. It doesn’t benefit from long simmering, but does ok with reduction after you remove the bones. For one carcass including the head, I fill the stock pot maybe halfway up and simmer on low heat for about 20-30 minutes, then I strain it in a collander to remove the bones, pressing to get all of the liquid out of the head/bones. Simmer to reduce until the flavor is strong enough. This makes about four quarts of stock, which I freeze up in individual quart containers and use it for fish soup, risotto, or my favorite concoction:
1 lb cubed fish steaks. Fish stores often offer a great deal on fish trim at a low price.
Onion, garlic, and whatever you like, chopped finely (“mirepoix”)
1 jar of broad beans, preferably Italian “gigantes”/butter beans (or even better, fresh broad beans, like “cranberry” beans).
1 quart fish stock.
Salt, pepper, herbs to taste
Saute the mirepoix in olive oil. Add the beans and simmer until they’re tender. Add the fish and simmer briefly until it’s just cooked. Season and serve forth.
(User seamunky suggests some bacon; you can fry it before adding the mirepoix)