I started fishing not that long ago, and have since felt a small sense of guilt when discarding a fish carcass. I never had this feeling before, but personally seeing to its end changed my outlook, and I do my best to use everything now. These are some ways I’ve learned to use every part of the fish.
For most fish, the meat from the body is going to primarily be found in the filet. Once you’ve cut this off of the fish, you’ll notice that there’s a lot of meat left in-between the bones in the skeleton. What I usually like to do then is take a short, sharp knife and scape as much meat off of there as possible. Depending on how well you can filet a fish, this might mean a lot of extra meat. You can make fish cakes with it, or just flake it onto something like your pasta.
When I cook a fish whole, such as trout, I actually peel the skin off and take the meat off of the bones. The meat that’s normally in all the nooks and crannies of the fish skeleton are very easy to remove after it’s cooked, so that’s an option. The best part of this meat is that it’s usually the more tender and flavorful than the filets (in a good way).
For me the fish head is one of the most underrated parts of an animal, and there’s no other way to prepare it than by slow-roasting. Something amazing happens as the head cooks slowly in the oven – it becomes more and more tender as the gelatin begins to melt away and seep into the remaining flesh. This means that while other parts of the fish are very sensitive to overcooking, this part actually favors the longer cooking times.
In larger fish, eating the cheek is a delicacy in its own right. The cheek meat, for many of the aforementioned reasons, is much more tender than filet meat. Even though I’m normally not fond of Alaskan Halibut, the cheeks are absolutely delicious.
Fish bones are actually an excellent source of calcium that rival even milk. I usually enjoy the softened bones of canned sardines, but for a fish that you just caught I can’t imagine gnawing on the leftover bones. Luckily it’s fairly well-covered that you can deep fry those bones into something palatable. There’s a good starter recipe at Serious Eats.
The other thing you can do with bones is make fish stock. This would generally be preferred for leaner fishes like Halibut, Rockfish or Lingcod. Fattier fishes can be used, but they tend to taste fishy.
One of my favorite treats is what I call “fish bacon”. With that leftover skin (assuming you don’t keep it on the filet), try seasoning/frying it in a pan with some oil – just like, yep, bacon. I prefer the fattier fishes like Salmon or Trout for this, but most scaled fish skins will work.
What’s left? The gills and stomach of course, but I’m not suggesting you eat these! The gills will not taste very good, as it’s where much of the blood in a fish is stored – which when left to touch the meat gives it the foul seafood flavor everyone hates. The guts, while you can use these for some things, shouldn’t be consumed in the vast majority of fish. Mercury and other toxins concentrate here more than anywhere else, so it’s safe to recommend discarding these.