Fish scraps, from heads and bones to collars and bellies, are far more useful than most people realize. You can turn them into rich stock, powerful garden fertilizer, homemade fish sauce, or compost. Some parts you might be throwing away are actually prized cuts at high-end restaurants. Here’s how to get the most out of every piece.
Eat the “Scrap” Cuts First
Before you toss anything, take a closer look. Several parts commonly discarded as scraps are actually some of the most flavorful bites on the fish.
The collar, the crescent-shaped section just behind the gills, is rich with fat and connective tissue that turns silky when cooked at high heat. In Japanese cuisine it’s called kama and is typically roasted or broiled. Rub it with a neutral oil, season with salt and pepper, and roast at the highest temperature your oven can reach (around 525°F) for six to eight minutes until it picks up some color. A quick sauce of soy, lemon juice, and chopped scallions is all it needs.
Fish cheeks, especially from halibut or cod, are tender medallions of meat that cook like scallops. They’re often significantly cheaper per pound than fillets from the same fish. A quick sear in butter with a squeeze of lemon is the classic approach. Salmon belly is another undervalued cut: fatty, deeply flavored, and usually sold for a fraction of the fillet price. Sear it skin-side down, finish in a hot oven, and serve with soy or lemon. Monkfish liver (ankimo) is considered a delicacy in Japanese cuisine, sometimes called the foie gras of the sea, and is typically steamed, sliced, and served with ponzu.
Make Fish Stock
Heads, spines, tails, and fin trimmings all make excellent stock. The key difference between fish stock and meat stock is time. Fish bones release their collagen quickly, and if you simmer them too long the result turns cloudy, bitter, and unpleasantly fishy.
Place your scraps in a pot, add enough water to cover everything by an inch or two, and pour in a splash of white wine along with aromatics like onion, celery, and a bay leaf. Bring to a gentle simmer and hold it there for 45 minutes to one hour, no more. Strain through a fine mesh strainer, and you’ll have a clean, golden stock that works as a base for chowders, risottos, paella, and pan sauces. You can freeze it in ice cube trays or quart containers for months.
One important note: avoid using scraps from strong, oily fish like mackerel or bluefish for stock. Lean white fish frames (halibut, snapper, cod, sole) produce the most versatile and neutral-flavored results. Shrimp shells work beautifully too.
Brew Homemade Fish Fertilizer
Fish scraps are packed with nitrogen, phosphorus, and trace minerals that plants love. You can make a liquid fish fertilizer at home through a simple fermentation process, and the result feeds both your plants and the beneficial microbes in your soil.
Start by cutting your scraps into small pieces, then blend or run them through a meat grinder. The finer the pieces, the faster and more complete the fermentation. Mix the ground fish with non-chlorinated water at a ratio of one part fish to three parts water. Chlorinated tap water kills the microbes that drive the process, so use well water, rainwater, or water that’s been left out overnight to off-gas.
Add sugar (raw cane sugar or molasses works well) at a ratio of roughly one part sugar to three parts fish. Then stir in about two tablespoons per liter of liquid from an active lacto-ferment like sauerkraut or kimchi brine. This introduces the bacteria that will break everything down. Pour the mixture into a bucket, cover it with cloth or mesh to keep insects out, and let it ferment for two weeks to over a month. You’ll know it’s done when the strong smell fades to a faint vinegar scent. Dilute the finished liquid before applying it to your garden.
Make Your Own Fish Sauce
The fish sauce you buy at the grocery store is essentially just fish scraps fermented in salt. The ancient Roman version, garum, was made the same way thousands of years ago, and the process is surprisingly straightforward.
Mix fish scraps (guts, heads, and small whole fish all work) with salt at a ratio of about 18% salt by weight of fish. That means roughly 180 grams of salt for every kilogram of fish. This salt concentration is high enough to prevent harmful bacteria while allowing beneficial enzymatic breakdown. Stir occasionally and let it ferment at room temperature for about four months, though depending on your climate the process can take anywhere from 20 days to six months. Warmer environments speed things up. Strain the liquid when it’s done, and you’ll have a potent, umami-rich condiment.
Compost the Rest
Fish scraps make nutrient-dense compost, but they need more care than vegetable scraps. The high protein content attracts animals and can create strong odors if not managed properly.
The most important factor is heat. A compost pile processing fish needs to reach and sustain internal temperatures of at least 55°C (131°F) to meet EPA guidelines for pathogen reduction. This means you need a large enough pile (at least three feet in each dimension) and plenty of carbon-rich “brown” material like wood chips, straw, or dried leaves to balance the nitrogen-heavy fish. Bury the scraps deep in the center of the pile rather than leaving them near the surface, which helps with both heat retention and keeping raccoons and other scavengers away.
Turn the pile regularly to maintain airflow, and expect the process to take several months. The finished compost is exceptionally rich and works well for vegetable gardens and fruit trees. If you only have a small backyard bin, consider the fertilizer route instead, since small compost systems rarely generate enough heat to safely process animal protein.
What You Shouldn’t Do
Tossing fish scraps into a lake, river, or the ocean might seem natural, but it’s regulated more than you’d expect. The EPA oversees ocean dumping of fish waste under the Marine Protection, Research and Sanctuaries Act. Dumping in harbors, enclosed coastal waters, or any area where it could endanger health or ecosystems requires a permit. Even in open water, commercial-scale disposal of fish processing waste may require approval.
For home cooks, the practical takeaway is simple: don’t dump scraps in local waterways. Concentrated fish waste in shallow or enclosed water depletes oxygen and harms aquatic life. The garbage disposal is an option for small amounts, but every method above gives you something useful in return, whether that’s a pot of chowder, a bottle of fertilizer, or a batch of fish sauce that would cost you $15 at the store.