Why fish shape and bone structure change how you fillet
The way you fillet a flatfish looks nothing like filleting a round fish, and trying to use one technique on both will leave you with meat still stuck to bone and frustration in your hands. Each fish family has a skeleton that works differently — where the backbone sits, how the ribs attach, whether the fish is compressed side-to-side or top-to-bottom — and your knife follows the fish's own structure, not the other way around. Learning to read the bone layout under your blade takes the guesswork out of filleting and means you waste less meat.
The three main categories — round fish (like bass, trout, and mackerel), flatfish (like flounder and sole), and firm fish with large bones (like catfish and pike) — each have a distinct approach. Once you understand why the technique changes, you can adapt it to any fish you catch, whether it is a species you have filleted a hundred times or one you are meeting for the first time.
Key Takeaways
- Round fish are filleted by cutting behind the gills and pectoral fins, then following the backbone down the length of the fish to separate the fillet from the ribs.
- Flatfish yield four fillets instead of two because their backbone runs down the center and ribs branch on both sides, so you cut along the centerline first, then fillet each side.
- Firm fish with large bones like catfish need a Y-shaped cut behind the head to navigate around the pectoral fins before you can follow the backbone down.
- The knife angle matters: you angle toward the backbone on round fish to follow the ribs, but keep the blade nearly flat on flatfish to separate the fillet cleanly from the skin side.
- A sharp filleting knife (five to six inches) and a cutting board with a groove to catch liquid are the only tools you need, and they make the work faster and safer.
Round fish: the most common catch and the standard technique
Most fish you catch — bass, trout, salmon, mackerel, snapper — are round fish, meaning their body is roughly cylindrical and their backbone runs down the center with ribs branching out on both sides. The fillet comes off in two pieces, one from each side of the body.
Start by placing the fish on its side on your cutting board. Make your first cut behind the gills and pectoral fins, angling the knife toward the head at about 45 degrees. Cut down to the backbone but not through it — you want to feel the blade hit bone and stop. Then turn the knife to run parallel to the board and cut along the backbone toward the tail, keeping the blade angled slightly upward toward the ribs. As you move down the fish, let the knife follow the curve of the ribs; you will feel them guide your blade. When you reach the tail, cut through the skin to free the fillet. Flip the fish and repeat on the other side.
The angle of your knife is what separates a clean fillet from one that leaves meat on the frame. If your blade stays too flat, you will cut into the ribs and leave meat behind. If you angle too steeply toward the backbone, you will waste the delicate meat along the rib cage. Practice the angle on a few fish and your hands will remember it.
Flatfish: four fillets from one fish
Flatfish — flounder, sole, halibut, turbot — are compressed side-to-side rather than top-to-bottom, and their backbone runs down the center with ribs branching upward and downward. This means you get four fillets from one fish instead of two, and the technique is different enough that it is worth learning separately.
Place the flatfish on the board with the dark side up. Make a cut along the centerline from head to tail, running your knife right down the backbone. Then, starting at the head, angle your knife at about 20 degrees — much flatter than you would use on a round fish — and cut from the centerline toward the outer edge of the fish, following the ribs as they branch away from the backbone. The blade should stay nearly parallel to the board; this shallow angle lets you separate the fillet cleanly from the skin without tearing the meat. Work from head to tail on one side, then repeat on the other side of the centerline. Flip the fish and repeat the whole process on the other side.
The key difference is the angle: flatfish fillets come away cleanly when your blade stays shallow and lets the ribs guide you outward, rather than cutting down into them. If you use a round-fish angle on a flatfish, you will cut through ribs and waste meat.
Firm fish with large bones: catfish, pike, and similar species
Catfish, pike, and some other freshwater fish have large pectoral fins and a bone structure that does not follow the standard round-fish pattern. The pectoral fins are thick and bony, and they sit in a way that blocks a straight cut behind the gills. You need to navigate around them first.
Place the fish on its side. Instead of cutting straight behind the gills, make a Y-shaped cut: start at the top of the head behind the gill cover, angle down and forward toward the pectoral fin, then angle back and down toward the belly, creating two lines that frame the pectoral fin. This removes the fin and the bone structure around it in one piece. Once the pectoral fin is out of the way, you can follow the backbone down the length of the fish just as you would with a round fish, angling your blade toward the ribs as you go.
The Y-cut takes practice to visualize, but it solves the problem these fish present: their pectoral fins are too large and too bony to cut through cleanly, so you remove them as part of the head section instead. After you have done it a few times, the motion becomes automatic.
Skin-on versus skin-off: when to separate them
Some fillets come off the fish still attached to skin, and some cooks prefer to remove the skin before cooking while others leave it on for flavor and texture. The timing of skin removal changes depending on how you plan to cook the fish and how confident you are with the knife.
If you want to remove skin immediately after filleting, place the fillet skin-side down on the board, hold the tail end firmly with your non-knife hand, and run a sharp knife between the skin and the meat at a very shallow angle — almost parallel to the board. Let the blade do the work; you are not sawing, just sliding the knife forward while the skin stays put under your fingers. This works best when the fillet is still cool and the skin is taut.
Many cooks prefer to leave the skin on during filleting and remove it just before cooking, or not at all. Skin-on fillets are easier to handle during cooking because the skin holds the meat together, and you can always peel it away at the table. If you are pan-frying or grilling, skin-on fillets often taste better because the skin crisps and adds flavor.
Removing pin bones from fillets
After you fillet a round fish, you will often find a line of small bones running down the center of the fillet — these are the tips of the ribs, and they need to come out. Run your finger down the fillet from head to tail and you will feel them as a slight ridge.
The fastest way to remove them is with a pair of fish tweezers or a small needle-nose pliers. Grasp each bone at the base and pull at a slight angle toward the head — pulling toward the tail will snap the bone and leave the base behind. Work from head to tail, one bone at a time. On a large fillet, this takes a minute or two. Some cooks use a knife to cut a thin V-shaped channel along the line of bones and remove them all at once, but this wastes a little meat; tweezers are more precise.
Flatfish fillets usually have fewer pin bones or none at all, depending on the species. Catfish and pike fillets may have a heavier line of bones that takes longer to remove. It is worth doing this work before cooking because nobody wants to find a bone while eating.
The tools that make filleting easier and safer
A good filleting knife is the only essential tool, and it should be sharp. A dull knife requires more pressure, which means your hand is more likely to slip, and you will tear the meat instead of cutting cleanly through it. A filleting knife is typically five to six inches long with a thin, flexible blade that bends slightly as you work — this flexibility lets you follow the contours of the fish without the blade binding in the ribs.
A cutting board with a groove around the edge catches the liquid that comes off the fish and keeps your work surface from becoming slippery. This matters for safety: a wet board is a board where your knife hand can slip. Some boards have a groove deep enough to hold the fish steady while you work, which is helpful but not essential.
Fish tweezers or small pliers for pin bones are inexpensive and make that task much faster. A damp towel under your cutting board keeps it from sliding around. Beyond that, you do not need much — a sharp knife, a good board, and clean hands are enough to fillet any fish well.
Frequently Asked Questions
Can I use the same filleting technique on all round fish?
Yes, the basic technique works for any round fish, but the angle and pressure change slightly based on size and bone density. Small trout need a lighter touch and a shallower angle than a large salmon. Start with the standard technique and adjust based on what you feel — let the ribs guide your blade, and you will naturally find the right angle for each fish.
What should I do with the head and bones after filleting?
Save them for stock. Fish heads and bones make a rich, flavorful broth that takes only 30 to 45 minutes to simmer. Rinse them first to remove blood and scales, then simmer gently with water, aromatics, and a splash of white wine. Strain and freeze the stock in ice cube trays for future soups and sauces.
How do I know if my filleting knife is sharp enough?
A sharp filleting knife should glide through the skin and meat with light pressure — you should not have to saw or force it. If you are tearing the meat or the blade is slipping sideways, it is time to sharpen. A steel honing rod keeps a sharp knife sharp between uses; a whetstone or professional sharpening restores a dull blade.
Why does my fillet have a lot of meat still stuck to the bones?
Your knife angle is too steep or too shallow for the fish you are working with. Round fish need the blade angled toward the ribs so you follow them cleanly; flatfish need a much shallower angle. Practice the angle on one or two fish and adjust based on how much meat you are leaving behind. A sharper knife also helps — a dull blade tears instead of cutting, which makes it harder to follow the bone structure.
Is it better to fillet a fish fresh or after it has been chilled?
A chilled fish is easier to fillet because the meat is firmer and the skin is taut, so your knife moves more cleanly. If you are filleting immediately after the catch, chill the fish in ice for at least 30 minutes first. The meat will be easier to work with and the fillet will look better on the plate.