Cooked fish flakes apart easily because its muscle fibers are much shorter than those in red meat. The fibers are held together by a thin layer of connective tissue called myocommata, which breaks down quickly with heat. As a result, the fibers separate and flake apart.
What makes fish flake apart faster? In red meat, a thick, sturdy layer of collagen surrounds the muscles, so the meat separates into whole pieces rather than individual fibers as it cooks. Fish has much thinner connective tissue, which nearly dissolves once its internal temperature passes a certain threshold. That explains why overcooked fish falls apart so easily. The type of fish matters too: delicate fish such as cod or sole are more prone to flaking, while firmer fish such as tuna or salmon hold together better. Turning fish too early or roughly with a stiff spatula can also pull the flesh apart before it has fully cooked and set.
How can you keep fish from falling apart? Taking fish off the heat as soon as its internal temperature reaches 63°C / 145°F is the most basic way to prevent overcooking. Fish that has changed color and flakes easily with a fork is usually done; leaving it on the heat beyond that point makes it fall apart faster. Flip it in a single motion with a wide, flat spatula to keep the underside from sticking to the pan and tearing as you lift the rest. Leaving the fish undisturbed in the pan until the crust is well browned and releases on its own serves the same purpose.
The cooking method affects the result too. Fish cooked gently by steaming or baking generally falls apart less than fish turned in a hot pan, because it heats more evenly and requires less handling. A skin-on fillet generally falls apart less easily than a skinless fillet; the skin acts as a natural support beneath the flesh and helps hold the fibers together.
Letting fish rest for 1–2 minutes after cooking, rather than moving it straight to a plate, can have a similar effect. This brief rest lets moisture between the fibers redistribute and helps the flesh firm up slightly. Fish that has been frozen and thawed falls apart more easily during cooking than fish that has never been frozen. Ice crystals formed inside the cells during freezing loosen and damage the fish's natural structure. By the time it is cooked, the bonds between its fibers are already weaker, so it falls apart more quickly.
The thickness of the fish also affects the risk. A thin fillet cooks faster and passes the critical internal-temperature threshold sooner, so it is more likely to break when handled or turned. A thick cut can hold together longer. Fish left at room temperature for a long time after being taken out of cold storage also becomes less firm. Fish taken out of the refrigerator just before cooking therefore stays a little firmer after cooking than fish left at room temperature for a long time.