What descale means
To descale a fish means to remove the hard, flat scales that cover its skin, so the skin is pleasant to eat and crisps well when cooked. It is usually done before cooking whole fish or skin-on fillets of fish such as sea bass, sea bream, red mullet, snapper, salmon and carp. Scales are attached at one end and overlap like roof tiles from head to tail, so they are removed by scraping in the opposite direction, from the tail towards the head. A purpose-made fish scaler, which has a ridged or toothed head, does the job quickly, but the back of a knife or a spoon also works.
The fish is held firmly by the tail and scraped along both sides, around the fins and along the belly, until the skin feels smooth. Scales tend to fly everywhere, so many cooks descale fish in the sink, under gently running water, in a bowl of water or inside a large plastic bag. Afterwards, the fish is rinsed and patted dry. Some fish, such as mackerel, trout and herring, have very small scales that do not usually need removing, and fish that will be skinned before cooking do not need descaling.
Fishmongers will usually descale fish on request. The word descale is also used for removing limescale from kettles. You will see descale in fish recipes and preparation guides.