Fish can be sold whole or cut into fillets, steaks or cutlets. Most fishmongers and supermarkets will skin, clean, bone and fillet any fish for customers. It is convenient to classify fish as white, oil-rich or shellfish. White fish are also called demersal fish. They live on or near the sea-bed and may be round or flat.
Round fish live near the sea-bed, e.g. cod, coley.
Flat fish live on the sea-bed, e.g. plaice. Oil-rich fish are also called pelagic fish. They swim near the surface of the sea and are found in shoals, e.g. herring, mackerel. This checklist summarises the main points to look for when choosing fish. Spoilage is mainly caused by the actions of enzymes and bacteria.