Fish can be categorised in a number of ways:
– freshwater fish like trout, pike and perch
– seafaring fish such as gurnard, tuna and turbot, although some species are native to both environments.
Another difference is between white fish (such as cod, in which the oil is contained within the liver) and oily fish (like salmon, where the oil is dispersed between the flakes of flesh).
A more useful distinction for chefs is the one between ‘round fish’ and ‘flat fish’. These two categories cover almost every species we cook with, and the same filleting technique can be used across the board. The big difference is this: round fish have a single fillet each side of the spine, whereas flat fish have a ridge of bone running along the body, separating the flesh into four fillets in total.