A member of the Cod family, the Saithe, Pollachius virens, is found widely around the UK. It is an active, shoaling fish growing up to about 15kg although this size is now unusual due to commercial fishing. It is an important food fish. Often confused with the Pollack (see later photograph), the Saithe has a dark green back, sharply becoming silver on its sides. Its lateral line is straight. The Pollack has a more dark brown/green back shading to yellowish brown on its sides and the lateral line has a sharp curve over the pectoral fin; the lower jaw protrudes beyond the upper.
Saithe have a large number of common names depending on which part of the country you are in - coalfish, coley, billet, black pollack, blackjack, glassen are all examples. Just to confuse the issue in the USA it is called the pollock!
St. Abbs Marine Reserve, North Sea