Common whitefish
From Cookipedia
The common whitefish (Coregonus lavaretus) is a species of freshwater fish in the salmon family (family Salmonidae). It is the type species of its genus.
This fish is very common in Russian Arctic lakes, where it is known as sig (Russian: сиг).
The gwyniad (C. pennantii) is sometimes considered to be a subspecies or population of C. lavaretus. The common whitefish is also sometimes called the lavaret or the powan, although the latter refers more properly to C. clupeoides.