see sources below

Whitefish is the name in English.

An alternative name is lavaret, but it is not so commonly used.
http://en.wikipedia.org/wiki/Coregonus_lavaretus

sik subst. siken sikar
• en välsmakande laxfisk som lever i sött eller bräckt vatten på norra halvklotet: siklöja; gråsik; gullspångssik; älvsik; vid Vätterns strand såldes nyrökt sik
(Norstedt)


Sik is a Salmonid, and in English, whitefish for the Swedish ´sik´ should always be written as one word, pronounced with the stress on white.

In the UK at least, white fish (two words, pronounced with roughly equal stress) is a generic term often used in recipes where you can use a variety of firm, non-oily fish, typically filleted. These are normally marine fish, though carp might work in some recipes.

´White fish´ in these recipes may be cod, haddock, hake, even monkfish... or in other cases plaice or flounders.
One or other type may be ideal, but you can use whatever is available or in season - the recipe still ´works´.
_______________

Lavarel - Austrian/Swiss Lake trout
is also mentioned in Charles Sinclair´s Dictionary of Food.