Pollock (or pollack, either spelling) is a white fish something like cod or haddock, although sometimes it can look a little greyish when raw.
So whatever the amount is for cod or haddock, you could use that.
We will be seeing more of it in the future as it is being pushed as a substitute for the more well-known fish which are becoming scarce because of overfishing.
And - if you have ever had any fish fingers, they were probably made with pollock (unless they specifically say otherwise).