One can also say "a" portion of fish
One could, but the Finnish says "yksi" so there's not much point changing it
Why is it "kalaa" partitiivi after yksi?
Because of annos. If there's some amount of something (for example portion of fish), then that something is in partitive (the expection being if preceded by yksi).
Yksi annos kalaa
Kaksi annosta kalaa