Is this a single fellet? If so, I'd thaw it just enough to break or cut it, then freeze the rest immediately in water. If they are multiple pieces, you can just thaw them enough to get them apart and then refreeze the left over.
I always freeze fish in water. When we had the boat, perch and walleye would last forever that way. Put the fillets in a heavy duty freezer bag and fill it with water. Work out all the bubbles and then zip it shut. Here is the trick... after you have worked out all the bubbles and sealed the bag, lay it on the counter with the opening over the kitchen sink. Open the 'zipper' just a little so the water can pour out. When almost all the water is out, but no air is getting in....zip up the bag again in midstream. No air will get in and you will have fresh fish frozen in just enough water to keep them from freezer burning. They don't take so long to thaw and don't take up as much room in the freezer either.