We do a quick (10hr+) freeze on our fish before we vacuum pack it. Freezing the fish first on cookie sheets will makes the liquids solid and when you place the item in the bag and run the vacuum there are no run away liquids that damage the seal or the vacuum.
We learned this when we where packaging blue berries and the vacuum packer would crush the fruit. Freezing them first made the fruit solid and then you could suck the package tight and remove all the air and the item remains in good shape.
Just watch out for any bones stick up or it will damage the vacuum bag