When you dry cure fish a lot of water comes out. Does the water include all the fat from the fish or does that remain inside?
It should be mostly just water, very little fat. Water is released pretty easily, fat not so much, especially without grinding things up. Most fish isn't that fatty to begin with, anyway.
In any case, fat and water don't mix without help, so just look at the liquid. If there's fat there you'll see it floating on top. I'm guessing with fatty fish there might be enough for a thin shimmery layer on top, but I doubt you'll ever see very much.