I once caught a 7 lb bass with one barb/hook of a treble hook sticking out of its arse. I always let the first fish go, but its belly/arse looked very red and sore, so I kept it and release the second fish instead.
When I came to gut it, the hook was rusty and in no way it was going to rust thru in any time soon. It must be days for the hook to flow thru its digestive system.