Where did you fish?
Even in the fishing place in the 'hometown' I was able to get IIRC rare items.

Gems, don't remember having gotten those. Fish-something whatever, yes, I might look it up, but not right now ;)
Rare fishes, yes, 1 out of 7 give or take a few ;)

Stat raising, again I don't think I ever caught one of those ;)

The chances, at least of what I read somewhere are rolled when entering the fishing areal and are not changed anymore, so you might be better off going to a different place instead.

But as I said, even in the hometown fishing pond, I did get some (more or less ;) useful stuff ;)

EDIT:

Found it ;)

Torchlight FAQ/Walkthrough

By Matt P

From gamefaqs:

He also mentioned it depends on your level as well.....