Giant Sturgeon in the lakes.
I don't know about 20 foot long ones, that would be extremely big.

I've seen 7 foot Sturgeon in fairly small streams before tho. You catch them by sight, see em and float the bait into their mouths basically.

Maybe they can get massive out in the lakes but don't think I have ever even heard of one that was 20 foot long, pretty sure I would remember that.

And they don't really go up on the surface, they're basically bottom feeders.