On the 'how do I eat without irrigating' question a bit back':
Cavern floors come naturally muddy, if you can handle (or wall off) the off-map entrances to the place. Actually, unlike the surface, you can build walls right against the side of the map underground, so it's far from impossible to completely seal off a cavern or two from visiting critters. Just remember to close off the space above the bottom wall, if there's extra z-space above 'em, and watch out for underwater entrances. Might have to wall off some of the pools, too.
Dunno if there's anything controlling it, but underground soil layers -- or at least sand -- seem to allow for natural underground plant growth; if your FPS can handle it, just mine out a few screens worth of empty space and run off of herbalism. Plenty of trees that way, too. I vaguely remember getting at least 50 plants a season with basically untrained herbalists, and that's probably massively misremembering (in an underestimation sense) the actual plant throughput that was going on.
Hunting's fairly viable, if you've got critters walking around the map. It's probably safer to get a small squad of military dwarves and issue them kill commands against the wandering fuzzies; it's a middlingly decent way to train novices, too.
Fishing is incredibly spotty, as has been noted. Sometimes you can, sometimes you can't. Gods only know what's the deciding factor involved; yeh probably shouldn't rely on it.
Trade's pretty easy, too. Ore density being what it is now, you could probably buy the caravans out just selling them gold nuggets, much less anything processed.