Using the forum search function and looking for 'warcarp' turned up a few threads...
The process seems to be roughly along the following lines.
1, build room near the source of carp or other fish.
2, fill it with cage traps, (and a kennel).
3, connect room to carp source
4, wait for fish to swim into room and get trapped in cages
5, drain the room of water
6, construct an alaborate complex system of floodgates and such and then reflood the room but only up to 3/7 water... enough so that the fish don't drown but the dwarfs don't either...
7, tame the fish, train them as war fish if you've edited the raws.
Fish won't air-drown in cages, even regular wooden ones, so this isn't an issue. Getting your dwarves to put them back in the water without killing them IS an issue, though.
I remember that thread and was searching for it, here it is:
http://www.bay12games.com/forum/index.php?topic=15371.0
The very last page was someone trying to have tame fish for food purposes, doesn't really seem to work.
Hrm, I suddenly have to experiement with something. *opens DF*
Actually, come to think of it, there's still the question of getting a tame fish back into a cage. Still going to run the experiement I was going to run.
Edit: Well, that worked. If you dump cages with tame fish through a hole above the center tile of a butcher's workshop, they'll be right there for the butcher to slaughter. The butcher can still access the fish, since the fish themselves can't be forbidden (as items are after being dumped), and said butcher doesn't have to drag the fish anywhere to slaughter them.
That still leaves the problem of getting tame fish back INTO a cage to slaughter them, though.
Edit 2: The cages would not even list the carp in 4/7 water as animals to be caged. I tried reducing the water to 3/7 (making the carp begin to air-drown), and it still didn't list them. The carp died, so I decided to give up on this. The only other option that I can see would be knocking them out so they trigger submerged cage traps. ...I may examine that possibility some other time.
However, in the process, I examined the myth that dwarves will path through 4/7 water for jobs. That myth is now BUSTED. They WERE quite happy to path through 3/7 water to a remove a wall that was next to 3/7 water, though.