(I need to find a way to streamline this 'execution' thing before I run out of cages).
Don't waste your time doing what I just tried.
Untamed tropical shrubland map, with a stream. Or minor river, I forget. Point is, there are aboveground pools that never dry up, and one of them is infested with carp. Six of them, in an area of about 20 square tiles.
Naturally (naturally!) I turned this into my execution pit. Built a platform 3 z-levels above it — think 10-meter diving platform — and tested it by pitting off a puppy.
Apart from the fact that the job got cancelled like five times due to carp interruption — seriously, guys, you're 30 feet above the pond here; man up — it worked well. The puppy (poor puppy) was stunned when it hit the water, and the carp tore it to pieces.
I set up a bunch of cage traps in front of my entrance and waited.
An ambush came, a goblin got trapped. Yay, time to test out my carp pond of dooooooooom.
First of all, getting the cage built on the platform was a pain in the BUTT because of all the interrupted-by-carp cancellations. THEN the dang carp didn't even have the decency to kill the goblin when I dropped him. He just drowned.
All in all, very disappointing. The platform remains as a monument to failed experiments, mostly because I can't get the dorfs to take it down because the fish scare them too much.