I realise that this is a huge necro-bump, but I'm having the same issue.
I started a sea-side fortress to breed merpeople, inspired by the thread about it.
I named one of the miners after myself, and pretty much made him the king of the fortress (oh, the irony).
Anyway, once I had my fortress capable to sustaining its own population, I started to create a channel to catch the merpeople. And when it was done, I actually did catch 7 mermaids and 1 merman: perfect for breeding!
However, just as I was about to pump the water out of the channel to take the cages, my miner guy fell in the channel and drowned. I "alt+f4"-crashed the game to load up where I was just before I had finished the channel.. but now the merpeople arent showing up anymore! In fact, for the last 2 years I havent had any creatures enter my map at all!
Any clues as to why this may happen?
edit: Oh, I did kill about 10 slugmen and 10 snailmen on my map, which were the first animals to arrive here. Also, I'm on an island; I'm not sure how that is going to affect widlife respawning.
edit2:
It would seem it has fixed itself. There was 1 last merman just.. sitting still on the bottom of the ocean. So I built a bridge over it and to scare it away. Once it was gone, a new wave of merpeople arrived (as well as a new trading caravan - finally).