(Apologies for necromancy, but it's hopefully no worse than creating another topic about this.)
In my quest to found the perfect fortress (v47.05 because I started before the v50 Linux build), I recently attempted to "transfer" captured moghoppers from a temporary embark to my chosen main site (which otherwise lacked little) via the medium of adventurer.
Unfortunately, it failed: when released into the wild, moghoppers eventually disappear like all vermin, but a new region population is not increased, unlike with native vermin. Probably because the entry for them doesn't exist to begin with!
I did learn some facts about moghopper capture: for one, the "capture live fish" job appears to ignore them, likely because they lack the [VERMIN_FISH] token (see also
this post).
Baited traps
might work, but there's apparently some internal vermin type order, and until all the vermin higher on the list are extinct, no amount of savescumming will yield moghoppers.
"Capture live land animal" is what I ended up using. Unlike the former two, it actually targets a vermin on the map (as opposed to abstract region populations), so I put a barrel of booze near each pool (which were conveniently evaporated at the time) and guided my trapper to every sighted moghopper by forbidding every barrel except the nearest one (and all other food, too). Awful lot of micromanagement, but burrows didn't work.
I ended up with 10 of them, and it took a
lot of effort! Shame it was for naught.
Another thing I noticed is when a spawned vermin disappears back into the region population, sometimes it's increased by more than 1. That's how they replenish, I suppose.
Oh, and I also tried to catch moghoppers in adventure mode first, but apparently pond vermin don't show up on their own there, and [L]ooking for them doesn't work because they're not [VERMIN_SOIL]. (You can dig up mussels as an adventurer, though, which is likely the easiest way to stuff the future fort with shells.)
That temporary embark also caught and tamed several giraffes -- I wanted to see if I can raid for them from the main fort, but after the moghopper failure I'm starting another world so I can't yet say if it's a viable strategy.