Observation my good people (while we wait for Aya to return from her scouting). I can't help but notice that the route down below was infested with those unsavory vermin. It also occured to me that there were but four ways out of that system of the sewers. One, the way we came in, which was locked, barring greater intelligence and a rather unlikely understanding of advanced lockpicking, I doubt said vermin got their sustenance from that direction. Two, the caved in area near their lair, and though it seemed sufficiently pourus enough to allow seepage, I find it unlikely that it allowed them egress for hunting. Three, the rushing, poorly maintained downstream flow, though surely they can easily navigate down the flow, unless they had a great deal more akin to salmon then they appeared, that would be one-way hunting.
And finally, four, the ladder we came up. Considering their humanoid form, I suspect that the ladder was navigable for them, thus they hunt the sick up here, which seem to have an open doorway to their predatory neighbors. Bearing these facts in mind, and noticing that they aren't paying undue attention to our means of entry, indeed they leave that very unguarded doorway open, this leads to an alarming conclusion. Someone, either the ill or the Slave Catcher himself disposes of enough unfortunates by this path, to feed that rather, formerly, sizeable host.
Merely a thought my stalwart companions, merely a thought. *smiling pleasantly*