If that's how it works it must dramatically lower the megabeast rate in the first place.
Oh mercy, don't decrease that, its one of the tried and true Fun aspects of DF that has been around for awhile. If anything bump megabeasts and semimegabeasts up to the hundreds and thousands, respectfully, in advanced worldgen.
I seldom breach a cavern wall, like you. But I enjoy lots of topside activity with so much megafauna in the world. Do forgottens even constitute megafauna? *shrug*
I smooth and fortify(murder hole) a leak into the three caverns. Try that.
The cracks in fortify seems to provide spores safely into a fortress, unless I'm mistakenly getting it from somewhere else.
We get zero subterranean water life any longer thru a murder hole/leaks. I don't even know if we get it normally in the underground lakes. Haven't fished one of those in a dozen embarks or so.
@vondus, I butcher the bejeebus out of anything that grazes, whether fully grown or small in status. Tame animals go to meeting areas, and mingle. Not sure why they don't go to where the food is if unobstructed, like real tame animals tend to do. They must die waiting for the dinner bell.
Way I avoid it, is to wall them in with brick or block, or slice down the ramps up to a highland knoll, they also makes a nice natural wall. This keeps them safe, somewhat, so that I wouldn't have to unpen them.