Beemer,

I can't recall being out there in October but I can say from first-hand experience that they SEEMED to be mostly gone by mid-November (it was a late freeze that year).  Still, I'm sure you're absolutely right.  

I was up on Cirrus at the tail-end of September a couple years back & the little devils were completely brazen.  I'd be about to dip my spoon into my oatmeal bowl sitting on rock beside the campfire &, instead, I'd end up tossing it at the gooped-up furry head that was poking around & staring back at me!!  Voles, I think.  You could find bunches of them just by pulling off a few of the campfire rocks.  They were everywhere!!  

Rumor has it that Phantom Jug uses live voles routinely as Northern Pike bait...just as I was sorely tempted to.  Being duly mindful & respectful of Park regulations re: use of live bait, of course, I'd never do THAT.

No, no... rather I'd just hook up some of the dead ones that I happened to find next to my campfire after each meal....

Jimbo