hog migration

I lnow for a fact wild hogs are in maryland. Saw a good sounder coming in to land on pax river. I ask a couple guys in the hanger bout them and said yep, mostly eurasian/domestic blooded hogs. He said they have been there for at least 30yrs, which is how long he has been working on the naval station and said he was told by some old timers when he got there, that wild hogs have been in that area for around 50yrs before he got there. There is also hogs in new york. Had a plt sgt that was born and raised in up state new york near ft drum, and his family (grand dad, dad and himself) hunted them for many yrs.

Now as for the possible reasoning as to the lack of popultation, i would say it has to do with the temperature mostly. Them cold harsh winter kill off the weak.
Now as to them being imported. Were these hogs of today not imported in the 1400s and were free ranged and many generations of people free ranged hogs and hogs getting out of game ranches that possibly were imported onto game ranches? Heck, canada has wild hogs, imported from europe many many yrs ago and have gotten out.