This BKaufman seems to imply the problem started happening in 1980`s.
Quote
The non-native animals were brought to Western Canada from Europe in the 1980s to be hunted for sport but were officially designated a pest in 2008 after numbers of them escaped from their enclosures.
I remember as a youngster back in the late '40`s, Dad having shot two on the farm we lived on. He said they were tame pigs that had escaped years earlier from some farms, turned wild and that there were more around.
I remember back in the mid 70`s seeing one south of Greencourt, deep in the bush while moose hunting.
So much for BKaufman`s theory.