What phenomenon occurs in highly regulated zones where owners still possess pitbull-type dogs?

Answer

A "shadow population" of dogs whose existence is kept out of public notice.

In areas with strict regulations, owners who continue to keep these dogs often take extreme measures to keep them hidden from public view, resulting in an uncounted population that exists but is not statistically visible.

What phenomenon occurs in highly regulated zones where owners still possess pitbull-type dogs?
locationDogpitbullbreedCanine