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.

Related Questions
Is "Pitbull" recognized as a single, officially recognized breed by the American Kennel Club (AKC)?What two types of dogs were crossed historically to develop the dogs commonly labeled as pitbulls?What gruesome sports were these dogs initially bred for?How did these dogs establish themselves in new territories like the United States historically?How is the contemporary presence of pitbull-type dogs described globally?What is the defining factor determining where pitbull-type dogs are not found openly?What phenomenon occurs in highly regulated zones where owners still possess pitbull-type dogs?Which facilities are primary locations where the public often interacts with pitbull-type dogs that have been surrendered or found as strays?What complicates the accurate mapping of pitbull populations regarding identification?How drastically does the legal standing of these dogs vary locally?