One of the most densely populated cities in the world is Manilla, Phillipines at 111,002 people per square mile. However, there is a small barrier island in the Sea of Chuckchi on the coast of Alaska. This is home to an estimated 10,000 Pacific Walruses.
An average walrus weighs 2,000 pounds while an average human weighs about 185 pounds. 2,000 divided by 185 is about equal to 11 people per walrus. With 10,000 walruses, there would be about 110,000 people in a small island less than a square mile.