In the 19th and 20th centuries, the penguins of Penguin Island were victims of dog attacks and shooting by holiday-makers. An informal assessment of the Penguin Island colony was made by Vincent Serventy in 1946. After several visits, he estimated the colony to number approximately pairs. In the 1940s concern was expressed for the viability of the penguin colony on Penguin Island, due to combined threats of human landing parties with guns and dogs, occasional fires, and an abundance of rabbits which were denuding the island of its former vegetation and accelerating its erosion. Rabbits were believed to have been introduced to the island in the 1920s, and numbered approximately four to five thousand in the late 1940s. By 1950, it had become an illegal act to take a dog to Penguin Island.