Currently, the greatest ecological threat is the longline fishing of Patagonian toothfish, which endangers a number of seabirds that dive into the water after baited hooks. The wildlife is particularly vulnerable to introduced species and the historical problem has been with cats and mice. House mice arrived to Marion Island with whaling and sealing ships in the 1800s and quickly multiplied, so much so that in 1949, five domestic cats were brought to the research base to deal with them. The cats multiplied quickly, and by 1977 there were approximately 3,400 cats on the island, feeding on burrowing petrels in addition to mice, and taking an estimated 455,000 petrels a year. Some species of petrels soon disappeared from Marion Island, and a cat eradication programme was established. A few cats were intentionally infected with the highly specific feline panleukopenia virus, which reduced the cat population to about 600 by 1982. The remaining cats were killed by nocturnal shooting, and in 1991 only eight cats were trapped in a 12-month period. It is believed that no cats remain on Marion Island today, and with the cats gone, the mouse population has sharply increased to "plague like" levels. In 2003, ornithologists discovered that in the absence of other food sources, the mice were attacking albatross chicks and eating them alive as they sat helplessly on their nests. A similar problem has been observed on Gough Island, where a mouse eradication programme is currently planned to begin in 2019, with the island expected to be mouse free by 2021. A programme to eradicate invasive rats on South Georgia Island was completed in 2015, and as of 2016 the island appears to be completely rat free. The geography of Marion Island presents certain obstacles not found on either Gough or South Georgia islands, particularly its large size, high elevations and variable weather. An assessment of the island was completed in May 2015, led by noted invasive species ecologist John Parkes, with the general conclusion that an eradication programme is feasible, but will require precise planning. Both Gough Island and the Prince Edward Islands also suffer from invasive procumbent pearlwort ("Sagina procumbens"), which is transforming the upland ecosystem and is now considered beyond control. Marion Island and Prince Edward Island were claimed for South Africa on 29 December 1947 and 4 January 1948 respectively, by a South African Navy force from HMSAS "Transvaal" under the command of John Fairbairn. On 1 October 1948 the annexation was made official when Governor-General Gideon Brand van Zyl signed the Prince Edward Islands Act, 1948.