An oil spill off the South Atlantic island of Nightingale has put nearly half of the world’s population of endangered northern rockhopper penguins (Eudyptes moseleyi) at risk.

The Maltese-registered ship MS Olivia ran aground on Nightingale Island on March 16. The New York Times reported Wednesday that more than 725 metric tons of fuel oil—half the cargo—had already leaked from the ship, surrounding the island in an 13-kilometer-wide oil slick.

Nightingale is home to between 40 and 50 percent of the world’s northern rockhoppers. A 2005 study counted 19,500 pairs on the island. According to the International Bird Rescue Research Center (IBRRC), at least 20,000 penguins have already been caught up in the oil, a number calculated by conducting a visual survey.

Other birds which nest on the island, including several albatross and shearwater species, can fly away from the spill, but penguins must swim through it and are therefore more vulnerable, according to IBRRC executive director emeritus Jay Holcomb, who said the penguins “cannot be removed from the islands and brought to the mainland due to disease-transmission concerns.”

Besides the oil, there’s another potential threat. The ship was also carrying 60,000 metric tons of soybeans, and conservationists fear that rats could have been on board. The island is currently rat-free, and introducing the voracious, egg-eating rodents into the habitat could be devastating to the birds that nest there.

Rescue ships are en route (the Olivia‘s crew has already been rescued), but getting to the penguins in time won’t be easy. Nightingale Island is part of Tristan da Cunha, the world’s most remote inhabited archipelago. A British Overseas Territory, it is located about 2,700 kilometers west of South Africa in the middle of the Atlantic Ocean. Fewer than 300 people live on the islands.

According to a study published in 2009 by BirdLife International, northern rockhopper populations have declined 90 percent since 1950 due to changes in marine ecosystems and overfishing of the penguins’ foods. Northern and southern rockhopper penguins (E. chrysocome) were considered the same species until 2006, when genetic tests differentiated them for the first time. The northern species was listed as endangered by the International Union for Conservation of Nature and Natural Resources in 2008.

Photo via Wikipedia