On 16 March 2011, the freighter MS Oliva ran aground on Nightingale Island, spilling tons of heavy fuel oil into the ocean, leaving an oil slick threatening the island's population of <answer>rockhopper penguins</answer>. Nightingale Island has no fresh water, so the penguins were transported to Tristan da Cunha for cleaning.
what animal was threatened with the fuel oil spill?
rockhopper penguins