The 45-foot-long (14-meter-long) fish, still largely a mystery to scientists, is considered a vulnerable species by the International Union for Conservation of Nature.

(See "World's Largest Shark Species at Risk, Expert Says.")

"This spill's impact came at the worst possible time and in the worst possible location for whale sharks," said biologist Eric Hoffmayer, who studies whale sharks at the University of Southern Mississippi's Gulf Coast Research Laboratory.

Sightings confirmed that the animals were unable to avoid the slick at the surface, where the giant fish may feed for seven to eight hours a day. The oil may have clogged the fish's gills, suffocating