LIMA, Peru — The carcasses of dead pelicans still litter the beaches of northern Peru, even as the last of nearly 900 dolphins are slowly being cleared away.

The mass die-offs have Peruvian scientists searching for a cause and environmentalists raising questions about the government’s ability to protect the Pacific nation’s marine life, among the world’s most abundant thanks to the Humboldt current that hugs most of its 1,500-mile coast.

After weeks of study, investigators say they think they know why the 4,450 pelicans have died: Hotter than usual ocean temperatures have driven a type of anchovy deeper into the sea, beyond the reach of many young pelicans.

But Peruvian scientists studying the deaths of dolphins and porpoises from early February to mid-April say it remains a mystery, due in part to the government’s slowness in investigating the phenomenon.