Scientists searching for the cause of the mass deaths of dolphins and pelicans off the coast of Peru think they know what killed the pelicans.

After weeks of study, investigators say they think they know why the 4450 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 scientists studying the deaths of hundreds 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.

They say the deaths of the dolphins and pelicans were unrelated and it was a coincidence that they happened around the same time.

Authorities were so late in gathering tissue from the mammals that crucial clues were lost, said the scientist heading the dolphin death probe, Armando Hung, head of the molecular biology lab at Cayetano Heredia University.