It's a mystery that has left scientists baffled. Over the last few weeks, 500 dead pelicans have been found along a 70km (40-mile) stretch of Peru's northern coast. Other dead marine creatures have also been discovered, including 54 boobies, five sea lions and a lone turtle. This follows the discovery of more than 800 dead dolphins in the same area between January and April.

Unravelling the mystery

The last time pelicans died in such numbers was in 1997: then, the birds had starved to death because of a shortage of anchovies associated with El Niño (a warming of the Pacific waters that happens every five years or so). But this isn't an El Niño year. The mystery is deepened by the fact that the pelicans all seem to have died on the beach, none at sea.

And what, if anything, links the pelicans to the dolphin deaths? Tests on the dolphins have, according to the Peruvian Maritime Institute, eliminated starvation, interaction with fisheries, pesticide poisoning, biotoxin poisoning and contamination by heavy metals. The deputy environment minister has said that the most likely cause of death is a morbillivirus. Similar viruses – related to measles – have been responsible for dolphin die-offs elsewhere.

But it may be that human activity lies behind all this. A virus is much more likely to prove deadly if the dolphins' immune systems have been weakened by high levels of pollution. And environmentalists have pointed out that the dolphins' deaths seemed to coincide with seismic oil exploration, which involves setting off explosions underwater: damage to the dolphins' delicate sonar organs, or sheer panic, might have killed them. However, the fisheries ministry says that initial reports have ruled this out.

Until answers are found, the possibility of more deaths remains – and uncertainty hangs over the future of Peru's fishing industry and its marine wildlife.

Read more: Learn more about the mysteries hitting the Peruvian coastline.