At least 500 dolphins were found dead in the past few weeks on different beaches in northern Peru and the cause of death is still under investigation, media reports said.
The Peruvian Sea Institute, or Imarpe, sent a team of scientists to investigate why the dolphins beached themselves in the northern regions of Lambayeque and Piura, the El Comercio newspaper reported.
Other beachings of dolphins occurred in the past two years in northern Peru.
The team covered a 142-kilometer (88-mile) stretch of coast on Jan. 28-29, traveling from Pimentel, a resort city in Lambayeque, to the southern part of the reserve in Illescas, located in Piura, the newspaper said, citing officials.
Experts found at least 400 beached dolphins, with the discovery coming after about 100 other dolphins beached themselves in recent weeks.
Fishermen told the Imarpe team that the dolphins were caught in nets regularly and drowned, the newspaper said.
The scientists, however, confirmed that some young and adult dolphins died at sea and others arrived on the beaches near death.
Tests conducted on tissue samples in Lima determined that the marine mammals were not poisoned by fishermen and did not die from the effects of extractive activities in the regions.
The marine mammals may have died from ingesting toxic algae, the head of Imarpe's Lambayeque office, Jaime de la Cruz, told El Comercio.
Officials are concerned about the deaths because dolphins approach the coast at this time of year to mate and feed, De la Cruz said.
In 2012, about 800 dolphins were found dead in central and northern Peru, with Imarpe concluding in a report that the animals died from natural causes.
Contact with fishing vessels, pesticides, pollution from heavy metals and seismic mining exploration were among the human causes ruled out.
The report, however, also ruled out some possible natural causes of the deaths, including lack of food, bacterial infections, viral infections and biotoxins.
Hundreds of seabirds were also found dead in northern and central Peru in early 2012.
The deaths of more than 1,200 pelicans were due to lack of food because of the migration of cold water fish as a result of the warming of ocean waters, officials said.
Hundreds of dead seabirds were found along a 200-kilometer (124-mile) stretch of beaches in the Piura and Lambayeque regions in early 2012, Peruvian media reported.