The approximately 800 dolphin deaths over the past few months in central and northern Peru were likely due to natural causes, the Peruvian Sea Institute, or Imarpe, said in a report.
"Human activities have been ruled out as the direct cause of death," but "the exact cause of death has not been determined," the 81-page report, which was posted on Imarpe's Web site on Tuesday, said.
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.
"Although the exact cause of these deaths has not been determined, it is presumed that they are related to natural causes, whether biotoxin poisoning or emerging diseases, of which there are no prior records," the report said.
"Atypical conditions have been present in the marine ecosystem" since the start of the Southern Hemisphere summer at the end of last year, and "persistent (algae) blooms" have appeared since January between the port of Callao and Paita, a port city in northern Peru, the report said.
The environmental group Orca released a report earlier this week that identified offshore oil exploration as the cause of the dolphin deaths.
The dolphins found on Peru's beaches died in the ocean from emphysema and had "air bubbles in vital internal organs, such as the liver, kidneys, spleen, bladder and lymph nodes," Orca said.
A multidisciplinary commission should be formed to investigate the deaths of the marine mammals and monitor "the continuous changes being experienced in the marine ecosystem," the Imarpe report said.
Hundreds of seabirds were also found dead in northern and central Peru earlier this year, prompting the government to order Imarpe to conduct an investigation.
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, Production Minister Gladys Triveño told Radio Programas del Peru, or RPP, on Tuesday.
"The majority of the ones found dead were very young pelicans that did not have much experience in gathering their own food," Triveño said.
Hundreds of dead seabirds were found along a 200-kilometer (124-mile) stretch of beaches in the Piura and Lambayeque regions earlier this year, Peruvian media reported.
The reports prompted fishermen and environmentalists to call on officials to investigate the deaths of the pelicans and other seabirds. EFE