The mystery continues.

Marine investigators in Peru say they still have no idea what is causing a mass die-off of dolphins and porpoises along the country’s coast. About 900 dolphins and 4,450 pelican carcasses have washed up onshore, triggering a massive investigation into the cause of deaths.

A final report by the Sea Institute ruled out viral and bacterial infections, human intervention, pesticides or heavy metals as causes for the deaths, first noticed on Feb. 7 and continued through mid-April. It speculated that biotoxins, algae blooms, or an unknown emerging disease could be to blame.

The Peruvian environmental group Orca, which first alerted the public to the deaths, insists that seismic testing used in oil exploration was likely the cause.

But the Institute said that experts found no evidence any of the deaths were a result of seismic soundings, which involve shooting compressed air at the sea floor: There were no signs of internal hemorrhages or brain lesions that would be compatible with damage from such tests. But it said it did notice damage to some plankton where the soundings were done.

Orca contested those findings in its own report on Tuesday, saying it had independently confirmed hemorrhages and middle-ear infections as well as the presence of air bubbles in internal organs and severe lung damage.

Several leading Peruvian scientists complained that the government agency was late in gathering samples, making it harder to determine the cause of death because the tissue tested was so badly decomposed.

The Sea institute based its findings on autopsies of just two dead dolphins, which were collected in mid-April, while Orca said it gathered the first of the samples it tested on Feb. 12.

Seismic testing in the area was conducted between Feb. 7 and April 8 by Houston-based BPZ Energy.

The Institute report said the testing occurred 50 to 80 miles (80 to 130 kilometers) off shore and that the equipment used was calibrated in those waters between Jan. 31 and Feb. 7.

It said testing also ruled out morbillivirus, a type of distemper that some government officials had suggested as a likely cause long before kits arrived from the United States to check for it.

Experts have said the pelicans are dying because unusually warm ocean temperatures is causing some anchovies to sink deeper into the ocean, beyond the reach of many young pelicans, causing them to starve.

Marine wildlife in Peru is among the most abundant in the world.

Based on reporting by The Associated Press.

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