The document asserts that the marine mammals did not die from lack of food, hunting by fishermen, poisoning by pesticides or due to heavy metal contamination.
But the local conservation group ORCA disputes the official findings, reasserting its allegations that powerful underwater sonar used by oil exploration is what killed the sound-sensitive animals.
“We found cells that had injuries due to bubbles that are associated with decompression sickness,” said Carlos Yaipén-Llanos, director of ORCA.
He and colleagues say the victims suffered from rapid decompression caused when they were scared to the surface by the sonar.
Houston-based BPZ Resources has said it conducted seismic surveys starting on Feb. 8 in part of the affected area.
But the company assures that it adhered to strict environmental standards used elsewhere around the world. It also says that the first deaths happened before it began those operations.
Thousands of dead seabirds have also been found along the same stretch of coast, and southward to central Chile during the same period.
But experts say climate-driven migration of anchovies appears responsible for those starvation deaths, and there is no link between them and the nearly 900 dolphin fatalities.