note that the operative phrase is "could have". In other words, they really don't know.
From Express.co.uk: DID SONAR KILL 3,000 DOLPHINS?
EXPERTS believe the deaths of nearly 3,000 dolphins washed up on beaches in Peru could have been caused by sonar blasts used by firms to find oil under the sea.
The biggest of a number of beachings involved 1,500 of the mammals. They were found by staff from a marine coastal reserve in Piura. Just two weeks later later 615 dolphins washed up dead on beaches in the city of Lambayeque last month.
Peruvian scientists are investigating acoustic surveying carried out by companies looking for oil. The sonar blasts can damage the dolphins’ ears, causing intense pain and difficulty breathing.
Dr Carlos Yaipen-Llanos, who is carrying out tests on the species, said: “Evidence points to acoustic impact but we are also looking to see if any diseases added to this unusual mortality event.” British environmentalist Mark Simmonds, of the Whale and Dolphin Conservation Society, believes fishing nets may also be to blame. He said similar incidents happened in Europe 30 years ago when dolphins suffocated after being caught in nets.
About 90 per cent of the dead creatures are long-beaked common dolphins. The rest are Burmiester’s porpoises that come inshore to calve.