In June 2014 on the Page Islands in southern Australia, conservationists were counting Australian sea lions as part of a monitoring programme.

While doing so, they came across four dead ones.

They were lying within three metres of each other. Two adults, a male and female, and two pups were found near a lighthouse tower 20 metres above sea level.

Caution: images of dead sea lion below

The pups were about four months old, the male adult was also still young. He was found with his mouth open and his tongue hanging between his teeth.

There was no evidence of any bullet holes or of blunt trauma

All four appeared in otherwise good health. Peter Shaughnessy of the South Australian Museum and one of the monitoring scientists, was at first puzzled by the cause of death.

"The four bodies looked very fresh, were in good condition and it had not been possible for anybody to get on the island for several days."

That's because there had been a terrible storm, making travel to the island impossible. The lighthouse tower was also hit by lightning and damaged.

Shaughnessy had heard of lightning killing caribou in Alaska in the early 1970s when he was a PhD student.

He therefore realised these sea lions could have suffered a similar fate. His suspicions were confirmed upon closer inspection of their bodies.

This is the first reported case of lightning killing a marine mammal

The dead male showed faint jagged linear marks consistent with burning, which would be expected from a lightning strike, Shaughnessy says.

"It was then a process of eliminating other possible causes of mortality. The four dead sea lions were all in good condition, there was no evidence of any bullet holes or of blunt trauma, nobody could have visited the island for several days before our visit because of bad weather."

As their bodies were on top of the island above sea level, it also ruled out the possibility that they drowned. It was doubtful they starved as their bodies would have been emaciated, as was the case with four other dead pups, which were found elsewhere on the island on the same day.

There was also no evidence that they died from toxic algal blooms - a known killer of marine creatures.

While lightning has been known to strike several animals, this is the first reported case of it killing a marine mammal. Shaughnessy and colleagues outline their findings in the journal Marine Mammal Science.

The bodies were too heavy to be transported for further analysis but the researchers are confident lightning was the cause of death.