The cause of death of the sperm whales which washed up on the Dutch and German Wadden Sea islands in January has not yet been established but many of the mammals had ingested large quantities of plastic, RTL said on Tuesday.

In total, 29 whales died in the North Sea, five of which stranded on the Dutch island of Texel.

‘One of the animals had a large net in its stomach,’ biologist Mardik Leopold told RTL. ‘Others also contained plastic rubbish.’ However, despite some suggestions that the plastic had caused their deaths by filling their stomachs, this is unlikely to be the case, Leopold said.

‘Sperm whales’ stomachs are so big that an adult male would fit inside,’ he said. In addition, the whales would vomit the plastic up in time, he said. ‘It would only be dangerous if it reached their intestines, where it could cause blockages.’

The whales which died on Texel were fat and had not died of starvation, he said.

Sperm whales eat mainly squid but tend to swallow whatever is in the water. According to a press release from the Wadden Sea National Park in Germany, the garbage the whales had eaten included a 13-meter-long shrimp fishing net, a plastic car engine cover, and the remains of a plastic bucket.