A study of whales and dolphins that have washed up dead in Greece over a 20-year period has found alarmingly high levels of plastic trash – mostly bags – in the animals' stomachs, which can condemn them to a slow and painful death.

In the worst case, a researcher said Friday, a 5.3-meter young sperm whale beached on the Aegean island of Mykonos had swallowed 135 plastic items weighing a total of 3.2 kilograms.

In a separate incident outside the scope of the Greek study, an 8-meter pregnant sperm whale was found dead off Sardinia in March with 22 kilograms of plastic in its stomach.

The study, which did not examine microplastics that are invisible to the naked eye, found that nine out of 34 whales, porpoises and dolphins washed up dead on Greek shores from 1993-2014 had swallowed plastic debris of considerable size.

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