In 2016, 13 sperm whales washed up on a beach in Germany.

Sadly, this was part of a trend: Throughout Europe, more than 30 sperm whales beached in the first three months of the year. Scientists have attempted to determine why the whales washed up in the North Sea, where they don’t typically hunt. As a part of their research, they performed an autopsy on the whales from the German beach.

What they found was a depressing reminder of man’s impact on the ocean. According to a press release from Wadden Sea National Park, four of the mammals had large amounts of plastic in their stomachs.

That include a 13-meter-long fishing net, a plastic cover from the engine of a car, and part of a plastic bucket. Scientists believe that the whales sometimes mistake plastic for prey; other times, they simply eat floating waste to stop themselves from feeling hungry. The large ocean animals can survive after eating some plastic, but the trash can cause them to starve over time.

However, the plastic waste probably doesn’t explain the mass beachings.

Currently, scientists don’t believe that the whales died due to the plastic. They note that the sperm whales didn’t have any problems with their internal organs; however, they had starved due to a lack of prey.

Humans aren’t off the hook completely, of course. Some scientists believe that the whales’ prey shifted habitats due to climate change. In particular, recent northeastern Atlantic storms may have caused calamari to leave the North Sea. Calamari are one of the main food sources for sperm whales.

The prevailing theory is that the sperm whales follows other prey into the North Sea, which isn’t as deep as the whales’ normal hunting grounds. Sperm whales are some of the deepest divers in the ocean, so they may have become confused in the shallower waters.

Some scientists have other theories. National Geographic reports that ships and drilling surveys may cause noise pollution that disorients certain species of whales, eventually causing beachings. Some even believe that changes in the Earth’s magnetic fields have caused the recent issues.

In any case, the large amounts of plastic found in these sperm whales’ stomachs still present a serious issue. Malnutrition is a growing problem among dwindling whale populations, and human trash affects every part of the ocean food chain.

“The plastic debris in [the whales’] stomachs is a horrible indictment of humans,” said Hal Whitehead, a whale researcher at Dalhousie University in Nova Scotia, Canada, to National Geographic.

Complicating matters further is the lack of information on total sperm whale populations worldwide. Because the whales dive so deep, scientists have trouble keeping track of them.

Current estimates suggest that there are at least 200,000 sperm whales worldwide and perhaps as many as 1 million. As such, sperm whales are classified as “vulnerable,” and their conservation outlooks is thought to be relatively positive when compared to other whales. Sperm whales are protected by conservation laws worldwide, and they’re no longer hunted in most parts of the world.