This article was originally featured on Hakai Magazine, an online publication about science and society in coastal ecosystems. Read more stories like this at hakaimagazine.com.
In the North Sea, nearly 100 meters underwater, the seafloor is littered with more than 40,000 shallow pits in the sand. The pockmarks, sometimes spanning more than 10 meters, come in a variety of sizes and odd shapes. While some look like long furrows, half-moons, or concentric circles of sand, others are ringed by mounds of sediment.
When he first saw the pockmarks, Jens Schneider von Deimling, a marine geophysicist at the University of Kiel in Germany, wondered whether they were evidence of methane seeping from the sediment. Methane seeps are often sites of unique seafloor communities that live off the gas the way plants live off sunlight. Methane is also a short-lived but potent, climate change–inducing molecule: over just 20 years, the greenhouse gas can trap 84 times more heat than carbon dioxide. So if a lot of methane were bubbling out of the North Sea, scientists would want to know about it.
But the physical appearance of these seafloor marks weren’t like those seen at typical methane seeps. Gas burping out of the seafloor and into the water tends to leave a distinctly circular pit with a conical bottom. Schneider von Deimling was puzzled. “The [pockmarks] looked really peculiar,” he says. It looked as if someone had disturbed the sand from above.
Schneider von Deimling’s team investigated by analyzing millions of preexisting scans of the area made with a multibeam echo sounder, a piece of equipment that shoots out sound waves and measures how they bounce back—much like how sonar works. The approach gave the scientists highly detailed images of the curious cavities, confirming the pits’ unusual shapes. And when the researchers filmed the seafloor, they couldn’t find any methane-reliant organisms living nearby. The team also made new scans to see how the area changed over a year: not only did new pits appear, but old ones widened or merged with neighbors, a change not usually seen with gas seeps.
Schneider von Deimling was stumped. But his colleagues who study marine mammals offered what is now the scientists’ most likely explanation for the seafloor pits: hungry harbor porpoises.
In previous research, scientists have found grains of sand in the stomachs of stranded harbor porpoises. They’ve also found the remains of sand eels, small fish that bury themselves in the seafloor. Perhaps porpoises are grubbing in the sand to scare sand eels out of hiding, creating these strange pits as they vacuum up their quarry?
So far, it’s just an idea. Researchers know harbor porpoises feed during their long dives, and they’ve seen captive porpoises digging in the sand. But no one has actually caught a wild harbor porpoise in the act of disturbing the seafloor.
Magnus Wahlberg, who studies cetacean biology at the University of Southern Denmark and wasn’t involved in the research, says harbor porpoises are skittish, hard to identify, and difficult to follow. But Wahlberg has seen harbor porpoises poking into stones and algae, likely to reveal small fish, and says the cetaceans change their foraging techniques depending on the available food.
The North Sea is home to many porpoises and many sand eels. “If I were a porpoise, I would definitely spend my time poking around in the sand for them,” says Wahlberg.
Schneider von Deimling says researchers have found similar pits around Ireland’s Aran Islands and in the English Channel, other places with harbor porpoises and sand eels but no underwater gas seeps. He’s now continuing his research studying the seafloor off Canada and New Zealand.
If this foraging behavior is as common as harbor porpoises—there are roughly 700,000 spread around the planet—then identifying porpoise habitat could be as simple as looking for the holes they dig.
This article first appeared in Hakai Magazine and is republished here with permission.
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