‘Sea unicorns’ are especially sensitive to human noise | Science

One of the ocean’s most legendary creatures is also one of the most sensitive to human noise. A new study finds the unicornlike narwhal is affected by ship sounds more than most other whales and dolphins. The noises—which are growing more frequent as ships push further into the narwhal’s Arctic habitat—could disrupt the whales’ hunting patterns.

“It’s a great study that shows how sensitive narwhals are to [human] noise,” says marine ecologist Kristin Laidre of the University of Washington, Seattle, who was not involved with the work.

Dolphins and whales—collectively known as cetaceans—rely on sound for nearly everything they do, including hunting and communicating. Narwhals, specifically, use echolocation to hunt for food, including fish, squid, and shrimp. They utilize a series of clicking sounds to locate their prey, increasing the click rate the closer they get. As the narwhals zone in on their meal, the clicking gets so fast that it sounds like a buzz.

Previous research suggested that, of seven large marine mammals in the Arctic, including polar bears and beluga whales, narwhals might be the most vulnerable to boat activity. But that conclusion was based on a smattering of data about animal-ship collisions and observations of how ship traffic interferes with mating, migrating, and foraging in the various species. Researchers had yet to test how narwhals respond to ship traffic—or other human activity—in the real world.

So a team led by ecologist Outi Tervo of the Greenland Institute of Natural Resources partnered with Inuit hunters to study a small group of narwhals in a fjord in East Greenland. Because scientists have observed this group for more than 10 years, the narwhals’ normal behavior was well-established and provided a good baseline.

The group captured six male narwhals by chasing them into nets from the shore. Next, the researchers attached small instruments with GPS receivers and acoustic monitoring systems to their backs. The equipment let the scientists monitor the whales’ buzzing and measure how their hunting activity changed in response to sound disturbances.

The team monitored the buzzing behavior of the narwhals ranging from hours to days and recorded their location for months. This was enough time to watch how the individuals reacted to ship noise and underwater airgun blasts—meant to simulate booming seismic surveys looking for oil and gas in the Arctic.

At 12 kilometers from the noisy ships, the narwhals buzzed, on average, half as much as normal, and all of them ceased buzzing entirely within 6 to 7 kilometers of the sound. But they changed their behaviors at even farther distances: At about 40 kilometers from the ships and airgun blasts, some of the animals started to slightly reduce their rates of buzzing. Those results rank narwhals with beaked whales and harbor porpoises as the cetaceans most easily perturbed by human sounds, the team reports in Biology Letters.

The finding came as a surprise to the scientists because the sound disturbances perceptible to the narwhals dipped below the sea’s ambient noise level within just kilometers from the source. It was assumed these sounds would be imperceptible to the narwhals beyond a few kilometers from the boat, Tervo notes. “But here we show that the narwhals could detect … these foreign sounds far, far away and at much lower levels than the background noise.”

The strong reactions of the narwhals are worrying, says ecologist Greg Breed of the University of Alaska, Fairbanks. He notes, however, that the experiment occurred during narwhal-hunting season—and the whales may have been on extra high alert, especially because they had recently been captured.

Tervo counters that because the narwhals behaved normally both before and after the sound disturbances, the noise is directly responsible for the animals’ reactions. Also, because the sounds used in the experiment were new to the narwhals, she argues, they had no reason to associate the noises with humans.

The results highlight the potential harm of ship noise to Arctic wildlife, Tervo says. She argues that boaters may need to consider slower ship speeds or quieter engines when passing through Arctic waters.