Narwhals hit moorings—questioning safety assumptions of oceanographic monitoring in the Arctic

Underwater passive acoustic recording is vital for researchers to monitor and study marine animals in their natural environment with minimal disturbance.

“Using passive acoustic monitoring to detect acoustically active animals helps to census biodiversity, understand animal behavior and habitat use, and reduce the negative impacts of human-made noise,” said Associate Professor Evgeny A. Podolskiy of the Arctic Research Center at Hokkaido University in Sapporo, Japan. “For these reasons, scientists increasingly rely on passive acoustic monitoring to answer fundamental ecological questions and manage conservation.”

Endemic Arctic whales, the narwhals, however, seem to have developed a strong interest in underwater passive recording devices. Over the course of a two-year study in Greenland, narwhals repeatedly approached, scanned, and hit deep-sea hydrophones.

In a study published in Communications Biology, Podolskiy and colleagues from Hokkaido University and the National Institute of Polar Research analyzed data from Inglefield Bredning Fjord in northwest Greenland. They collaborated with local Inughuit hunters to deploy and retrieve three underwater acoustic recording devices to depths ranging from 190 m to 400 m.

The devices recorded sounds between August 2022 and May 2024, including those of narwhals repeatedly knocking and rubbing against them, of echolocation clicks, and of the foraging ‘buzz’ of narwhals, which got louder as they approached the devices.

Altogether, there were 247 incidents of narwhal hits in more than 4,000 hours of audio records. These hits were detected on the two deepest recording devices located 25 km apart. Given that the recordings were not continuous and had ~15 min pauses, the researchers estimated that the true number of narwhal hits on the two devices could have been as much as 484-613 over the two months of narwhal presence in the area, representing an average of 10-11 hits a day, mostly during daytime hours.

“Our results suggest that narwhals repeatedly dived to visit the moorings out of playful curiosity or, more likely, due to confusion with potential prey,” Dr. Podolskiy said.

The study included an analysis of the stomach contents of 16 narwhals, caught in the same area in August 2022 and 2023 by Inughuit hunters as part of their subsistence harvest. The analysis revealed that narwhal diet was primarily composed of cod, with smaller amounts of shrimp and squid. The team also found stones in the stomachs.

The researchers speculate that the narwhals might confuse the recording devices with cod or halibut near the seafloor, although they noted that narwhals might be able to distinguish fine differences in texture and density through echolocation, as other toothed whales.

They also noted that the devices sometimes picked up a prolonged ‘rubbing’ sound after the foraging buzz and hit, which they suggested could be the sound of the animal’s skin sliding past the microphone. “Though little is known about molting in narwhals, mooring rubbing could be the associated behavior,” they wrote.

“Inughuit hunters were not surprised by the discovered interaction: they are familiar with narwhal entanglement in unattended gear. They also believe that narwhals like to play and are told so by their parents, and joked that narwhals might scratch their backs, like cats. While this is possible, and other arctic whales are known to rub their bodies over rocks, it is unlikely due to the high energetic costs of deep diving,” Dr. Podolskiy said.

The findings raise questions about whether passive acoustic monitoring is actually as non-invasive as it’s thought to be. The paper demonstrates that narwhals are not deterred but rather attracted to scientific moorings, showing that the presence of these artificial devices can affect the behavior of this species. The authors suggest that short mooring lines might be a simple precaution to minimize inadvertent effects of observations indispensable for conservation and management.

“Understanding animals’ interaction with industrial and scientific infrastructure can help reduce impacts on wild animals and improve our ability to implement and interpret autonomous field observations,” Podolskiy said.