Narwhals struggle to escape the stress of ship noise

Narwhals, who are often isolated from human disruption, are now impacted from the loud noises of oil exploration. They live in high Arctic waters where sea ice has isolated them from human disturbance for millions of years. Recent declines in polar sea ice have made the region more accessible to human activity like shipping and natural resource exploration.

Narwhals respond to the noise of seismic air guns by swimming away to escape the noise, and this has been found disruptive to their physical capabilities. 

“They’re swimming as hard as they can to get away, and yet their heart rate is not increasing – we think because of a fear response. This affects how much blood and oxygen can circulate, and that’s going to be problematic,” said Terrie Williams, a professor of ecology and evolutionary biology at UC Santa Cruz who led the new study.

The study is a first look at the impact of seismic noise on the physiological responses of a deep-diving cetacean. In narwhals, the combination of low heart rates, increased heart rate variability, and high-intensity exercise presents a significant physiological challenge. This will only intensify as oil exploration, and other human activity continues. 

The researchers recorded extremely low heart rates during noise exposure and increased variability, with heart rates switching rapidly between extremely low rates associated with fear and fast rates associated with intense exercise. 

Reduced heart rate, or bradycardia, is a normal part of the mammalian dive response. Narwhals and other deep-diving marine mammals usually save energy by gliding rather than actively swimming as they descend to depth.

During noise exposure, it was found that narwhals performed 80 percent less gliding, their swimming strokes exceeded 40 strokes per minute, their heart rates dropped below 10 beats per minute, and their breathing at the surface was 1.5 times faster. This abnormal reaction is very costly in terms of energy consumption, explained Professor Williams.

“Not only is the reaction costly in terms of the energy needed for diving, the escape time will also take away from time spent foraging for food and other normal behaviors,” she said.

The research was conducted in Scoresby Sound on the east coast of Greenland, where study co-author Mads Peter Heide-Jørgensen, has been studying the East Greenland narwhal population for more than a decade.

Over the past two decades, noise from human activities such as military sonar has been linked to mass strandings of deep-diving cetaceans, mostly beaked whales. These deep-diving species are extremely difficult to study. By partnering with Indigenous hunters, the research teams were able to attach monitoring devices to narwhals.

“Most of the potential impacts on the animals take place underwater, so it’s really difficult to study,” said Professor Williams. “We are fortunate to have this technology to show what’s happening at depth where these animals live in order to understand how their biology may be disrupted.”

This work was supported by the U.S. Office of Naval Research, Greenland Institute of Natural Resources, the Environmental Agency for Mineral Resource Activities of the Government of Greenland, the Danish Ministry of Environment, and the Carlsberg Foundation.

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By Katherine Bucko, Earth.com Staff Writer