The response of narwhals to the loud noise of seismic air guns used in oil exploration involves disruption of the normal physiological response to strenuous exercise as the animals attempt to escape the noise. The overall effect is a sharp increase in the energy cost of diving while a paradoxically reduced heart rate impairs blood and oxygen circulation.
“They swim as hard as they can to escape, and yet their heart rate doesn’t increase – we think because of a fear response. It affects the amount of blood and oxygen that can flow, and that’s going to be problematic,” said Terrie Williams, professor of ecology and evolutionary biology at UC Santa Cruz, who led the new study.
Published on July 8 in the Journal of Functional Ecology, the study provides a first insight into the impact of seismic noise on the physiological responses of a deep-diving cetacean. According to Williams, the combination of extremely low heart rates, increased heart rate variability, and high-intensity exercise during deep dives presents a significant physiological challenge for narwhals, especially if disturbances are prolonged, such as this. would be likely during prolonged oil exploration activities.
Narwhals live year-round in High Arctic waters where sea ice has isolated them from human disturbance for millions of years. But the decline of the polar ice cap makes the region more accessible to navigation, the exploration of natural resources and other human activities.
In a previous study, Williams and coauthors showed that narwhals released after becoming entangled in nets set by native hunters exhibited a similar physiological response, with extremely low heart rates during intense exercise in a series of dives. of escape. The difference between a capture event and noise, Williams said, is the potential duration of the disturbance.
“When they escape from the nets, their heart rate returns to a more normal rate within three or four dives, but with the seismic vessel moving and the sound bouncing, the escape response occurred on a longer long time,” she said. .
Researchers not only recorded extremely low heart rates during noise exposure, but also increased variability, with heart rates rapidly changing from extremely low frequencies associated with fear to fast rates associated with intense exercise. A reduced heart rate, or bradycardia, is a normal part of the mammalian diving response, but during normal dives heart rate always increases with exercise. Additionally, deep-diving narwhals and other marine mammals generally save energy by gliding rather than actively swimming when descending to depth.
During noise exposure, the narwhals glided 80% less during dive descents, their swimming strokes exceeded 40 strokes per minute, their heart rate fell below 10 beats per minute, and their respiration fell to the surface was 1.5 times faster. Overall, this unusual reaction is very costly in terms of energy consumption, Williams said.
“Not only is the reaction costly in terms of the energy needed to dive, but the escape time will also reduce time spent foraging for food and other normal behaviors,” she said.
The studies were conducted at Scoresby Sound on the east coast of Greenland, where co-author Mads Peter Heide-Jørgensen, a research professor at the Greenland Institute of Natural Resources, has been studying the East Greenland narwhal population for over of a decade.
Williams’ group at UC Santa Cruz has developed instruments that allow researchers to monitor the exercise physiology of marine mammals during dives. The instruments were attached to narwhals with suction cups and dropped after one to three days, floating to the surface where they could be retrieved by scientists.
Over the past two decades, noise from human activities such as military sonar has been linked to mass strandings of deep-diving cetaceans, primarily beaked whales. These deep-diving species are extremely difficult to study, and it was only through a partnership with native hunters that Williams and Heide-Jørgensen’s teams were able to attach tracking devices to the narwhals.
“Most of the potential animal impacts take place underwater, so it’s really hard to study,” Williams said. “We are fortunate to have this technology to show what is going on deep down where these animals live to understand how their biology can be disrupted.”
In addition to Williams and Heide-Jørgensen, co-authors of the paper include Susan Blackwell of Greeneridge Sciences, Outi Tervo and Eva Garde of the Greenland Institute of Natural Resources, Mikkel-Holger Sinding of the University of Copenhagen, and Beau Richter of the UC Santa Cruz. . This work was supported by the United States Office of Naval Research, the Greenland Institute of Natural Resources, the Environmental Agency for Mineral Resource Activities of the Government of Greenland, the Danish Ministry of the Environment, and Carlsberg Foundation.