When seismic air guns are used for oil development, the loud noise causes narwhals to respond. As they attempt to get away from the noise, the animals’ usual physiological response to vigorous exercise is disrupted. The overall result is a significant increase in the energy expenditure of diving, but a paradoxically lowered heart rate affects blood and oxygen circulation.
“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, which was published on July 8, 2022, in the Journal of Functional Ecology, offers the first insight into how seismic noise affects a deep-diving cetacean’s physiological reactions.
Williams claims that the combination of very low heart rates, increased heart rate variability, and high-intensity exercise during deep dives poses a serious physiological challenge for narwhals, particularly if the disruptions are prolonged as would be the case during prolonged oil exploration activities.
Since there has been sea ice in the northern Arctic for millions of years, narwhals have been protected from human influence. However, the arctic sea ice is receding, opening the area up to shipping, resource development, and other human endeavors.
Williams and her coauthors previously demonstrated that narwhals freed from the nets left behind by local hunters exhibited a similar physiological reaction, with incredibly low heart rates after strenuous effort in a series of escape dives. The possible duration of the disruption, according to Williams, is what distinguishes a capture event from noise.
Most of the potential impacts on the animals take place underwater, so it’s really difficult to study. We are fortunate to have this technology to show what’s happening at the depth where these animals live in order to understand how their biology may be disrupted.
Professor Terrie Williams
“When they escape from the nets, their heart rate comes back up to a more normal rate within three or four dives, but with the seismic ship moving through and the sound bouncing around, the escape response occurred over a longer period,” she said.
The researchers observed greater variability with heart rate swings between extremely low rates associated with anxiety and fast rates associated with vigorous exercise, in addition to extremely low heart rates during noise exposure.
The mammalian dive response is normally characterized by bradycardia, yet during typical dives, the heart rate still rises with activity. Narwhals and other deep-diving marine mammals typically conserve energy by gliding rather than swimming aggressively when they drop to depth.
The narwhals’ gliding during diving descents decreased by 80% after noise exposure. They also swam at a rate of over 40 strokes per minute, had heart rates below 10, and breathed more quickly at the surface. 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 for diving, the escape time will also take away from time spent foraging for food and other normal behaviors,” she said.
The research was carried out at Scoresby Sound on Greenland’s east coast, where co-author Mads Peter Heide-Jrgensen, a research professor at the Greenland Institute of Natural Resources, has been researching the East Greenland narwhal population for more than ten years.
Instruments created by Williams’ team at UC Santa Cruz allow scientists to track the exercise physiology of underwater mammals. The suction cups used to connect the instruments to the narwhals broke off after one to three days, floating to the surface where the scientists could retrieve them.
The past two decades have seen a rise in the number of deep-diving cetacean strandings, primarily of beaked whales, which have been connected to noise from human activities like military sonar. Williams and Heide-Jørgensen’s teams Jrgensen’s were only able to observe narwhals thanks to a collaboration with local hunters because these deep-diving species are very challenging to examine.
“Most of the potential impacts on the animals take place underwater, so it’s really difficult to study,” Williams said. “We are fortunate to have this technology to show what’s happening at the depth where these animals live in order to understand how their biology may be disrupted.”
Williams and Heide-Jrgensen are joined by Susan Blackwell from Greeneridge Sciences, Outi Tervo and Eva Garde from the Greenland Institute of Natural Resources, Mikkel-Holger Sinding from the University of Copenhagen, and Beau Richter from the University of California, Santa Cruz as coauthors on the paper.
The 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 all provided funding for this research.
