The response of narwhals to the loud noise of seismic air guns used in the search for oil is to disrupt the normal physiological response to intense exercise as the animals try to escape the noise. The overall effect is to greatly increase the energy expenditure of diving, while the paradoxically reduced heart rate alters the circulation of blood and oxygen.
“They swim as hard as they can to get away, and yet their heart rate doesn’t increase – we think it’s because of the fear response. That affects how much blood and oxygen can circulate, and that’s going to be problematic,” said Teri Williams, a professor of ecology and evolutionary biology at UC Santa Cruz, who led the new study.
Published on July 8 in Art Journal of Functional EcologyThe study provides the first insight into the effects of seismic noise on the physiological responses of deep-diving cetaceans. According to Williams, the combination of extremely low heart rates, increased heart rate variability, and high exercise intensity during deep dives presents a significant physiological challenge for narwhals, especially if disruptions are prolonged, as would likely be the case during extended oil exploration operations.
Narwhals live year-round in the high Arctic waters, where sea ice has helped isolate them from humans for millions of years. But declining polar sea ice makes the region more accessible to shipping, natural resource exploration and other human activities.
In a previous study, Williams and her co-authors showed that narwhals released after becoming entangled in nets set by local hunters showed a similar physiological response, with extremely low heart rates, during intense exercise in a series of rescue dives. The difference between a seizure event and a noise event, Williams said, is the potential duration of the disturbance.
“When they escape from the nets, their heart rate returns to more normal within three or four dives, but because the seismic ship was moving through it and the sound was bouncing around, the escape response took longer.” she said. .
The researchers recorded not only extremely low heart rates during noise exposure, but also increased heart rate variability, in which heart rates rapidly switched between extremely low rates associated with fear and rapid heart rates associated with intense exercise. A decrease in heart rate, or bradycardia, is a normal part of the mammalian response to diving, but during normal dives the heart rate still increases with exercise. In addition, narwhals and other deep-sea marine mammals usually conserve energy by gliding rather than actively swimming, sinking to the depths.
During the noise exposure, narwhals made 80% less sliding during diving descents, their swimming speed exceeded 40 beats per minute, their heart rate dropped below 10 beats per minute, and their surface breathing increased 1.5 times. Overall, this unusual reaction is very expensive in terms of energy consumption, Williams said.
“This response has a cost not only in terms of the energy required to dive, but the escape time will also take away time spent on foraging and other normal behaviors,” she said.
The research was conducted in Scarsby 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 mink population for more than a decade.
Williams’ group at the University of California, Santa Cruz, has developed tools that allow researchers to monitor the exercise physiology of marine mammals during dives. The instruments were attached to the narwhals with suction cups and fell off after one to three days, floating to the surface where scientists could retrieve them.
Over the past two decades, noise from human activities such as military sonar has been linked to mass strandings of deep-sea cetaceans, mainly beaked whales. These deep-sea species are extremely difficult to study, and it was only through a partnership with local hunters that Williams and Heide-Jørgensen’s teams were able to attach monitoring devices to the narwhals.
“Most potential exposures to animals occur underwater, so it’s very difficult to study,” Williams said. “We’re fortunate to have this technology to show what’s going on at the depths where these animals live, to understand how their biology can be disrupted.”
In addition to Williams and Heide-Jørgensen, the paper was co-authored by Susan Blackwell of Greeneridge Sciences, Outti Terva and Eva Garde of the Greenland Institute of Natural Resources, Mikkel-Holger Sinding of the University of Copenhagen, and Bo Richter of the University of California, Santa Cruz. This work was supported by the US Office of Naval Research, the Greenland Natural Resources Institute, the Greenland Government’s Environmental Protection Agency for Mineral Resources, the Danish Ministry of the Environment, and the Carlsberg Foundation.