Narwhals lower their heart rate when more active

Anti-clickbait : when the article is more interesting that the headline suggests.

The researchers worked with the hunters to find narwhals already entangled in nets. They released each animal, attaching a tag to its back with a suction cup, before pushing it into the deep water of the East Greenland fjords.

"The very first heart rate measurement was - as you would imagine fairly high," recalled Dr Williams. "When the animals were just sitting there, it was about 60 beats per minute - about the same as our resting heart rate. But the moment those animals took off, their heart rate immediately plunged down to three or four heart beats per minute - 15 to 20 seconds between each beat."

This reduction in heart rate, the scientists suggest, could help explain some whale strandings. If animals are moving quickly to escape a threat, but their heart rate is very low, this could deprive their brain of oxygen and leave them disorientated. Long periods of this low blood flow and reduced oxygen supply to the brain might even cause permanent damage.

So presumably the animals are "fleeing" after being released from the nets, but for how long can they monitor the heart rate ? How does it change while hunting or moving at high speed for other reasons ? Is is normally at 60 bpm while stationary, or was this artificially high caused by the stress of being trapped in a net which then drops when they're released because the stress level drops ?
http://www.bbc.com/news/science-environment-42259289