A severely entangled North Atlantic right whale was given a shot of sedatives in the open sea that allowed rescuers to remove thick fishing line cutting in to the whale’s upper jaw and left lip.

Following the unprecedented treatment, the whale’s prognosis is uncertain but scientists say that the exercise could prove invaluable in helping other whales that face severe injury or death when they become entangled in fishing line.

Scientists have long been frustrated in trying to help entangled whales. The animals tend to avoid boats and if researchers do get close enough, they have to be careful that the whales – which can way up to 40 tonnes – don’t capsize the boat.

Over the past decade, the Woods Hole Oceanographic Institution, in Massachusetts, USA, in collaboration with the US National Marine Fisheries Service and two veterinary schools at the University of Florida and the University of Wisconsin have developed a sedation system to slow the animals and make them more approachable by rescue boats.

The injured animal was first sighted entangled east of Brunswick, Georgia, on January 14, the Boston Globe reported. An initial disentanglement failed when the whale evaded attempts to cut the line. The next day researchers tried again with the addition of a sedation dose, delivered by remote syringe and needle, but it still didn’t work.

However, the dose given appeared to make the animal feel less pain and rescuers tried – and failed – again on February 1. Then on March 5, rescuers ventured near the whale again, slowly increasing the dose they had used in January. Rescuers felt comfortable with the sedatives because they started with low doses that were known to be safe with dolphins, beluga and killer whales – and increased the dose very slowly. It still didn’t work that day but the sedative appeared to cause the whale to take shallower, more frequent breaths.

On Friday, they tried again with a little higher dose. The whale tolerated repeated close encounters with a boat while researchers worked quickly to cut about 90 per cent of the line from the 40-foot, 40,000 pound animal.

The animal remains in very poor condition and has a guarded prognosis, but the disentanglement will give it a better chance for survival, officials say.

The North Atlantic right whale is the most endangered great whale, with a population of less than 400. Human activity?particularly ship collisions and entanglement in commercial fishing gear?is the most common cause of their deaths.