HALIFAX -- Marine biologists are on the lookout for one of the most productive North Atlantic right whales left in the world, after an apparently unsuccessful attempt to free it from thick rope.

Heather Pettis of the New England Aquarium said the team spotted the older whale -- named Kleenex -- near Stellwagen Bank off Boston last week and decided to try to cut a length of yellow rope that was snarled around the upper portion of its mouth and near its blow hole.

Pettis said the whale, which has given birth to eight calves over several decades, appeared thin and in poor health, raising concerns that the species could be losing an animal that is responsible for 22 right whale descendants.

It's estimated that Kleenex is the mother, grandmother or great-grandmother of nearly five per cent of the remaining 430 North Atlantic right whales.

"She's got a reproductive span of 32 years, so she's a really important animal in the population," she said in an interview. "She is in the top six reproductive females in the history of what we know about this population."

About 18 right whales were killed in U.S. and Canadian waters in the last year. It's suspected that many of them died after getting ensnared in fishing gear or hit by large vessels.

Bob Lynch of the Center for Coastal Studies in Provincetown, Mass., was on the boat with the disentanglement crew that used a crossbow to shoot a cutting arrow at the rope, managing to slice a bit of it as Kleenex dipped below the water's surface.

Lynch said the exercise was particularly challenging since they couldn't get too close to the "elusive" 15-metre whale, which often disappeared under the water.

"It's not an easy task and it takes a whole village," he said with a laugh. "There was an aerial survey team that spotted the whale and it takes the driver putting the shooter in the right position and the people on the boat to help see the whale when it's coming up."

He said it wasn't clear whether the line was cut enough to lead to its release.

"What we do know is that we improved the chances for that individual by nicking it," he said, adding that they may try another disentanglement if they see her again.

Still, he said it's unlikely that she would breed while entangled.

"It's likely she's not going to be reproducing, so that basically removes a female from the reproductive population," he said.

Pettis said Kleenex was thin, had a concave slump in her back and marks around her blow hole, indicating that she was in poor health.

She said the whale was first seen in 1977 with a calf and is a critical member of the species since it's believed there are only 100 breeding females left in a population of about 430 North Atlantic right whales. Only a couple of other right whales have produced more calves.

The whale, who was named Kleenex because one of her calves had white scars that reminded scientists of a child with a runny nose, was seen entangled in gear in 2014. Pettis said she can still eat with some difficulty, but is expending extra energy because of the drag caused by the rope.

"Having gear attached to her is requiring an expenditure in energy," she said.

The development is the latest setback for a species that suffered record losses last year. There were also no new calves spotted this season.