Conservation and animal-rights groups filed a lawsuit against the U.S. government on Thursday for failing to protect the critically endangered North Atlantic right whale from lethal fishing-gear entanglements after an unprecedented spate of deaths in Canadian and U.S. waters last year.

"This has really become a crisis," said Kristen Monsell, a senior lawyer at the Center for Biological Diversity, which is based in the United States.

"Right whales are hanging on the brink of extinction and the [U.S.] federal government needs to do everything in its power to prevent whales from dying painful deaths in entanglements in fishing gear. We just can't let the species go extinct on our watch."

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Twelve of the 17 right whales found dead in Canadian and U.S. waters last year were discovered in the Gulf of St. Lawrence, a semi-enclosed fishing and shipping area bordering the Atlantic provinces and Quebec. The total deaths account for nearly 4 per cent of the world's population, of which there remains only about 450. Scientists say the species may be extinct in two decades unless protective measures are taken.

The plaintiffs, which also include Defenders of Wildlife and the Humane Society of the United States, allege the National Marine Fisheries Service, the agency in charge of the U.S. lobster fishery, violates the Endangered Species Act and the Marine Mammal Protection Act. The action was filed in federal court in Washington. The groups are not seeking damages, but mitigation action.

The U.S. National Oceanic Atmospheric Administration declined to comment, saying it cannot discuss continuing litigation.

Ms. Monsell said she's hoping the lawsuit prompts action from the U.S. government, such as mandating the use of ropeless fishing gear or rope that is more easily severed, so that when a whale does come into contact with gear, it's less likely to cause serious injury or death.

"We don't need more studies. We need action," Ms. Monsell said. "We know the right whale is in trouble. We know entanglements are the main culprit in the decline of the species and we know that the agency has a legal and moral obligation to make changes and protect the species."

In Canada, a recently updated report detailing the cause of deaths of right whales in the Gulf showed a fishing-gear entanglement killed the seventh whale to be necropsied. The two-year-old female was found tightly wrapped in ropes and trailing a rusted snow-crab trap on Sept. 15 off the coast of New Brunswick. The report also said likely five other right whales died from vessel strikes and one from a long-term entanglement in fishing gear.

Last summer, Fisheries and Oceans Canada responded to the crisis by closing snow-crab fisheries in the Gulf two days early. In August, the government imposed a temporary lower speed limit in the western Gulf for vessels 20 metres or more. The speed limit was lifted Jan. 11 because the whales had moved on. Transport Canada said 14 vessels were issued $6,000 in penalties for non-compliance.

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Dominic LeBlanc, Minister of Fisheries, Oceans and the Canadian Coast Guard, met last November with representatives from fishing organizations, marine-transportation industries, Indigenous peoples, scientists and the NOAA to discuss the crisis. The fisheries department says it's working on many of the proposals raised at the roundtable, including ways to adjust fishing gear, adjust seasons around the right whales' presence and enhance whale sighting and detection information. The Government of Canada says it's also looking into imposing speed limits and adjusting shipping lanes based on whale sightings.