A ballet of three North Atlantic right whale tails break the surface in Cape Cod Bay near Provincetown, Mass. The endangered whales increasingly are frequenting the bay, enticed by the fine dining possibilities of its plankton-rich waters. (Stephan Savoia/AP)

The ability of an endangered whale species to recover is jeopardized by increasing rates of entanglement in fishing gear and a resultant drop in birthrates, according to scientists who study the animal.

The population of North Atlantic right whales has slowly crept up, from about 300 in 1992 to about 500 in 2010. But a study in the journal Frontiers in Marine Science said the number of right whales born every year has declined by nearly 40 percent since 2010.

Study author Scott Kraus, a scientist with the New England Aquarium in Boston, said the whales’ population suffers even when individual animals survive entanglements in fishing gear. He said data suggests that those entanglements have long-term negative physical and reproductive effects.

“They are carrying heavy gear around, and they can’t move as fast or they can’t feed as effectively,” Kraus said. “And it looks like it affects their ability to reproduce, because it means they can’t put on enough fat to have a baby.”

Entanglements have surpassed ship strikes as a leading danger to right whales in recent years. Forty-four percent of diagnosed right whale deaths were caused by ship strikes and 35 percent by entanglements between 1970 and 2009, the study said. From 2010 to 2015, ship strikes caused 15 percent of diagnosed deaths and entanglements caused 85 percent, it said.

There is reason to believe the entanglements could harm conservation efforts despite recent signs of the whales’ recovery, Kraus said. Researchers said this year that they were beginning to see more of the whales in Cape Cod Bay, and that was a good sign.

Stormy Mayo, a senior scientist at the Center for Coastal Studies in Provincetown, Mass., said the drive to make fishing gear safer for the whales could be key to saving them.

“There’s a great deal of work being done to try to change the configurations of various kinds of fishing gear or the methods of fishing to reduce entanglement,” he said.

North Atlantic right whales are among the most endangered species of whales in the world. They spend the warm months feeding in areas off the Northeastern states and Canada and spend the winter off Southern states, where they give birth.

They are called right whales because they were hunted relentlessly during the whaling era, when they were considered the “right” whale to hunt because they were slow and because they floated when killed.