PROVINCETOWN — Only 12 years ago, marine biologists and whale watchers on the Cape had something to celebrate: the endangered North Atlantic right whale population was making a rebound, from fewer than 300 individuals in the 1990s to nearly 500 by 2010. But that success was short-lived.

With fishing-line entanglements and ship strikes taking a heavy toll, the right whale is in crisis, with deaths outpacing births. Their total numbers are now thought to be down in the low 300s.

Sean Brilliant, a senior conservation biologist at the Canadian Wildlife Foundation, sees stark similarities between the brutal overhunting of right whales in past centuries and the present-day population loss.

“Seventy years ago, we were shooting explosives into their bodies to kill them,” said Brilliant. “Now we are still doing the same thing, but in a nastier sort of way. They are not dying instantly anymore.”

Entanglements and ship strikes are now the leading causes of death for these whales. National Oceanographic and Atmospheric Administration (NOAA) scientists estimate that 85 percent of right whales have been entangled at least once. Repeated run-ins with fishing equipment and speeding boats injure and slowly kill the animals.

Brilliant believes humans are directly responsible for these injuries and deaths because the whales live along the world’s most industrialized, urbanized coastline.

“It’s the busiest, in terms of fishing and shipping,” he said. “It’s a really tough place to grow up.”

Brilliant attributes the recent decline in right whale numbers to a shift in their migration track.

Traditionally, North Atlantic right whales winter in the waters off the Carolinas, Georgia, and Florida and migrate north in the spring. After the long and difficult journey up the coast, they linger in Cape Cod Bay and nurse their calves before swimming further north to Canada.

The whales have shifted their distribution, moving away from the areas established to protect them and north toward the Gulf of Saint Lawrence.

The effects of this change went unnoticed at first. According to Sean Hayes, chief of the Protected Species Branch at NOAA Northeast Fisheries Science Center, it took scientists five to six years to identify the problem that began emerging in 2010.

Charles “Stormy” Mayo, founder of the Right Whale Ecology Program and co-founder of the Center for Coastal Studies in Provincetown, estimates there are now only 336 North Atlantic right whales. About three-quarters of them come to Cape Cod Bay in the late winter and spring, he says.

With a recent average of 20 whale deaths per year and just half that number of births, the population is not rebounding. Only 2 percent of mature females are reproducing each year. “Right now, every single vital statistic is negative,” Mayo said. “Everything points to extinction.”

Mayo said the ecological repercussions of a North Atlantic right whale extinction are “unknown at present,” but the whales have historically played a key role in the ecosystem: they fertilize the ocean.

In a process known as the “whale pump,” marine mammals dive down to consume nutrients and then poop in the surface layer, fertilizing it. This phenomenon nourishes plankton and therefore fish in the sea.

Hayes believes diminishing fish stocks are connected to the decline in whale populations.

“It’s almost like they are the canary of the ecosystem,” Hayes said. “Offshore wind development, vessel speeds, commercial fishing — there are a thousand species that could be impacted by these things. But one species gets 99 percent of the attention, and it’s North Atlantic right whales.”

Hayes said that fishing boats unintentionally kill hundreds or even thousands of marine mammals in New England every year. Actions to protect the right whale will also protect other marine species.

One example is ropeless lobster fishing. Traditionally, lobster traps on the sea floor are attached with long ropes to floating markers on the surface — a hazard for right whales. To reduce entanglements, scientists are developing lobster traps that rest on the bottom of the ocean and release their lines to the surface only when lobstermen approach.

Ship strikes are another problem. The name “right whale” was given to the species by early whale hunters because they were the “right whale” to hunt — they swim slowly, near the surface, and their bodies float. These movement patterns now make them highly susceptible to being struck by boats.

Although they have many sensory mechanisms, the whales are unable to easily escape from oncoming ships because of their size. Nancy Downes, a senior field representative for the nonprofit group Oceana, says that you can “reduce vessel strikes by 90 percent if you just slow down.”

Scientists haven’t given up on the North Atlantic right whale. Although there is a process in the Endangered Species Act to allow giving up on a species if there is no hope of population recovery, there is no such process in the Marine Mammal Protection Act. As a result, the right whale will be protected until the very last animal dies.

Nonetheless, Mayo is pessimistic about the future. “They are going extinct unless we change something,” he said.

Mayo believes that outcome would be felt most on the Outer Cape where Provincetown is currently the center of right whale research and residents flock to the beach to watch the whales make their annual visit.

Addressing a Provincetown audience at a viewing of the new documentary The Last of the Right Whales, Mayo was emphatic about that connection.

Right whales are “your whales,” he told the group.

The Last of the Right Whales will be shown again at 7 p.m. on Thursday, Aug. 11 at the Salt Pond Visitor Center in Eastham.