It didn’t go well. The first one caught, a juvenile, was quickly released after veterinarians said it showed signs of stress. The second, a breeding-age female caught last weekend, died just a few hours after being placed in a protective floating pen. Instead of making strides toward salvation, the joint U. S.-Mexico rescue team, Vaquita CPR, had brought the population one individual closer to extinction.

In a statement, Vaquita CPR said it was “heartbroken by this devastating loss,” and this week it decided to suspend the capture program indefinitely. Now the team is reckoning with what, if anything, can be done to prevent the disappearance of a species before our eyes.

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The project has for the time lowered its sights, focusing on counting the remaining animals by using underwater listening devices and taking photographs of their dorsal fins, each of which carries unique scars and markings. The Mexican government might give each one identified a name, officials said, in hopes that personalizing the cute cetaceans — known as “pandas of the sea” for the black rings around their eyes — might galvanize public attention.

“We do know we won’t be attempting any more captures” anytime soon, said one Vaquita CPR leader, Randy Wells, director of the Chicago Zoological Society’s dolphin research program in Sarasota, Fla. But maybe, he added, broadcasting that “there are so few of them that you can essentially give them all names is something that will cause more people to embrace their plight.”

The mission’s quick derailment was not a surprise to some groups that said they had not supported the idea from the start. Sea Shepherd, an activist group that patrols the vaquitas’ habitat to deter fishermen whose illegal nets sweep them up, said the project had “contributed to the possible extinction” of the porpoise and should be called “Vaquita RIP.” The Washington-based Animal Welfare Institute called for an “immediate halt” to the capture effort.

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“I looked right at them and said, ‘You’re going to kill an animal.’ And nobody likes to be right about something like that,” said Naomi Rose, a marine-mammal scientist with AWI, recounting conversations with some of the team’s members. “Vaquitas are fragile. They are vulnerable to stress.”

Stepped-up enforcement to counter illegal fishing needed to be done “yesterday,” Rose said. “Sometimes there’s no answer. They’re going to go extinct, and I hate that with every fiber of my being.”

Population trend lines certainly make it look that way. The number of vaquitas, which live only in a small section of the upper Gulf of California, also called the Sea of Cortez, plummeted from 567 in 1997 to 245 in 2008. By 2016, only about 30 were swimming the gulf’s muddy waters.

Tragically, the vaquita’s precipitous decline happened by accident: It is the victim of illegal trade in another endangered animal, the totoaba fish, whose bladders fetch thousands of dollars in Asia. The gill nets used to catch totoaba also ensnare vaquitas, and a Mexican government ban on gill nets and fishing in the area has done little to stem the problem.

Vaquita CPR members said the threat illustrated by those plunging numbers — imminent extinction — was the only reason they agreed to try capturing some animals. Andy Read, a Duke University marine biologist, said he never would have entertained the idea two decades ago, when he joined an international vaquita recovery committee. But by this year, he said, the group determined that the threat of gill nets outweighed the risks with capture efforts.

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“Our initial thought was that [poaching] is a harder nut to crack than bringing them into captivity,” Read said. Now that conclusion is in question, he added, and “it’s 11:59:59. We feel that urgency very much.”

Starting last month, the team — including trained U.S. Navy dolphins — headed out into the gulf to search for vaquitas, which are so small and shy that, Read said, they can be spotted only when waters are “mirrorlike.” The chosen capture method was a lightweight surface net that had successfully been used to capture harbor porpoises, and the team hoped it might also work for their smaller cousins.

When a Vaquita CPR net caught the female Saturday, Mexico’s environment secretary, Rafael Pacchiano, tweeted that it was “a great achievement that fills us with hope.” But the rejoicing did not last long.

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Frances Gulland, a senior scientist with the Marine Mammal Commission in California and the mission’s lead veterinarian, has worked with stranded dolphins, and she said she was surprised to see that the vaquita was calm in the net and during the hour-long boat ride to the 65-foot-diameter, mesh-sided sea pen where she would be released. With a steady heart rate and breathing, “she was really a one out of five in terms of signs of discomfort or distress,” Gulland said.

They first released her into a smaller, 30-foot-wide pen within the larger one. That’s when things changed. She swam rapidly, hitting the net, then began going back and forth, doing “quick turns, like an Olympic swimmer doing laps,” Gulland said. The porpoise calmed down for about an hour — and then suddenly went limp in the middle of the pool. Gulland said the team decided at that point to release the vaquita back into open waters.

The porpoise panicked, swimming away rapidly and then making a quick U-turn back toward the pen. To prevent her from crashing into it, several team members jumped in to catch the animal, and they found she wasn’t breathing; she’d probably had a heart attack, Gulland said. On a boat, the veterinary team tried to resuscitate the vaquita — using chest messages, emergency drugs, oxygen and intravenous fluids — but she never began to breathe on her own. After three hours, the vaquita had another cardiac arrest, and “we just declared her dead, essentially,” Gulland said.

The death profoundly shook those present and those nervously watching from afar.

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“It just seems like they’re not as robust as harbor porpoises,” Read, who had left before the capture, said of vaquitas. “I was heartbroken, both for the animal itself and also because that would likely end the attempts to establish an ex-situ population.”

And it angered some observers, including Rose, who said it heightened her frustration over an expedition she said used valuable resources and manpower for a Hail Mary pass with a “very, very, very low probability of success.”

Read rejected the idea that Vaquita CPR isn’t worth it, citing examples of other species pulled from the brink of extinction by captive breeding programs, including the California condor and the northern elephant seal. And it’s not as though the mission’s resources could have easily been transferred to some other species, he argued — they came together for the vaquita and only the vaquita.

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“As a conservation scientist, I just never give up. I’m not willing to consign a species to extinction while there’s still hope,” he said. But he does regret that it wasn’t attempted in the 1990s, when a few deaths wouldn’t have made such a dent.

“One of the big lessons for me is not to wait until the very last minute — to try conservation interventions like this and to be bold,” he said. “Maybe we weren’t bold enough.”

Team members said conversations with the Mexican government are now focusing on enforcement against poachers. Veterinarians are also awaiting results from blood samples taken from the vaquita at different points during her brief captivity. They might show the point at which her hormones and other markers of stress changed, and together with information from ultrasound exams, they might signal whether it was the capture, transport or enclosure that made her panic, Gulland said.

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Knowing that, she said, could help Vaquita CPR decide whether to modify the plan and try again at some point in the future — if there are any vaquitas left to capture.