On a windy afternoon last weekend, Cynthia Smith was on a boat in the Gulf of California, searching for one of the rarest marine mammals in the world.

Never before had a vaquita porpoise been successfully captured and cared for by humans. But Dr. Smith, a veterinarian with a group known as Vaquita CPR, knew that the porpoises — whose black-rimmed eyes and dark noses have earned them the moniker “panda of the sea” — were likely to disappear without human intervention. Scientists estimate there are fewer than 30 vaquitas left in the wild. It was a last resort.

When Dr. Smith and her team spotted a pair of porpoises, and managed to haul in the female, they were hopeful: the vaquita was calm, her vital signs promising. But soon after they moved her into a sea pen she began swimming too rapidly, then abruptly slowed down. By the time the veterinarians released the vaquita back into the wild, it was too late. “She was no longer breathing,” Dr. Smith said. “We just couldn’t bring her back.”

In the past five years, the vaquita population — which lives in only a sliver of water between Mexico’s mainland and Baja California — has plummeted by 90 percent. Humans are to blame, but they are not even hunting for the vaquitas themselves.

The animals, the world’s smallest porpoises, get tangled and drown in nets set illegally to catch another endangered species, a fish called the totoaba. The poachers’ bounty is an organ from the totoaba called the swim bladder, which is considered a delicacy and status symbol in China and can sell for up to $50,000 on the black market. It has been dubbed “aquatic cocaine.”

Mexico banned totoaba fishing in 1993, but it was only in 2015 — when vaquita numbers dwindled to about 100 — that the government also banned most gill nets, including those used for catching shrimp and other kinds of fish. The gill nets catch the totoaba, and also trap the vaquitas.

Fishermen are still allowed to use a different type of net, intended for catching corvina fish, that is not supposed to pose a threat to the vaquitas. But because the corvina and totoaba fishing seasons overlap, fishermen can hide the totoaba nets beneath the corvina nets on their boats, said Lorenzo Rojas-Bracho, the director of Vaquita CPR. “You can use a corvina as a cover-up, and that’s what happened,” he said.

Most of the region’s fishermen, who have been out of work since the ban, poach totoaba swim bladders, which fetch $3,000 to $10,000 per kilogram, said Andrea Crosta, the director of a group that investigates the totoaba trade. An equivalent catch of shrimp might earn a fisherman $200 at best.

“If you’re out here on the water, you’re not risking jail time to catch shrimp,” said Dave Bader, a spokesman for Vaquita CPR.

After the death of the captured vaquita, the scientists agreed to shut down the program. “The evidence today is vaquita are not good candidates for being protected in this way,” Dr. Smith said. “It’s heartbreaking.”

The scientists had hoped to keep the animals in captivity temporarily, and possibly even breed them, until gill nets were eliminated from their habitat. While any wild animal is expected to experience some stress when captured, the group had hoped the vaquita could cope with being handled. (Last month, the scientists released a vaquita calf they had pulled from the waters when they determined it was too young to be separated from its mother.)

Other conservationists had been critical of the plan. “I was telling them that this was going to happen, you’re going to stress out the animals,” said Paul Watson, the founder of the Sea Shepherd conservation group, which also works to protect vaquitas. “They are very elusive, they are very shy; you’re going to kill one of them.”

Mr. Crosta said that while he admired the work of the biologists, they couldn’t possibly solve a problem “that starts in San Felipe and ends in a shop in China.” The fate of the vaquita, he said, was “entirely in the hands of law enforcement.”

In the past two years, Sea Shepherd and local groups have recovered more than 25,000 yards of gill nets from the gulf, weighing about 50 tons. Now that efforts to capture the vaquitas have failed, conservationists say hauling in the poachers’ nets is the best bet to save the species from extinction.

The Mexican government, conservationists say, also needs to crack down on illegal fishing, while providing sustainable alternatives for the community. Financial compensation from the government often never makes it into the hands of the fishermen, diverted by corruption, Mr. Crosta said.

Local economies are an important consideration in conservation biology, Dr. Rojas-Bracho said. “You cannot save vaquita, and drive fishers to extinction.”