SAN FELIPE, Mexico — In the shallow sea waters of the Gulf of California swims a porpoise that few have seen, its numbers dwindling so fast that its very existence is now in peril.
Known mostly by its Spanish name, the snub-nosed vaquita is the world’s smallest cetacean, a miniature porpoise with a cartoonlike features and dark smudges around its eyes. The species lives only in the fertile waters of the gulf’s northern corner.
The size of its population has always been precarious, but now voracious demand in China for a fish that shares the vaquita’s only habitat has pushed the tiny porpoise to the brink of extinction.
No more than 30 vaquitas are left, according to a November estimate based on monitoring of their echolocation clicks. Half of the vaquitas counted a year earlier have disappeared.
This calamity has hardly gone unnoticed. The vaquita has been vanishing in plain sight, to the despair of conservationists who have been advising the Mexican government on how to save it. All of the resources brought to bear, including the
 protection of the Mexican Navy, have proved to be no match against the illegal wildlife trade.
“If we continue on the path we’re on, we’ll have no vaquitas in two years,” said Barbara Taylor, a marine mammal expert at the National Oceanic and Atmospheric Administration.
The vaquita are simply bycatch, trapped and drowned in curtains of illegal gillnets set for an endangered fish called the totoaba. The fish’s swim bladder is dried and smuggled to China, where wealthy diners pay thousands of dollars for the delicacy, believing it to have medicinal powers.
To feed that appetite, totoaba poachers have killed 90 percent of the vaquita population since 2011, according to the acoustic monitoring program led by Armando Jaramillo Legorreta at the Mexican government’s National Ecology and Climate Change Institute, known as INECC.
With so few vaquitas left, experts advising the Mexican government have proposed capturing several specimens and holding them in a sea pen as a way of conserving the species until the threat to its habitat is removed. It’s a last-ditch measure that conservationists had hoped they would never have to resort to.
“We had always been opposed to captivity,” said Lorenzo Rojas Bracho, a marine mammal expert at INECC and the chairman of an advisory group, the International Committee for Vaquita Recovery. But nobody expected that the population would decline so quickly.
“There are risks,” Dr. Rojas Bracho said of the capture plan. “But they are fewer than leaving them with the fishing as it is.”
The plan would entail training United States Navy dolphins to locate vaquitas, capturing them for transfer to a temporary pool and then to a sea pen to be built in their habitat along the Gulf of California coast. The majority of vaquitas would remain in the wild.
But the unknowns loom large. “We don’t know whether they find them,” Dr. Taylor said of the dolphins. “We don’t know whether we can catch them. We don’t
 know how they will react.”
“If you get a negative result in any one of these steps,” she added, “it’s basically game over” for the capture plan. Even in the best of scenarios, breeding in captivity is unlikely to restore the population. A female vaquita gives birth to one calf every two years on average.
If the proposal goes forward, the vaquita would join other species at the brink of extinction — like the California condor and the golden lion tamarin, in Brazil — that are being closely managed in some form distinct from their natural setting. It would be the first such effort for a marine mammal.
A very small population can be pulled back from the edge, but “it requires outside-the-box thinking,” said Samuel Turvey, a research fellow at the Zoological Society of London who studies conservation management for highly threatened species.
A managed-conservation plan designed with the expectation that the animals can eventually be returned to the wild “is not a permanent solution,” he added. “It’s an emergency stopgap with an exit strategy.”
Nor would it be a quick fix. “It requires intensive sustained efforts for decades to recover species from these catastrophic low levels,” said Richard Young, the head of conservation science at the Durrell Wildlife Conservation Trust .
Dr. Turvey speaks from experience. He witnessed the first human-caused extinction of a cetacean, the Yangtze River dolphin. Like the vaquita, the baiji, as it was more commonly known, occupied a limited habitat in small numbers and was decimated as bycatch in local fisheries.
For a decade, researchers discussed removing individual baiji to a semi-natural reserve as a short-term conservation measure. But when Dr. Turvey and other researchers led an expedition down the Yangtze in 2006 to look for specimens, they found none.
The baiji “only became a story when it was gone,” he said, adding that “it was really dark and upsetting.”
 It is an experience that Dr. Taylor, who was on the expedition, hopes not to repeat. “It’s idealistic to think that we’re going to effect the significant changes in fisheries and enforcement practices in the wild in time to save vaquitas,” she said.
If anything, those efforts have reached a nadir.
Two years ago, the Mexican government imposed a two-year ban on all gillnets across 5,000 square miles of the vaquita habitat and sent its Navy to enforce it. To support the communities of the upper Gulf, which depend on fishing and shrimping, the government allocated $74 million in compensation over the two years.
The hope was that the military could halt the totoaba trade and that two years would be long enough to complete development of vaquita-safe trawl nets to substitute for shrimp gillnets. (Even before the totoaba trade surged, legal gillnet fishing had depleted the vaquita population.)
But local fishermen argue that the new nets’ catches are too meager to provide a living, and the authorities have been sympathetic. “While there is no alternative to fishing practices, nobody will give up their gillnets,” Dr. Rojas Bracho said.
The promised enforcement also has fallen short. That was evident this month aboard the Sam Simon, a 57-meter antipoaching vessel operated by the Sea Shepherd Conservation Society, an environmental organization.
In an agreement with the Mexican Navy, Sea Shepherd has been patrolling the vaquita habitat, pulling illegal nets out of the water and spotting poachers. “We see illegal activity almost every day,” said Oona Layolle, the leader of the Sea Shepherd campaign, called Operation Milagro (Spanish for “miracle”).
At about 4 p.m. one afternoon this month, a fishing boat pulled up just a few hundred meters from the Sam Simon carrying four men guided by a hand-held GPS device. One of the men dragged a hook in the water, looking for a gillnet they had hidden there.
The Sam Simon sent a drone over the small boat and it sped off, only to return with six men aboard, who threw objects at the drone before leaving again. Despite a call to the Mexican Navy, nobody came.
 Even when arrests are made, conservationists say, the prosecution is too slapdash to win a conviction for a serious crime.
Last year, Mexican Navy patrols succeeded in scaring off the totoaba poachers by day, forcing them to haul in their nets at night. But this year, the poachers work openly during the day, some wearing balaclavas, apparently undeterred by desultory government patrols. Some poachers even post photos of their weapons on Facebook.
At the same time, four boats belonging to Mexico’s environmental prosecutor are parked on a side street running above the dock here, their motors broken or simply unused because fuel is in short supply.
The nets tell a similar tale. Over 10 weeks last spring, Operation Milagro pulled 42 totoaba nets from these waters. In the fall, a broad government-sponsored survey succeeded in finding 36 totoaba nets, 28 of which were in use.
In mid-December, Operation Milgro resumed and found 56 more totoaba nets in nine weeks. Almost all were new, and some were set in the same places that the government effort had cleared just weeks earlier.
During night patrol aboard the Sam Simon last week, the crew pulled up yet another totoaba net of wide blue mesh, its unweathered red buoys evidence that the net was brand new.
“The situation is so dramatic that we have to take huge measures,” Ms. Layolle said. “It is a desperate time.”
Mexico’s environment minister, Rafael Pacchiano Alamán, promised this month to send 45 federal police officers to patrol the beaches and to dismantle poachers’ camps.
But he did not respond to the main recommendation of conservationists: a permanent gillnet ban. The legal fishing season for corvina has begun, which means dozens of small boats will be out on the water, giving cover to poachers.
Despite a promise last year by President Enrique Peña Nieto, the government has yet to act on the gillnet ban. Without that, warn conservationists, there is no way
to begin to save the vaquita.
“If you can’t remove the threats, the population keeps declining,” Dr. Turvey said. “You don’t have time for complacency.”