Species don’t come much more endangered than the vaquita, a tiny porpoise that’s frequently killed in commercial fishing nets in the northern reaches of Mexico’s Gulf of California. Earlier this year, a dire report from experts estimated that just 60 individuals remain, solidifying Phocoena sinus’s status as the world’s most endangered marine mammal. That grim assessment now has biologists pondering a controversial strategy: removing a handful of vaquitas from the wild and breeding them in captivity.

The vaquita population has been declining for decades, in large part because the diminutive, 1.5-meter-long cetaceans are prone to becoming entangled in gillnets and drowning. Still, conservationists had long rejected captive breeding as too risky. As the vaquita’s collapse has accelerated, however, it has become impossible to dismiss so-called ex situ conservation, the practice of preserving species by removing them from wild habitats and managing them in artificial settings.

“Given the crisis we’re in, we need to explore all of our options,” says Barbara Taylor, a biologist with the National Oceanic and Atmospheric Administration’s Southwest Fisheries Science Center in San Diego, California, and member of the International Committee for the Recovery of the Vaquita (CIRVA). “Keeping some individuals in a sanctuary is one of those options.”

Still, some vaquita advocates are leery. “I don’t like this idea at all,” says Omar Vidal, director general of the World Wildlife Fund (WWF) Mexico in Mexico City. “The risk of killing a vaquita while catching them is very high. With only 50 or 60 animals left, we can’t play with that.”

Hummingbirds of the sea

Just a decade ago, biologists faced insurmountable obstacles to capturing and caring for porpoises. Although some cetaceans, like bottlenose dolphins, thrive in captivity, porpoises are ill-suited for confinement. “They’re very sensitive to stress and noise, and they have high heart rates,” says Frances Gulland, a senior scientist at the Marine Mammal Center in Sausalito, California, who also serves on CIRVA. “We think of them as the hummingbirds of the marine mammal world.”

In recent years, however, scientific teams in Denmark, Japan, and elsewhere have developed techniques for capturing and maintaining porpoises. In the Netherlands, an aquarium called the Dolfinarium Harderwijk has housed, rehabilitated, and even bred harbor porpoises that have been entangled in nets or stranded on beaches. Those advances in husbandry convinced Gulland that an ex situ conservation program for vaquitas was worth exploring, and in September 2015 she convened a team of international experts in the Netherlands to assess its feasibility and recommend protocols.

So is captive breeding vaquitas possible? The short answer: We don’t yet know. Each phase of the process poses problems, the panel concluded.

The first, and likely most difficult, stage is locating and catching the elusive cetaceans in the choppy, murky waters of the gulf. To help with the hunt, the panel even suggests exploring the use of trained bottlenose dolphins, such as those kept by the U.S. Navy. Once the porpoises have been found, the team envisions herding vaquitas into lightweight surface gillnets—a tactic that’s been used to satellite-tag dozens of harbor porpoises in Greenland without any fatalities. But “whether vaquitas will respond the same as harbor porpoises is really hard to say,” notes Andy Read, a marine biologist at Duke University in Durham, North Carolina. “We know so little about the behavior and physiology of these animals.”

Even if scientists were to successfully capture vaquitas, they would face a daunting litany of logistical obstacles. They would have to transport their captives in a moist stretcher, then house the animals, first in a soft-sided net pen and likely later in a large artificial pool along the gulf’s coast. They’ll also have to figure out how to provide care and food for a species with unknown veterinary requirements; and how to get the creatures to breed in their enclosure. Finally, they’d have to release both parents and naïve offspring into the wild. “If you fail at any one of those steps, it’s game over,” Taylor says.

Conservation promise or political peril?

Despite the challenges, ex situ conservation has been used to recover other species. Captive breeding programs have famously saved black-footed ferrets and California condors, whose populations had both dwindled to about 20 animals. But the strategy has never before been put into action with a cetacean—although there have been false starts. In 2006, for instance, Taylor joined an international research team that cruised China’s Yangtze River in search of the baiji, an endangered freshwater dolphin. The team hoped to relocate baiji to sheltered oxbow lakes, but the plan was hatched too late: The dolphin went extinct before any could be captured. (In 2015, scientists did succeed in translocating eight finless porpoises, another endangered Yangtze River dweller, to the safety of the lakes, although it is not yet clear if they will reproduce.)

Unlike ferrets and condors, whose entire populations were gathered up for breeding, Taylor is adamant that vaquitas will never face complete removal from the wild. Instead, the plan is to potentially house several individuals as an insurance policy against extinction until Mexico implements more stringent measures to protect the animal from dying in nets. “This is not a condor situation,” Taylor says.

Still, some conservationists worry captive breeding will sap political will for protecting vaquitas from gillnets. “Species need to recover in the wild,” says WWF’s Vidal. He points to the return of the Guadalupe fur seal, a Mexican pinniped that was hunted nearly to extinction in the 19th century, as proof that imperiled marine mammals can rebound without captive breeding.

Vidal and CIRVA’s scientists also feared that discussion of captive breeding would undermine efforts to extend a temporary ban on gillnetting in the vaquita’s range which the government imposed in 2015. Those concerns were partly alleviated on 22 July, when the Mexican government announced that gillnets would be permanently banned from the area.

But legal gillnets are only partly to blame for the crisis. The most pressing existential threat to the vaquita is poaching—not of the porpoises themselves, but of a fish called the totoaba, whose bladders earn up to $20,000 apiece in China. The totoaba’s spawning grounds coincide closely with vaquita habitat, and fish poachers often snag porpoises in illicit nets. Three vaquitas—representing about 5% of the remaining population—were killed by totoaba fishermen this past March.

At the behest of the Mexican government, an international team of porpoise experts will begin to evaluate the prospects for ex situ vaquita conservation within the next 6 months. “The first attempts to capture a vaquita could take place in 2017,” if visits to the Gulf suggest that it’s indeed possible to catch and house vaquitas, CIRVA wrote in a recent assessment. The most likely initial target for capture would be a young male, Gulland says, because a loss in that demographic group would be least harmful to the population as a whole if capture goes awry.