The vaquita is one of the most endangered species on Earth. Researchers estimate there are likely fewer than 30 of the mini-porpoises surviving in Mexico’s Gulf of California. So in October, the Mexico government and conservation groups spearheaded an ambitious plan to save them, dubbing the effort Vaquita CPR. Teams would collect the remaining animals from the wild to relocate and protect them from fishing vessels.

But not everything is going to plan, reports Mark Stevenson at the Associated Press. Over the weekend, researchers captured one of the elusive porpoises—but she died soon after.

According to a press release, the Vaquita CPR team located and captured the female, who was not pregnant or lactating, on Saturday. She was transferred to a floating sea pen known as El Nido or The Nest in the north of the Gulf of California. The animal was monitored by veterinarians, the team says, from the moment of capture. But after spending some time in the pen, the researchers notice the vaquita was showing signs of stress and released the animal on Sunday. It was too late. She passed away soon after.

As Stevenson reports, the team caught a vaquita calf in October, but they were also forced to release the porpoise after it began showing signs of stress.

“We are deeply saddened to learn that the vaquita captured on Saturday has died. We are confident that the experts involved in the capture did their best,” Alejandro Olivera, the Mexico representative for the Center for Biological Diversity, which has worked for several years to protect the vaquita, tells Stevenson. “However, this should be a reminder for the Mexican government that 'Plan A' should never be forgotten. To truly protect these incredible little porpoises, the Mexican government must once and for all get deadly gillnets out of the vaquita's habitat.”

The little “panda of the sea” was not even discovered until the 1950s, Nick Pyenson wrote for Smithsonian Magazine earlier this year. Since 1997, however, populations have declined by over 90 percent. In 2012, researchers estimated there were 200 animals left. That dropped to 100 in 2014, 60 in 2016 and just 30 during a census in February 2017.

The deaths aren't because of hunting or disease. Almost all of the loss is attributable to illegal gill-net fishing in pursuit of another endangered species: the totoaba fish. These nets also entangle the vaquita, preventing them from surfacing for air and causing them to drown. The fishermen sell the totoaba swim bladders to Chinese traditional medicine practitioners, who pay thousands of dollars for the dried pieces of collagen. Illegal shrimp fishing has also impacted the porpoise.

Though the Mexican government has made some efforts to stop the illegal fishing, implementing a two-year gill-net ban in many parts of the Gulf, it has not been enough to stop the illegal fishing. As Stevenson reports, the government has stepped up enforcement by seizing four miles of nets and five metric tons of illegal shrimp and one shrimp boat in the second half of October alone.

Still, researchers worried that enforcement would not be quick enough to save the last 30 vaquita. Instead conservationists and the Mexican government launched the relocation plan, with the idea that the creatures would live in a protected sanctuary until the gill-net problem in the Gulf of California could be controlled. Then they would be released back into their natural habitat, hopefully with a few new members of the pod.

“The rescue project is, quite literally, the last chance to save the vaquita,” Dan Ashe, President and CEO of the Association of Zoos and Aquariums said in a statement when the capture plan began in October. “We know and accept that the rescue plan is risky, but if we do nothing, extinction of the vaquita is certain.”

The death, however, has called the entire plan into question, and as the AP reports, the Animal Welfare Institute is calling for a halt to the captures, saying “these tiny porpoises do not respond well to the stress of capture, and not a single additional vaquita should be deliberately put in danger in this way.”

According to Vaquita CPR, the team is waiting for results of a necropsy and will review the incident before deciding what to do next.