MEXICO CITY (AP) — Researchers were thrilled to have captured one of the few remaining vaquita porpoises, but announced Sunday that the adult female died after a few hours in captivity in a floating pen, raising questions about the last-ditch effort to enclose the world's smallest porpoises to save them from extinction.

Critics, and even supporters of the international rescue effort, knew the plan was full of risks: The small marine mammals native to Mexico's Gulf of California have never been held in captivity, much less bred there.

But with estimates of the remaining population falling below 30, the international team of experts known as Vaquita CPR felt they had no choice. In late October, researchers captured a vaquita calf but quickly freed it because it was showing signs of stress and was too young to survive without its mother.

On Saturday, the team felt its luck had turned when it caught a female in reproductive age. Mexico's environment secretary, Rafael Pacchiano, tweeted a photo of the vaquita in a net sling late Saturday, saying: "This is a great achievement that fills us with hope."

The vaquita was taken to a protected floating pen in the Gulf of California in the hopes that it would survive, and possibly breed if more vaquitas could be captured.

But on Sunday, the team said, "Veterinarians monitoring the vaquita's health noticed the animal's condition began to deteriorate and made the determination to release."

"The release attempt was unsuccessful and life-saving measures were administered," the team wrote. "Despite the heroic efforts of the veterinary team, the vaquita did not survive."

"The entire rescue team is heartbroken by this devastating loss" wrote the team, which is using U.S. Navy-trained dolphins to help find the elusive species in the upper Gulf of California, also known as the Sea of Cortez. "The risk of losing a vaquita during field operations was always acknowledged as a possibility, but it was determined that it was unacceptable to stand by and watch the vaquita porpoise disappear without a heroic attempt at rescue."

If it proves impossible to safely capture vaquitas, experts say an all-out effort will be needed to save them in their natural habitat. Vaquita populations have been decimated by illegal nets used to catch totoaba fish, whose swim bladder is prized in China.

"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," said Alejandro Olivera, the Mexico representative for the Center for Biological Diversity. "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."

Mexico has moved to ban gillnet fishing in the area, mounted a campaign to confiscate nets, and is trying to stop illegal fishing. But given the enormously high prices that totoaba swim bladders fetch on the black market, fishermen have used go-fast boats and stealth tactics that are hard to stop.

The environmental protection unit of the federal Attorney General's Office said Sunday that authorities had seized four miles (6.4 kilometers) of nets, 5 metric tons of shrimp, and one shrimp boat as part of enforcement efforts in the second half of October. Authorities also seized three smaller boats and four vehicles.