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.