MEXICO CITY (AP) — Researchers trying to catch and enclose the last survivors of the vaquita porpoise species captured a calf but released it because it was too young to survive without its mother.

Mexico's Environment Department said veterinarians determined the calf was too young and experts said it was showing signs of stress after capture. The experts with Mexican-led international effort known as VaquitaCPR still saw hope in the calf's capture.

"The successful rescue made conservation history and demonstrates that the goal of VaquitaCPR is feasible," said Environment Secretary Rafael Pacchiano. "No one has ever captured and cared for a vaquita porpoise, even for a brief period of time. This is an exciting moment and as a result, I am confident we can indeed save the vaquita marina from extinction."

Lorenzo Rojas, the lead scientist on the effort, said "while we were disappointed we could not keep the vaquita in human care, we have demonstrated that we are able to locate and capture a vaquita."

A photo released by the Environment Department showed the calf swimming in a pen surrounded by team members. The statement did not say how long it was in captivity.

The U.S. Navy trained dolphins to help find vaquitas and research boats are searching their habitat, the Gulf of California.

It was unclear if the dolphins were even needed; the team said "scientists spotted several vaquitas using visual search methods and acoustic monitoring."

The vaquita population has dropped to less than 30 because of illegal nets set for the totoaba fish, whose swim bladder is prized in China.

After a health evaluation and the taking of some tissue samples, the calf was returned to the area where it was captured and where several other vaquitas had been sighted. The team did not say whether it had rejoined its mother.

The effort started Oct. 12 in the Gulf, also known as the Sea of Cortez, and will continue.

Once caught, the vaquitas are to be held in protected floating pens with hopes they will reproduce and could eventually be re-released into the wild.