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."

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.

The team said several other vaquitas were sighted in the area where the calf was captured. 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.