A last-ditch attempt to save the world’s most endangered marine mammal, the vaquita, by taking them into human care has been abandoned. The chances that this rare species of porpoise will become extinct are now extremely high, researchers have warned.

They had hoped to catch a few of the planet’s last 30 vaquitas – which are only found in one small area of the Gulf of California – and protect them in a sanctuary where they could breed safely. But last month, the $4m (£3m) rescue plan by an international team of more than 60 scientists and divers ran into trouble after only a few days, when the first vaquita they caught had to be released when it began to display dangerous signs of stress.

Shortly after that, a second vaquita was caught but died a few hours after capture. The team then decided that catching any more animals presented too much risk to the species and further attempts were suspended.

“This is a very, very serious setback,” said project scientist Barbara Taylor, of the US National Oceanic and Atmospheric Administration. “Taking vaquitas into human care was always an extreme measure, but it was virtually our only option. Now even that has gone. The vaquita is now facing extinction unless illegal fishing can be curtailed.”

The vaquita, Phocoena sinus, which reaches a maximum length of only 5ft, has suffered a major population crash in recent years as a result of illegal catching of the totoaba fish. Flesh from the totoaba’s swim bladder can fetch more than £74,000 a kilo in China and this has generated a vast illegal fishing industry.

Unfortunately, gill nets designed to catch totoaba are also the perfect size for trapping vaquitas, which become tangled and drown. The Mexican government has recently tightened its laws against illegal fishing but the rewards for totoaba catches are so high there has been little respite.

As a result, vaquita numbers have plummeted from around 600 individuals 20 years ago to a few dozen today, leaving the animal hovering at the edge of extinction. “Our last hope was that we could capture enough vaquitas to start a captive breeding colony and restore numbers,” said another of the project’s scientist, Lorenzo Rojas-Bracho, of Mexico’s National Institute of Ecology and Climate Change.

“We had no experience in capturing these animals and it now turns out they respond badly to being taken in nets and being kept in captivity. Really we should have acted a decade ago when we still had a few hundred vaquitas left and the loss of one or two would not have been so critical.”