Scientists are finalising plans to make a last-ditch attempt to save the world’s most endangered marine mammal, the vaquita porpoise. They believe there are now fewer than 30 of these distinctive cetaceans left in the Gulf of California. Only by catching the remaining creatures and protecting them in a sanctuary can the vaquita be saved, it is argued.

The $4m rescue plan will involve conservationists patrolling the gulf with the help of dolphins trained by the US navy to pinpoint other cetaceans. The idea is that the animals will then be captured and transported to a sanctuary in San Felipe, Mexico.

But the attempt carries risks. No one has ever tried to capture, transport or care for a vaquita before and scientists do not know how they will react. “Some porpoises, like the harbour porpoise, don’t seem to mind too much when captured, but others, such as the Dall’s porpoise, go into shock,” said Barbara Taylor, of the US National Oceanic and Atmospheric Administration. “We don’t know which it is going to be. It is a nerve-racking prospect.”

However, scientists insist they now have no choice. “Vaquita numbers are so low it is clear that if we do nothing it will go extinct very soon,” said Lorenzo Rojas-Bracho, of Mexico’s National Institute of Ecology and Climate Change. “However, if we capture the last few and try to protect them we have a chance to save the species.”

The vaquita, Phocoena sinus, is small, reaching a maximum length of only 5ft, and has a grey back and white belly. Its home territory – a 900-square-miles section of the northern Gulf of California – is the smallest occupied by a whale species. Twenty years ago, there were about 600 in the region. However, the population has since crashed as a result of illegal fishing of a species called the totoaba. Flesh from its swim bladder can fetch prices of more than $100,000 a kilo in China, where it is prized for its medicinal properties. “Quite simply, it commands a higher price than cocaine,” said Rojas-Bracho.

The gill nets designed to catch totoaba are also the perfect size for capturing vaquitas, which get 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 and vaquita numbers have continued to plummet.

“The population dropped to 30 last year, but there have been more deaths so I expect we’ll lose about half of this number this year,” said Taylor, a member of the International Committee for the Recovery of the Vaquita . “At this rate it will not last much longer. That is why our task is so urgent.”

As part of the rescue project – which has received $3m backing from the Mexican government and $1m from the US Association of Zoos and Aquariums – researchers will use acoustic sensors over the next few months to find the vaquitas and then, in October, they will try to catch individual specimens in nets. “We plan to use a couple of trained dolphins to help us,” said Taylor. “It remains to be seen how effective they will be.”

Once the vaquitas are caught, they will be carried to floating pens and – if they respond to the ordeal in a relatively stress-free manner – they will be taken to a sanctuary being built in San Felipe.

“Ultimately, we would like to begin a captive breeding programme with the aim of restoring numbers and finally returning vaquitas to the wild, although we obviously cannot do that until we have dealt with the problems that are causing them to be wiped out at present,” said Taylor.

Ten years ago Taylor was involved in an attempt to survey numbers of a similar cetacean, the Yangtze river dolphin – also known as the baiji. Its population was known to be threatened by the illegal laying of fishing nets. What Taylor’s team found turned out to be far worse. “We didn’t see a single baiji or hear one whistle,” she told the Observer. “We were too late.” The baiji is now officially listed as extinct.

“I resolved then that the vaquita would not suffer a similar fate,” Taylor said – although she accepts the recent dramatic decline in its numbers puts it in a very perilous position. “It is always risky taking an animal into captivity, especially one with which we have no previous experience and who are made up of the last few individuals of that species. But we have to do this.”

In the past, other species have been pulled back from the brink of extinction, Rojas-Bracho said. Hunting had reduced numbers of the northern elephant seal to a few dozen in the 19th century. Today, protected by law, there are more than 170,000 of them. “A similar story concerns the southern sea otter, which was reduced in number to around 50 but which has since bounced back to around 2,500 creatures,” he said. “This sort of thing can be done.

“Certainly, we are not where we would want to be when it comes to saving the vaquita – but we have to do our best or it will be lost to the planet for ever.”