The vaquita porpoise is considered the world's most endangered marine mammal.

A 1997 survey put their numbers above 500. In 2008 their numbers had halved, with estimates suggesting there were about 245 left.

Now there are believed to be just 50 or fewer individuals.

The vaquita is endemic to the Gulf of California off the Mexico's north west coast. It is the country's largest endemic species, and at the same time the world's smallest living cetacean, the group that includes all whales, dolphins and porpoises.

In an unusual conservation tactic, researchers are eavesdropping on this rare species, which is helping them get a more accurate picture of their decline.

Porpoises are sometimes mistaken for dolphins. They do look fairly similar – the two are closely related – but porpoises have shorter beaks and flat, rather than conical, teeth. They also tend to be shorter than their leaner, larger cousins.

Porpoises are at risk of being accidentally caught in fishing nets

The vaquita is the smallest of six known porpoise species.

Unfortunately, their diminutive size is not the only reason they are so hard to see; they have been declining at an alarming rate.

There are several reasons why. Like many other cetaceans, porpoises are at risk of being accidentally caught in fishing nets, so-called bycatch.

The small area the vaquita inhabit is a prime fishing spot, shrimp and fish are abundant. Fishing is also the main income from many small fishing vessels.

Gillnets used to catch fish and shrimp are particularly damaging as the porpoises can easily become entangled. This has already contributed to the extinction of the China's Yangtze river dolphin. The vaquita is now alarmingly close to following a similar fate.

Illegal trade for the swim bladders of the endangered totoaba fish has also been rising in the last three years. They sell on the black market in China for up to $5,000. Porpoise bycatch is an unintended consequence of this.

A 1995 report found that of 128 vaquita fish accidentally captured between 1985 and 1992, 65% were caught in nets intended for totoaba.

To better understand the rate of their population decline, researchers have recently been using acoustic monitoring techniques.

The calls vaquitas emit from year to year are a good indicator of how many are in the area, especially given that they echolocate on a near-continuous basis to locate prey.

Armando Jaramillo Legorreta of Mexico's National Institute of Ecology and Climate Change, and colleagues, placed 48 passive acoustic monitors in a known vaquita refuge in the Upper Gulf of California. They monitored the porpoises for four years, between 2011 and 2015.

Porpoises emit calls at frequencies too high for the human ear to hear

After each season, from June to September, his team collects the monitors to download the data. "These clicks are then checked by expert analysts to determine the validity of every identification," says Legorreta.

The sounds made are nothing like dolphins clicks, which the human ear can easily identify. Research shows that porpoises emit calls at frequencies too high for the human ear to hear, about 135 kHz. Humans can only process sounds up to about 20 kHz. "There are not many similar acoustic signals in the Upper Gulf," Legorreta adds.

This makes the identification of vaquita calls easy to process.

In the four years his team listened in, they found that the calls declined about 34% per year, an unprecedented rate. The findings are published in the journal Conservation Biology.

"The fact is that as time goes on, vaquitas are acoustically detected less frequently than in the past. In other words, in the same sampling site we used to have several detections of vaquitas per day. Nowadays, we have only few or none."

A species that is more Mexican than the Chile or tacos

His results on population decline follow patterns found by others.

The most recent indication of their numbers indicates that only 50 remain. This was discovered from a still unpublished survey undertaken by the International Committee for the Recovery of Vaquita, in December 2015.

If the species is lost, it would have a lasting impact on the region and the people that live there, says Jorge Torre of the Mexican conservation and biodiversity NGO, COBI. If funding to conserve them disappeared, fishermen may stop receiving additional compensation and begin to overfish. If as a result, fish numbers dramatically decline, their main livelihood would be lost.

The ingredients are there for a possible recovery

It is a complex situation. "On the one hand you have a very tiny population of a species that is more Mexican than the Chile or tacos," says Torre, and yet he says that even many Mexicans are unaware of their plight.

Torre is also critical that for three decades conservation efforts have focused on a small pool of 1,500 fishermen, who are tired of being blamed.

There needs to be more focus on the vaquita's ecosystem and on its symbolic importance for Mexico, he says. That's why Torre proposes the idea of a national campaign to make the vaquita a key Mexican symbol.

For similar reasons, the president of Mexico announced an emergency conservation plan in April 2015.

As of May 2015 gillnets have been banned in known vaquita areas. Local fisherman have been compensated for any loss of income. "Alternative fishing gears are being developed and tested to replace gillnets, and the ones used for shrimp are already prohibited," Legorreta explains.

Aerial surveys have recently discovered that there are more vaquitas present near the Colorado River Delta. This means concentrated efforts by the Mexican Navy in this area could prevent vaquita being caught by illegal fishing vessels.

This means that all hope is not lost. The ingredients are there for a possible recovery, says Legorreta.

But until the use of illegal gillnets is completely stopped, a high risk to this rare cetacean remains. Fifty lone individuals may not represent enough for vaquita numbers to bounce back safely.

Mexico is therefore in danger of losing its largest endemic mammal.

The researchers say that unless other countries follow suit, "the vaquita will not be the last cetacean to go extinct in the near future".

Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter.

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