The world’s rarest marine mammal is on the verge of extinction due to the continuing illegal demand in China for a valuable fish organ, an undercover investigation has revealed.

There are no more than 30 vaquita – a five-foot porpoise – left in the northern Gulf of California today and they could be extinct within months, conservationists have warned. The population has been all but eradicated by pirate fishermen catching the large totoaba fish and killing the vaquita in the process.

The totoaba, which is itself highly endangered, is caught for its swim bladders which are smuggled to China for sale on the black market. Undercover investigators found the swim bladders, called maws, for sale in Shantou in Guandong province, at an average price of $20,000 per kilogram. The cost has led to the maws being dubbed “aquatic cocaine”.

“The demand is still strong and stable – it is not going down – and prices are climbing again,” said Andrea Crosta, from the Elephant Action League, an intelligence-led group now targeting all wildlife crime and which conducted the totoaba investigation.

“Because it is very expensive, it remains a product for wealthy people,” he said. “The law enforcement is very weak because it is not top priority and probably because it involves rich and powerful people.”

One trader in the illegal maws told the investigators: “When the government comes to check, they call and inform us earlier and we will hide them when they come.” However, the trade is less open than it once was. Chinese buyers of maws prefer those from domestic waters but these are exceedingly rare now, having been intensively fished for many decades.

The demand for totoaba maws is driven by its use as a business gift, investment or wedding dowry, as well as its supposed medicinal benefits, the investigation found.

The fish are captured in gillnets, which the Mexican government has banned although illegal fishing persists. The ban will expire at the end of May and the conservation group WWF called on Tuesday for Mexico to introduce and enforce a permanent ban on all gillnets.

The dried swim bladders have an average price of $20,000 per kilogram, causing them to be dubbed “aquatic cocaine”. Photograph: Anthony Wallace/AFP/Getty Images

“Time is rapidly running out for the vaquita – we could tragically lose [it] in a matter of months,” said WWF’s Chris Gee. “The last hope for the species is the Mexican government immediately putting in place and properly enforcing a permanent ban.”

The WWF is also urging the Chinese and US governments to collaborate with Mexico to intercept and halt the illegal transport and sale of totoaba maws, which are thought to be moved via drug-smuggling routes or hidden in legal fish products.

In 2015, Mexico’s federal environment agency Profepa revealed the totoaba maws were more valuable than cocaine in the country, with a kilogram selling for the same as 1.5kg of the drug. The price uncovered by EAL investigators make the swim bladders 10 times more valuable than caviar and 30 times more than elephant ivory.

With the extinction of the vaquita looming, an $4m emergency plan is being put in place to round up a few individuals, with the help of US navy-trained dolphins, and place them in a sanctuary. However, capturing vaquita has not been attempted before, meaning the success of the plan is uncertain.

• This article was amended on 16 May 2017. An earlier version gave the location of the vaquita as the Gulf of Mexico. This has been corrected to say the Gulf of California.