Critically Endangered porpoise can bounce back from edge of extinction

The world's most endangered species of mammal can bounce back if we stop fishing it to extinction.

There are estimated to be fewer than 10 vaquita, a species of porpoise found only in the Gulf of California, Mexico, left in the world. Thousands of vaquita are thought to have been killed after being caught by illegal gillnet fisheries, pushing the mammal to the brink of extinction.

There had been concerns that its low population size would cause the species to die out even if the nets were removed. However, a new study published in the journal Science shows that the vaquita's genetics are remarkably healthy, paving the way for a strong recovery if the threats are dealt with.

Dr Jacqueline Robinson, who led the study with Christopher Kyriazis, says, 'If we can allow these animals to survive, they can do the rest. They still have the genetic diversity that let them thrive for hundreds of thousands of years, until the gillnets arrived.'

Christopher adds, 'While we now know that the species' ability to recover is not limited by their genetics, vaquitas have very little time left. 

'If we lose them, it would be the result of our human choices, and not their inherent genetic factors.' 

What is the vaquita?

Meaning 'little cow' in Spanish, the vaquita is the smallest of all the cetaceans, the group of animals which include dolphins and whales. Males measure around 1.5 metres long at their largest, while females are around five to ten centimetres shorter.

The species was unknown scientifically until the 1950s, when the skulls of two vaquitas were found and described. It wasn't until the 1980s that the first detailed descriptions of the species were made when vaquita were accidentally caught by scientists trying to assess the population of another Gulf of California resident, the totoaba fish. 

The fortunes of the vaquita are closely linked to this fish. The swim bladder of the totoaba is considered a delicacy and used in traditional medicine in southeast Asia. As a result, the fish can fetch thousands of pounds and drives illegal fishing in Mexico.

The fishing is caried out using gillnets, which are designed to hook themselves around a fish's gills. But the vaquita is also caught in these nets, often leading to their death.

While in the 1950s there were thought to be thousands of porpoises in the Gulf, the fishing of the totoaba is thought to have caused a sharp decline in the vaquita population. By 1997, the first large census found 567 vaquita, and the number has continued to decline until only 10 are thought to survive today.

This decline has continued despite efforts to try and conserve the species. A fishing ban across the vaquita's habitat was declared by the Mexican government in 2017 but has been repeatedly flouted.

Meanwhile, efforts to capture the remaining vaquita to establish a captive breeding population have been called off. This follows an attempt in 2017 which resulted in the death of an adult female vaquita from the stress associated with capture. 

With the population now estimated to be fewer than 10 individuals, there had been concerns the vaquita could be entering an extinction spiral, with inbreeding and a build-up of harmful genes dooming the species regardless of final conservation attempts.

However, the new research demonstrates that the vaquita is much more resilient than we'd expected. 

How can the vaquita recover from the edge of extinction?

The study examined tissue samples of vaquitas taken between 1985 and 2017, resulting in 20 sequenced genomes. These were then used to peer into their genetic history and investigated for signs the species may be suffering from inbreeding.

The scientists found that the vaquitas genetics were relatively healthy for their population size. While they had the highest proportion of harmful gene variants of any cetacean, they had the lowest number overall.

The vaquita's ability to cope in small numbers is likely the result of a population bottleneck around 25,000 years ago, when around a third of the species is estimated to have been wiped out. 

The process of genetic purging, where individuals with low fitness die and remove harmful gene variants from the species, is then thought to have ensured the population remained stable until gillnets were introduced.

'Genetic diversity in vaquitas is not so low that it constitutes a threat to their health and persistence,' Jacqueline explains. It simply reflects their natural rarity, essentially making them the marine equivalent of an island species. 

'The vaquitas' naturally low abundance has allowed them to gradually purge highly deleterious recessive gene variants that might negatively affect their health under inbreeding.

'This means that, relative to other species, the vaquita has a higher chance of rebounding from an extreme population crash without suffering severe genetic consequences.'

The potential of the species to recover is high, with the scientists demonstrating that if fishing stops immediately, a population size of almost 300 vaquita could be achieved in the next 50 years. Reducing fishing by 90% sees a slower recovery, to around 50 vaquita by 2070, while any lower declines are likely to end in extinction.

Co-author Dr Phillip Morin adds, 'The reality is that there is no predetermined outcome here.

'The survival of the individuals, and the species, is in our hands. There is a high probability that they can recover if we protect them from gillnets and allow the species to recover as soon as possible to its historic numbers.'