WASHINGTON: The most comprehensive genetic assessment to date of the vaquita, the world's rarest marine mammal, is offering a glimmer of hope that this small tropical porpoise native to Mexico's Gulf of California may avoid extinction despite its population dwindling to about 10.
Researchers said on Thursday (May 5) genome data from 20 vaquitas showed that while the species possesses low genetic diversity - differences in the DNA among the various individuals - the number of potentially harmful mutations that could endanger its survival through inbreeding was quite low.
The vaquita, first described by scientists in 1958 and now deemed critically endangered, is the smallest cetacean, the group including whales, dolphins and porpoises, reaching about 1.5 metres long and 54kg. Its torpedo-shaped body is gray on the top and white on the underside with a dark ring around the eyes.
Computational simulations performed by the researchers to predict extinction risk showed that vaquitas, whose population has fallen more than 99 per cent since the outset of the 20th century due to human activities, have a high chance of rebounding if fishing gillnets are completely eliminated from their habitat. Gillnets, large curtains of netting that hang in the water, are used to catch fish and shrimp but have killed many vaquitas that get entangled and drown.
"Our key findings are that the vaquita is not doomed to extinction by genetics, as some have begun to assume," said UCLA ecology and evolutionary biology doctoral student Christopher Kyriazis, co-lead author of the study published in the journal Science. "These findings are important because they provide hope for a species that is at the brink of extinction, one that many are now giving up on."
A particular threat is gillnet poaching of an endangered fish called the totoaba. Totoaba swim bladders, purported to be a fertility enhancer, are prized in China.
"Dried totoaba swim bladders are traded on the black market in China for traditional medicinal purposes, and fetch a higher price than cocaine," said study co-author Phillip Morin, a research geneticist at the US National Oceanic and Atmospheric Administration's Southwest Fisheries Science Center.
Vaquitas, still actively reproducing despite their small numbers, inhabit the northern Gulf of California, also called the Sea of Cortez, between mainland Mexico and the Baja peninsula.
"Gillnet fishing in the vaquita's habitat has been banned, but the ban has not been enforced and vaquitas continue to perish in nets," said study co-lead author Jacqueline Robinson, a postdoctoral researcher at the University of California, San Francisco's Institute for Human Genetics.
The first population estimate, conducted in 1997, found there were approximately 570 vaquitas. The population has since declined by up to about 50 per cent annually.
The researchers gauged the genetic health of the species, which diverged evolutionarily from its closest relatives about 2.5 million years ago, by examining samples from 20 individuals obtained between 1985 and 2017, mostly archived from vaquitas that had died. One worry with such a small population is that inevitable mating among closely related individuals could increase deleterious mutations harmful to species survival.
The genome data indicated that the vaquita population already was relatively small - about 5,000 individuals - for hundreds of thousands of years before the crash caused by human activities, making low genetic diversity a natural feature of the species.
It also showed there has been relatively little inbreeding among vaquitas and very few harmful recessive mutations that may lead to congenital deformities when inbreeding that could imperil species survival - lower that 11 other cetacean species assessed, including the blue whale.
One cetacean species already appears to have been driven to extinction by humans in recent decades: the baiji, or Chinese river dolphin.
"Because of its shy nature, there is very little known about the vaquita," Robinson said. "The species is in danger of going extinct before we will even fully know what we are losing, and there is no replacing it once it's gone."