The vaquita porpoise, the world’s smallest marine mammal, is on the brink of extinction, with 10 or fewer still living in Mexico’s Gulf of California, their only habitat. But genetic analysis by a team of UCLA biologists and colleagues has revealed that the critically endangered species remains relatively healthy and can potentially survive – if illegal ‘gillnet’ fishing stops soon.
“Interestingly, we found that the vaquita is not doomed by genetic factors, such as harmful mutations, which tend to affect many other species whose gene pool has shrunk to a similar extent,” said Christopher Kyriazis, a UCLA doctoral student in ecology and evolutionary biology. co-lead author of the research. “Illegal fishing remains their biggest threat.”
The small porpoises, which are 4 to 5 feet long, often become entangled and die in the large-mesh gillnets used by poachers hunting totoaba, an endangered fish prized in some countries for its medicinal properties. perceived. While Mexico has banned totoaba fishing and made it illegal to use these nets in vaquita habitat, many say the bans aren’t always enforced.
The researchers analyzed the genomes of 20 vaquitas that lived between 1985 and 2017 and ran computer simulations to predict the species’ risk of extinction over the next 50 years. They concluded that if gillnet fishing ceases immediately, the vaquita has a very high chance of recovering, even when inbred. If, however, the practice continues, even moderately, the prospects for recovery are less optimistic.
The research is published in the journal Science.
“Compared to other species, the vaquita is more likely to bounce back from extreme population crashes without suffering serious genetic consequences from inbreeding,” said co-lead author Jacqueline Robinson, postdoctoral researcher at the UC San Francisco who earned her Ph.D. in biology at UCLA. “The genetic diversity of vaquitas is not so low that it poses a threat to their health and persistence. This simply reflects their natural rarity.
Genetic diversity is a measure of the differences that exist across the genome between individuals in a population. Large populations tend to have many differences, while naturally smaller or depleted ones have fewer, resulting in more genetically similar individuals. This similarity can often lead to a greater incidence of harmful mutations that endanger the population, as individuals are more likely to inherit the same mute gene from both parents, said lead author Kirk Lohmueller, associate professor of ecology. and evolutionary biology at UCLA and human genetics.
“A prevailing view in conservation biology and population genetics is that small populations can accumulate deleterious mutations,” Lohmueller said. “However, our finding that the vaquita likely has fewer highly deleterious mutations hidden in the population means it is better positioned to survive future inbreeding, which bodes well for its overall recovery.”
So what protects vaquitas from the genetic perils of inbreeding? This is largely because they have always been a small population in a very small habitat at the northern end of the gulf, the researchers said. Although their historical numbers are unknown, the first full survey in 1997 counted around 570 porpoises – a number that has steadily declined over the past 25 years but was initially not large.
“They’re basically the marine equivalent of an island species,” said Robinson, who noted that the species has survived for tens of thousands of years with low genetic diversity. “The naturally low abundance of vaquitas allowed them to gradually purge highly deleterious recessive gene variants that could negatively affect their health if inbred.”
In fact, Robinson said, of the 12 species of marine mammals — including vaquitas — the researchers genetically analyzed, vaquitas had the fewest number of potentially harmful mutations.
While the interplay between small population size, inbreeding and harmful genetic variation is complex, the approach used by the team in this study may help shed light on these dynamics.
“With genomic datasets, we now have the ability to deal with this complexity,” Robinson said. “Species can vary in their levels of harmful genetic variation, and not all will be affected in exactly the same way by reduction in population size or inbreeding. There are now many examples of species recovering from extreme decline.
“We hope that our analysis will be useful not only in demonstrating the recovery potential of the vaquita,” Kyriazis said, “but also in highlighting a novel genomics-based simulation approach for threatened species.”
Encouragingly, surviving vaquitas in the northern Gulf of California are actively breeding and appear healthy. But poachers’ gillnets continue to pose an existential threat to the species, and unless further action is taken to protect the porpoises, there is a real possibility that they will become extinct. The loss would be a great tragedy, said study lead author Robert Wayne of UCLA.
“The vaquita is symbolic of the unique diversity found in the Gulf of California, which was described by John Steinbeck in his marvelous 1951 book ‘The Log From the Sea of Cortez,'” said Wayne, a prominent professor of science. Ecology and Evolutionary Biology and Professor at the Howard Hughes Medical Institute. “It represents a unique evolutionary line – there is no similar species anywhere in the world – and its loss would deprive the ecosystem of an important predator adapted to this unique ecosystem.”