New Delhi: The world's smallest porpoise is at the brink of extinction, thanks to unchecked gillnetting. Roughly 10 vaquitas remain in the Gulf of California in Mexico. However, enough genetic resilience has been retained for the species to ensure that illegal gillnetting for a valuable fish does not wipe them out, a new study has found. 


The findings of the new genetic analysis were recently published in the journal Science. 


There is a contention that inbreeding among the few remaining vaquita will lead to extinction regardless of whether gillnetting ends. The new study contradicts that belief, and expands on similar findings in 2020 based on DNA from a single vaquita. 


What Are Vaquita Porpoises?


Vaquita is a small porpoise found only in the northern Gulf of California, Mexico, and is the world's rarest marine mammal. A porpoise is specifically any of seven species of toothed whales distinguishable from dolphins by their more compact build, generally small size, and curved blunt snout. Porpoises are among the smallest members of the cetacean family, which includes whales, porpoises and dolphins. The scientific name of the vaquita porpoise is Phocoena sinus. 


The vaquita is the world's smallest cetacean, and is unique among porpoises as it is the only species of that family found in warm waters. Vaquita means "little cow" in Spanish. The most striking feature of a vaquita is a dark ring around the eyes, along with a proportionally large dorsal fin. Since the vaquita is the only species among the family of porpoises round in warm waters, it is unique among the porpoises. Also, the size of the dorsal fin is believed to be an adaptation to the warm waters, allowing for extra body heat to dissipate.


The plight of cetaceans is exemplified by the rapid decline of the vaquita in Mexico, with about 10 individuals remaining, according to the World Wildlife Fund (WWF). This makes the vaquita the most endangered marine mammal in the world.


The vaquita is listed as a critically endangered species by the International Union for Conservation of Nature (IUCN).


The little porpoise, which was not discovered until 1958, is often caught and drowned in gillnets used by illegal fishing operations in marine protected areas within Mexico's Gulf of California. The estimated size of the original vaquita population in 1997 was 600. In the last few years, the population has dropped drastically. Between 2011 and 2016 alone, there has been a 90 per cent decline in the vaquita population, according to porpoise.org.


Vaquitas tend to be shy and elusive, and avoid boats when approached.


What Genetics Reveal About The History Of Species


An international team of researchers examined the genetic patterns of vaquitas from tissue samples collected by Mexican researchers since the 1980s. The genomes of 20 vaquitas that lived between 1985 and 2017 were analysed. 


In a statement issued by NOAA (National Oceanographic and Atmospheric Administration) Fisheries West Coast Region, Jacqueline Robinson, a co-author of the study, said if humans can allow these animals to survive, they can do the rest. She explained that genetically, they still have the diversity that let them thrive for hundreds of thousands of years, until the gillnets arrived.


Lorenzo Rojas-Bracho, a co-author of the new study, said who would have thought that several decades later, the tissue samples of vaquitas collected in the 1980s could reveal so much. 


He explained that genomics gives researchers clues into the species' past but also lets them peer into the future. Despite the small numbers, vaquita porpoises could recover if humans stopped killing them, he said.


The species emerged roughly 2.5 million years ago, eventually adapting to the shallow, highly productive waters of the northern Gulf of California, genetics reveal. According to the earlier analysis, the species' abundance fluctuated from a few thousand to around 5,000 over the last 2,50,000 years, making vaquita porpoises naturally rare compared to many other marine mammals.


How Has The Relatively Small Size Of Vaquita Population Helped The Species?


According to the study, the relatively small size of the vaquita population appears to have reduced the risk associated with inbreeding. Since smaller populations have less genetic variation from one animal to another, the chances of harmful mutations are few. When two animals with harmful traits occasionally mated, they produced compromised offspring that likely died. As a result, many harmful traits were purged from the population.


Larger populations have greater genetic variation, which results in more harmful genetic variants. The impacts of those harmful variants rarely appear because an animal must get the same harmful variant from both parents for it to be expressed. However, this does not happen because the variants are rare.


The odds of close relatives mating increases when a large population shrinks rapidly, resulting in an increase in the likelihood that both parents carry the same harmful mutation. Offspring of these parents suffer from a condition known as "inbreeding depression", which undermines their health.


In a statement issued by University of California, Los Angeles, Robinson said that relative to other species, the vaquita has a higher chance of rebounding from an extreme population crash without suffering severe genetic consequences from inbreeding. She explained that genetic diversity in vaquitas is not so low that it constitutes a threat to their health and persistence, and simply reflects their natural rarity. 


What Protects Vaquitas From The Genetic Perils Of Inbreeding?


Genetic diversity is a measure of the differences that exist across the genome among individuals in a population, with large populations mostly having many differences, and smaller or decimated ones having fewer. When there are fewer differences in the genome, the individuals tend to be more genetically similar. The similarity often results in a greater incidence of harmful mutations that endanger the population, according to senior author Kirk Lohmueller. This is because the individuals are more likely to inherit the same mutated gene from both parents.


Lohmueller said that vaquita porpoises are essentially the marine equivalent of an island species, and noted that the species has survived for tens of thousands of years with low genetic diversity. He explained that the vaquitas' naturally low abundance has allowed them to gradually purge highly deleterious recessive gene variants that might negatively affect their health under inbreeding. 


The fact that vaquitas have always been a small population in a very small habitat in the northern tip of the gulf is one of the reasons why the species is protected from the genetic perils of inbreeding, according to researchers. 


Lohmueller also said that a prevailing view in conservation biology and population genetics is that small populations can accumulate deleterious mutations. He added that their finding that the vaquita likely had fewer strongly deleterious mutations hiding in the population means they are better poised to survive future inbreeding, which bodes well for their overall recovery.


Robinson said that of the 12 marine mammal species the researchers genetically analysed, vaquitas had the lowest number of potentially harmful mutations. 


According to the researchers, the deteriorating health of the offspring can compound itself and propel the species into what some call an "extinction vortex". 


How Gillnets Pose A Larger Risk Than Inbreeding


Gillnets are set for shrimp and finfish, including totoaba, and the IUCN Red List of threatened species, with which vaquitas share their habitat. Totoaba are captured for their swim bladders, which are valued as financial investments and are used for traditional medicinal purposes in China.


However, the gillnets stretch through the water like giant tennis nets to catch the fish entangle vaquitas, due to which the small porpoises drown.


In this way, gillnets set for totoaba rapidly decimated vaquitas, killing them too fast for inbreeding to develop among the survivors. Though Mexico has outlawed totoaba fishing and made the use of those nets in the vaquitas' habitat illegal, many say the bans are not always enforced. According to the study, the few vaquita left still reflect the broader genetic diversity of the larger population they came from.


Phillip Morin, a co-author of the research, said the reality is that there is no predetermined outcome here, and that the survival of the individuals and the species is in our hands. He explained that there is a high probability genetically that vaquita porpoises can recover, if humans protect them from gillnets and allow the species to recover as soon as possible to historical numbers. The researchers concluded that if gillnet fishing ends immediately, the vaquita has a very high chance of recovery. 


Are The Species Likely To Recover?


Based on the genetics of archived vaquita samples, the scientists ran computer simulations to project how the population would fare under different scenarios for their protection. Immediate and complete elimination of mortality from gillnets led to a high probability that the species will recover, the scientists found. 


However, even low levels of continuing gillnet mortality rapidly reduced the species' chances of survival.


Recent surveys observed some of the elusive marine mammals that remain, and found that the few survivors looked healthy, and some had calves reflecting recent reproduction. 


Christopher Kyriazis, who co-lead the new study, said that while scientists know that the species' ability to recover is not limited by their genetics, vaquitas have very little time left. 


Vaquitas Are Not Doomed By Genetic Factors


He said that interestingly, researchers found that the vaquita is not doomed by genetic factors, like harmful mutations, that tend to affect many other species whose gene pool has diminished to a similar point. According to him, outlawed fishing remains the greatest threat to vaquita porpoises.


He said that the researchers hope their analysis is useful not only in demonstrating the potential for the vaquita to recover, but also in highlighting a novel genomics-based simulation approach for endangered species. 


"If we lose them, it would be the result of our human choices, not inherent genetic factors," Kyriazis said.


Surviving Vaquitas Appear Healthy


Robert Wayne, the senior author of the study, said that encouragingly, the surviving vaquitas in the northern Gulf of California are actively reproducing and appear healthy. However, poachers' gillnets continue to pose an existential threat to the species, and unless further measures are taken to protect the porpoises, there is a distinct possibility they may go extinct, Wayne said. The loss would be a great tragedy, he added.


Wayne also said that the vaquita is symbolic of the unique diversity found in the Gulf of California. He noted that the vaquita represents a unique evolutionary lineage, for there is no similar species anywhere in the world, and its loss would rob the ecosystem of an important predator adapted to this unique ecosystem.