Vaquita porpoises have long been considered one of the most endangered species in the world. They are small sea mammals related to dolphins and are found in the Gulf of California in Baja, California, Mexico. Compared to their other dolphin and whale relatives, vaquitas are small, averaging 4.5 feet in length. Their small size and proximity to fisheries threaten their population. Often vaquitas get caught in gillnets used to target other fish for both commercial and illegal fishing in a process called bycatch. As of February 2022, scientists estimate that there are only ten vaquitas left in the wild – this could spell danger for the population.
A small wildlife population is just as big of a threat as hunting or bycatch because of a genetic phenomenon called inbreeding depression. Imagine a large group of vaquitas (the largest recorded population was about 600 individuals). In this group, every organism has unique genes passed on from its mother and father – just like humans. So when there are lots of different individuals, there are more genetically distinct mates to choose from and more genetic diversity in the population. But why does this matter?
One reason is that more genetic diversity means the population is more prepared for an ecological catastrophe. If something in the environment drastically changes, like warming and acidifying oceans, then perhaps due to the high genetic diversity, several organisms have genes that help them cope with that disturbance and they can mate and pass that coping mechanism onto their offspring so the population survives. When a population gets too small, there are fewer genetically distinct individuals to mate with and the genetics become too similar amongst the population. This can lead to genes that cause disease to become more common and is another threat to the population and this is inbreeding depression.
A common human example is of the royal families in Europe who sought to maintain the royal bloodline by only marrying other royals – many of whom were close relatives. They had children and those children also married into the bloodline and this ultimately caused hemophilia, a blood disorder, to become uncharacteristically common amongst royals. The proliferation of genetic diseases is also a threat to small wildlife populations.
Threatened populations become small → small populations have inbreeding → effects of inbreeding causes the population to get smaller → and the cycle repeats. This is often referred to as the “extinction vortex.
But, a recent study by Dr. Jacqueline Robinson and her co-authors indicates that there is hope for the vaquita.
In this study, the researchers wanted to determine the genetic threats to the vaquita. Since the population is so small, is it in this extinction vortex? And if so, is there anything we can do to save the species?
To answer these questions, the researchers obtained archived tissue samples from vaquita collected from 1985 to 2017 spanning three vaquita generations. They were able to sequence the DNA from each sample, then compare the samples across the generations and between individuals. By comparing the DNA and looking for similarities between the samples, the researchers can estimate how much inbreeding is occurring within the population. Inbreeding depression usually occurs in populations with less than 25 breeding individuals. But, they did not see as much inbreeding as they expected. This is good news for the vaquita! The researchers think this is because the initial population size of the vaquitas (~600) was large enough to prevent genes with negative effects from becoming common in the population and passed down to other generations so far.
Using their genomic data, the researchers simulated future vaquita population growth with varying levels of bycatch estimates to understand if the fishing threats outweigh the genetic threats. Their results showed that the vaquita isn’t doomed to extinction due to inbreeding but may be doomed if the harmful fishing practices continue. Their models showed that if there are no bycatch deaths, 94% of the models predict a population increase and only 6% predict extinction. But, no bycatch deaths is a tall order. If bycatch death rates are reduced by 80%, only 38% of the models predict that the vaquita population will survive.
The take-home message here is that the vaquitas are not genetically doomed. Despite their small population size, they are not suffering from inbreeding. This provides a ray of hope for their conservation and for the conservation of other wildlife populations in similar circumstances. However, their survival now depends on human actions, mainly halting the use of gillnets and reducing vaquita bycatch in the Gulf of California. There are gillnet bans in place and a gillnet exclusion zone in the gulf, but illegal fishing still occurs. Now, the next step for conservationists and policymakers is to figure out how to reduce the human threat to the vaquita and other organisms.
Robinson, Jaqueline A. et al. The critically endangered vaquita is not doomed to extinction by inbreeding depression. 2022. Science (376), 635-639. DOI: 10.1126/science.abm1742.