Acoustic monitoring of the critically endangered vaquita

What species are out there in the big blue ocean? How many individuals are there? These are some of the biggest questions scientists face when studying populations. Specifically, when a particular species is in danger of becoming extinct. In marine science, visual tracking alone is not always practical. Our first post on the vaquita was massively popular, and our readers asked for more content on the vaquita, especially on how to track their population. This post will discuss an acoustic method monitoring the vaquita population and the successes and limitations that come with it.

The issue with endangered species: few individuals

Animals are elected to the Endangered Species List. Elections are based on factors like the presence of any immediate or hazardous threats and current population size. There are four categories of endangered species: vulnerable, endangered, critically endangered, and extinct in the wild. Critically endangered populations have less than 250 individuals. Scientific studies that count identified communities are an important aspect when considering what species are considered endangered.

So how do scientists measure such populations? Surveys are typically based on visual sightings. However, those methods are not as reliable when your target is small and shy like the vaquita. Getting an accurate count is difficult when populations are dangerously small and hard to track. In 1997, the first precise estimate of their population was 567. By 2008, the population was down to approximately 245.

One solution to this problem is to utilize Passive Acoustic Monitoring (PAM). PAM is exactly what it sounds like. Scientists use underwater microphones, or hydrophones, to record and analyze underwater sound. This lets researchers passively listen to the animals around them without disturbing them. Because sound travels further than light underwater, it is a good tool to use in these studies. Sound can give a more complete snapshot of the underwater world, without needing to actually see it.

Listen to some clicks recorded from a Vaquita here!

The invisible becomes visible

As stated previously, vaquita are evasive and difficult to track down visually. However, as a member of the toothed whale family, they use echolocation to find their prey (small fish, crustaceans, and cephalopods). This constant soundtrack of clicks and whistles makes acoustic methods of tracking very reliable. 

In this example, researchers used detectors that could automatically identify sequences of clicks, also known as click trains, that were likely from vaquitas. Then, researchers manually confirmed those sequences. Analysis was based on the average number of identified clicks within 24 hours. This benchmark, when combined with statistical models, painted a robust picture of the present vaquita population.

One problem with this method is the potential loss of equipment. Hydrophones are threatened by the same dangers that vaquitas face. Passing boats working with fishing gear can entangle and destroy recording equipment. In this case, that meant that researchers could only study in areas and at times with limited gillnet and trawl fishing: inside the Vaquita Refuge from June through September.

After this study, a two-year gillnet ban was introduced to try and stop the damage these nets cause. This was a great step, but more work needs to be done. The more we understand, the more detailed our actions can be to protect those animals and their habitats.

Continued devastating loss

From 2011 to 2015, annual rates of change of acoustic activity decreased: -17%, -3%, -46%, and -35% respectively. In other words, acoustic activity went down every year. The above figure shows that areas at the beginning of the study with a higher volume of clicks (yellow areas with over 1,000 clicks) were down to much lower levels in the later years (teal areas with 10-50 clicks). It leads us to believe that there were fewer animals in those areas during those years.

Even with limited resources, this study showed a substantial population decline. Researchers estimated that over this time period, the population decreased by 34% per year! Tragically, the current estimated population is fewer than 20 individuals.

A solution for the vaquita?

The situation is complicated, but it must end before the damage is irreversible. There are multiple steps we can take to fix this problem. First, countries need better enforcement against banned drift nets. Second, they must aid in efforts to retrieve abandoned ghost nets and other harmful pollution. Third, we must support and develop alternative fishing gear and support sustainable fishing.

The Vaquita is not the only species suffering from illegal fishing, and they are not the only cetacean species close to extinction. Unfortunately, the most endangered populations of porpoises and dolphins around the world suffer from similar gillnet threats. Over 600,000 marine mammals/ year are expected to be lost to global bycatch.

This is a problem, but it has a solution. There is still hope to save the vaquita and other endangered marine mammals! If everyone comes together to make the right choices, who knows what kind of world we can build.

Thank you for reading!

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Sources and further reading

 Check out our other posts on the effects of noise on to marine mammals.

• Jaramillo-Legorreta, A., Cardenas-Hinojosa, G., Nieto-Garcia, E., Rojas-Bracho, L., Ver Hoef, J., Moore, J., … Taylor, B. (2017). Passive acoustic monitoring of the decline of Mexico’s critically endangered vaquita. Conservation Biology, 31(1), 183–191. https://doi.org/10.1111/cobi.12789

• This article discussing the continued threats that gillnets cause.
• See how NOAA uses PAM to track marine mammals in Alaska.

Vaquita, the most endangered cetacean in the world