Summary
The authors report visual surveys in the Zero Tolerance Area in 2019 and 2021 and used expert elicitation to judge how many different vaquitas had been seen. They concluded that between 7 and 15 unique individuals were seen in 2019 and between 5 and 13 in 2021, which is more than a projection from the 2018 estimate would predict. These are counts of animals seen in one small area, and they are not population estimates.
Why it matters
It was the first published evidence that more vaquitas survived after 2018 than the declining trend implied, and that calves were still being born. It also explains why full population estimates were no longer feasible and what replaced them.
Key findings
- 2019 survey: 7 to 15 unique vaquitas judged plausible, most likely 11, and 84% belief in 9 to 13 (p.230).
- 2021 survey: 5 to 13 judged plausible, most likely 7 or 8, and 78% belief in 6 to 10 (p.230).
- Calves: most likely 3 in 2019 (97% chance of 2 or more); median 1.5 in 2021 (about equal chance of 1 or 2). Calves were seen in both years and all vaquitas appeared healthy.
- Projecting the 2018 estimate forward gave a probability of only 0.06 (2019) and 0.07 (2021) of exceeding the elicited minimums (p.230).
- Survey time was 18 days in 2019 and 17 days in 2021, but most days were lost to wind too strong for sighting and tracking.
Caveats
The numbers are the experts' judgement of unique animals seen within the Zero Tolerance Area, not an estimate of the whole population, and the authors say the 2021 figures carry more uncertainty than those for 2019. Total population size could not be estimated because a full line-transect survey was unaffordable and the acoustic detectors were being stolen.
Original abstract English
In 2018, it was estimated that fewer than 20 of Mexico’s endemic vaquita porpoise Phocoena sinus remained, and the species was declining by 47% yr−1. Entanglement in gillnets is the sole threat to the species, and since the last population size estimate, gillnetting has increased in the small area where most vaquitas remain — a 12 × 24 km area in the Gulf of California near San Felipe, Mexico. We conducted research efforts in 2019 and 2021 in that area to estimate the minimum numbers of adults and calves and look for any signs that vaquitas are unhealthy. Through expert elicitation, we estimated between 7 and 15 unique individuals were seen in 2019 and 5−13 were seen in 2021. Calves were seen in both years, and all vaquitas appeared healthy. Population projections from the last full survey indicated that more vaquitas have survived than expected. We suggest that these surviving adult vaquitas may have learned to avoid entanglement in gillnets. These vaquitas and their calves provide hope that the species can survive. However, given the high levels of illegal gillnetting and the theft of equipment which hindered our monitoring efforts, and with only around 10 individuals remaining, survival can only be assured if vaquita habitat is made gillnet-free.