Summary
The authors combined a ship-based visual line-transect survey with an array of underwater click detectors (C-PODs), run at the same time in autumn 2015, to estimate how many vaquitas were left across the species' whole range. They estimated 59 vaquitas (median of the posterior distribution, 95% Bayesian credible interval 22 to 145), a decrease of 92% since 1997 (95% credible interval 80 to 97%). They also projected the population forward under two scenarios, one with the gillnet ban kept and enforced and one with a return to the 2011-2015 rate of decline.
Why it matters
It is the estimate for the whole range at the start of the two-year gillnet suspension of 2015, and the source of the figure of about 60 vaquitas. Its projections tie the outcome directly to whether gillnets are removed from the vaquita's habitat.
Key findings
- Visual survey 28 September to 4 December 2015: 14 usable sightings over 1,305.9 km of effort, all inside the core sub-stratum; none outside it.
- Total 2015 estimate 59 (95% credible interval 22 to 145): 47 seen in the core area (18 to 115) and 12 in the shallow area (5 to 30), the latter from acoustic data.
- Probability that fewer than 50 remain is 36%; probability that fewer than 100 remain is 87%.
- In the core sub-stratum, estimates were 557 (1997), 166 (2008) and 47 (2015), a 92% decrease from 1997 to 2015 (95% credible interval 80-97%).
- Population grows only if bycatch is under 2.2 vaquitas a year (credible interval 0.9-5.5); with a return to the 34% annual decline, quasi-extinction (under 10) is expected by 2022.
Caveats
The estimate is imprecise (CV 0.50), and the paper calls it an estimate of what remained in autumn 2015, not a count. The shallow-stratum figure assumes clicks are detected equally well there and in the calibration area, an assumption the authors say has only weak support, though they judge its effect on the total to be negligible. The projections ignore inbreeding depression, Allee effects and stochasticity, and the authors say they could be overly optimistic. This paper does not report the later upward revision of the 2015 figure.
Original abstract English
The number of Mexico's endemic porpoise, the vaquita (Phocoena sinus), is collapsing primarily due to bycatch in illegal gillnets set for totoaba (Totoaba macdonaldi), an endangered fish whose swim bladders are exported to China. Previous research estimated that vaquitas declined from about 567 to 245 individuals between 1997 and 2008. Acoustic monitoring between 2011 and 2015 showed a decline of 34%/year. Here, we combine visual line transect and passive acoustic data collected simultaneously in a robust spatial analysis to estimate that only 59 (95% Bayesian Credible Interval [CRI] 22-145) vaquita remained as of autumn 2015, a decrease since 1997 of 92% (95% CRI 80-97%). Risk analysis suggests that if the current, temporary gillnet ban is maintained and effectively enforced, vaquitas could recover to 2008 population levels by 2050. Otherwise, the species is likely to be extinct within a decade.