Summary
A line-transect survey in October and November 2008 combined visual searching from a large research vessel with an acoustic array towed by a sailboat in the shallowest areas. It estimated 245 vaquitas (95% CI 68 to 884), which is 57% lower than the 1997 estimate of 567. The authors found an 89% probability that the total population declined between the two surveys.
Why it matters
It is the second range-wide abundance estimate and the first to show, with a stated probability, that the population fell after 1997. It also found that about half the population lay inside the Vaquita Refuge.
Key findings
- 2008 total abundance: 245 animals (CV = 73%, 95% CI 68 to 884); 57% below the 1997 estimate, an average decline of 7.6% per year.
- Bayesian analysis gave an 89% probability of decline in total population size over the 11 years, and 100% in the central part of the range.
- Probability of detecting a vaquita acoustically on the trackline was 0.41 (CV = 108%), estimated from simultaneous visual and acoustic data in a calibration area.
- Vaquita Refuge: 121 vaquitas (95% CI 59 to 250), about half of the 245.
- In the central area sampled the same way in 1997 and 2008, the 2008 estimate was 41% of the 1997 estimate (88 sightings in each year).
Caveats
Both the 1997 and 2008 estimates have wide confidence intervals that include or nearly include the other point estimate, and the 2008 estimate rests on very few acoustic detections (a single detection in the North stratum gave an estimate of 72 vaquitas). The acoustic part depends on an assumed group size of 1.9 and on a calibration with a CV of 108%.
Original abstract English
A line-transect survey for the critically endangered vaquita, Phocoena sinus, was carried out in October–November 2008, in the northern Gulf of California, Mexico. Areas with deeper water were sampled visually from a large research vessel, while shallow water areas were covered by a sailboat towing an acoustic array. Total vaquita abundance in 2008 was estimated to be 245 animals (CV = 73%, 95% CI 68–884). The 2008 estimate was 57% lower than the 1997 estimate, an average rate of decline of 7.6%/yr. Bayesian analyses found an 89% probability of decline in total population size during the 11 yr period, and a 100% probability of decline in the central part of the range. Acoustic detections were assumed to represent porpoises with an average group size of 1.9, the same as visual sightings. Based on simultaneous visual and acoustic data in a calibration area, the probability of detecting vaquitas acoustically on the trackline was estimated to be 0.41 (CV = 108%). The Refuge Area for the Protection of the Vaquita, where gill net fishing is currently banned, contained approximately 50% of the population. While animals move in and out of the Refuge Area, on average half of the population remains exposed to bycatch in artisanal gill nets.