A case study made by WWF of passive acoustic monitoring for the Vaquita:
Vaquita (phocoena sinus) are shy purpoises endemic in the Gulf of California, Mexico. Colateral victim of illegal and also endangered Totoaba fisheries, their population was very difficult to establish.
In response to their increasingly rapid declines, 44 C-PODs (passive acoustic sensor to detect specifically clicks from toothed whales ) were deployed from 2011 to 2015 to monitor the vaquita refuge set up by the Government of Mexico (Jaramillo-Legorreta et al. 2016).
Visual surveys had previously been used to monitor the population, however this method of monitoring becomes increasingly expensive with small populations. C-PODs were deployed in a grid of 48 points across the refuge, including 14 buoys around the perimeter, recording continuously for three months per season. Due to loss of sensors, data were collected from 46 points. In order to estimate vaquita density and the population trend, the number of identified clicks in 24 hours was used as a metric.
Trend analysis of these data revealed a mean annual decline of -34% per year in the vaquita population between 2011 and 2015 (Jaramillo-Legorreta et al. 2016). These trends are virtually identical to those from previous visual and acoustic surveys of the vaquita, indicating their validity. A 2-year gillnet ban was enforced by the Mexican Government following preliminary results of the acoustic surveys in 2014.