The Expert Panel on Vaquita Acoustic Monitoring had its second meeting on April 28, 2015 to analyze the first four seasons (2011-2014) of the acoustic monitoring program. Results indicate an estimated 67% decline in vaquita acoustic activity in the sampled area from 2011 to 2014. The average rate of decline of 31% per year (95% Bayesian Credible Interval -51% to -10% per year) is considerably worse than the previously estimated 18.5% per year (95% Bayesian Credible Interval -46% to +19% per year) for the 2011-2013 sampling period. These worsening results were caused by the very low number of detections in 2014, which resulted in an estimated rate of decline from 2013 to 2014 of 42%. The Panel found it is virtually certain that the acoustic activity has declined between 2011 and 2014 (prob. = 0.996) with very high probability of a rate of decline greater than 10% per year (prob. = 0.976).
The Panel considered the monitoring design to be sound but analyses were complicated by the loss of some monitoring devices (CPODs) in some years (2011, 2014) and low numbers of recording days for numerous CPOD devices in 2013. Several analyses were developed to account for the uneven sampling; all indicated substantial declines similar to the agreed estimate of 31% per year. The Panel agreed that year-to-year variation in the proportion of vaquitas present within the monitoring area could not be estimated both because the time series is short but also because the bycatch rate has likely changed from year-to-year. An earlier concern that the highest density acoustic detections were along the southeast boundary of the study area was addressed by adding CPODs further to the south in 2014. The new locations had approximately 10 times fewer detections than those to the northwest, which is consistent with the spatial pattern found in earlier visual survey data. The consistent spatial pattern of vaquita densities (both using acoustic and visual data) lends support to the conclusion that the decline is more likely a decline in vaquita population size rather than a shift in distribution. (SC/66a/SM/25)