The vaquita is now so rare, and lives in waters turbid enough, that acoustic monitoring (detecting echolocation clicks) and visual survey vessels give estimates that disagree year to year, and an unknown number of individuals likely go undetected by either method entirely.
What would resolve this Sustained, well-funded acoustic and visual monitoring across several consecutive seasons, combined with genetic sampling to estimate the population from DNA left in the environment, a technique already piloted for this species.
OpenMarine mammal conservation biology and bioacousticsIUCN Red List of Threatened Species