The most recent estimate of total population size comes from a shipboard line transect survey in 1997 that was stratified to provide relatively intensive coverage of the core area but that also sampled adjacent areas to the south, east and north, including the shallow marginal bays and the even shallower Colorado River delta. Although imprecise because of variable sighting rates and other factors, the estimate of 567 (95% CI 177 to 1,073) was a great improvement on previous estimates and stands as the best currently available.

There is no immediate means of estimating trend, so it is necessary to impute the direction and rate of population change by reference to a population model laden with assumptions.

Naturally rare (Taylor and Rojas-Bracho 1999) and very difficult to detect and count (cryptic) (Gerrodette et al. 1995). No population subdivision is known or suspected, i.e. no subpopulations, but also no fragmentation. Most recent estimate of total population size (1997 shipboard line transect survey): 567 (95% CI 177 to 1,073) (Jaramillo-Legorreta et al. 1999).

Given the difficulty of sampling the vaquita population, generation time and percent mature (i.e., capable of reproduction) can only be estimated crudely and by analogy with the life history and population biology of the better-studied harbour porpoise (Phocoena phocoena). Thus reasonable default values for porpoises (phocoenids) would be 10 years and 55%, respectively.

Ongoing decline inferred from available information on abundance and bycatch rate. Even an unrealistically optimistic scenario - high end of 95% CI for population size (1,073), lower of two estimates of annual bycatch mortality for a single fishing port (39 porpoises) - indicates likely decline (Rojas-Bracho and Taylor 1999).

Decreasing