Mortality in gillnets of various mesh size has long been recognized as the most serious and immediate threat to the vaquita?s survival (Vidal 1993, 1995; Reeves and Leatherwood 1994; IWC 1995:87, 167; Rojas-Bracho and Taylor 1999; Rojas-Bracho et al. 2006). The only available estimates of the vaquita bycatch rate are 39 (using one method) and 84 (using a different method) animals killed per year by boats from a single port (D?Agrosa et al. 2000). This alone would represent 7 or 15%, respectively, of the estimated total population size (Rojas-Bracho and Jaramillo-Legorreta 2002). Other potential threats that have been suggested but that appear not to be significant risk factors at present include inbreeding depression, pesticide exposure and ecological changes as a result of reduced flow from the Colorado River (Taylor and Rojas-Bracho 1999). The last of these may be important in the long term and deserves investigation.