Marine Mammal Sci (1999)

DOI: 10.1111/j.1748-7692.1999.tb00875.x


Uncertainty about the magnitude of various risks facing endangered species can paralyze conservation action. The vaquita is a naturally rare porpoise that has declined to the low hundreds of individuals because of gillnet mortality over the past 57 years. No variability in mitochondrial DNA (mtDNA) was found in vaquitas (n= 43). Because reducing gillnet mortality will require strong conservation action, the question was raised whether vaquitas are doomed because of inbreeding depression and whether, therefore, mitigation efforts would be futile. We use simulations to investigate the “doom hypothesis” by first asking whether the current level of genetic variability results from the recent decline or from historical factors. If fixation was historical then deleterious alleles could have been selected out of vaquitas over thousands of years, reducing concerns about inbreeding depression. Simulations showed that fixation most likely resulted from historical rather than recent loss. Of 1,000 simulations done at plausible abundances and mutation rates, 247 (84.3%) fixed before and 46 (15.7%) fixed during the recent decline. Fixation correlates with historical abundance, making it more likely that because vaquitas are fixed, they are also a naturally rate species. However, because studies on purging deleterious alleles have not shown purging to be universally beneficial we also examine the doom hypothesis using data on the response to inbreeding of a wide variety of captive animals. Responses are so variable that the doom hypothesis cannot be affirmed. We further explore whether more data from vaquitas would lead to conclusive results and found that the data required, such as the adult survival rate, will be impossible to obtain. We conclude that because the doom hypothesis cannot be affirmed this risk factor should not delay conservation actions.