Examining the risk of inbreeding depression in a naturally rare cetacean, the vaquita (Phocoena sinus)

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 mor tality over the past 57 years. No variability in mitochondrial DNA (mtDNA) was found in vaquitas (n = 43). Because reducing gillnet mortality will req uire strong conservation action, the question was raised whether vaquitas a re doomed because of inbreeding depression and whether, therefore, mitigati on efforts would be futile. We use simulations to investigate the "doom hyp othesis" by first asking whether the current level of genetic variability r esults from the recent decline or from historical factors. If fixation was historical then deleterious alleles could have been selected out of vaquita s over thousands of years, reducing concerns about inbreeding depression. S imulations showed that fixation most likely resulted from historical rather than recent loss. Of 1,000 simulations done at plausible abundances and mu tation rates, 247 (84.3%) fixed before and 46 (15.7%) fixed during the rece nt decline. Fixation correlates with historical abundance, making it more l ikely that because vaquitas are fixed, they are also a naturally rare speci es. However, because studies on purging deleterious alleles have nor shown purging to be universally beneficial we also examine the doom hypothesis us ing 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 r esults and found that the data required, such as the adult survival rate, w ill be impossible to obtain. We conclude that because the doom hypothesis c annot be affirmed this risk factor should not delay conservation actions.