• Barbara L. Taylor,

    1. Southwest Fisheries Science Center, P. O. Box 271, La Jolla, California 92038-0271, U. S. A. E-mail: taylor@caliban.ucsd.edu
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  • Lorenzo Rojas-Bracho

    1. Universidad Autónoma de Baja California (UABC)/Programa Nacional de Mamiferos Marinos Facultad de Ciencias Marinas, Instituto Nacional de la Pesca (INP), Km 103 Carretera Tijuana-Ensenada, Ensenada 22800, Baja California, Mexico
    2. Southwest Fisheries Science Center, P. O. Box 271, La Jolla, California 92038-0271, U. S. A.
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  • 1

    Ken Norris was for us not only important for his contribution to the recognition of the vaquita as a species but also as an inspired thinker. He was a role model for the hybrid vigor contributed to many fields by the creative thinker who will boldly cross from one area to the next pollinating our imaginations. With Ken in the field of marine mammal science there was never any danger of inbreeding depression. We therefore dedicate to Ken this manuscript, which we hope uses Ken's style of bringing all possible tools to bear on a problem by blending lessons from the genetics of captive animals, the population dynamics of wild marine mammals, and computer simulations of evolution to reject the hypothesis of certain doom of the species he brought to the attention of science.


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.