Biological Conservation 25 (1983) 209-232 Genetic Diversity in California Sea Otters: Theoretical Considerations and Management Implications Katherine Rails, Jonathan Ballou Department of Zoological Research, National Zoological Park, Smithsonian Institution, Washington, DC 20008, USA & Robert L. Brownell, Jr US Fish and Wildlife Service, National Museum of Natural History, Washington, DC, 20560, USA ABSTRACT The California sea otter population was reduced to a small number of animals by fur hunters in the 18th and 19th centuries. The population has partially recovered but is still threatened, largely because of its vulnerability in the event of a major oil spill. The translocation of enough animals to establish a second colony outside the present range has been suggested as a means of reducing the vulnerability of the population. Any population that has been reduced to a small number and then allowed to increase may have lost some of its original genetic diversity. The loss of genetic diversity often results in deleterious effects, such as increased juvenile mortality and reduced fertility. It is therefore of interest to determine the degree of genetic diversity which the California sea otter population should have theoretically lost during its population 'bottleneck' and the number of otters which should be translocated in order to avoid substantial loss of genetic diversity in the new colony. Application of some of the concepts of population genetics to the California sea otter indicates that the current population should theoreti- cally retain a large proportion (77%) of whatever genetic diversity 209 Biol. Conserv. 0006-3207/83/0025-0209/$03'00 © Applied Science Publishers Ltd, England, 1983. Printed in Great Britain