Southern sea otters (Enhydra lutris nereis) have made a slower than expected recovery after hunting for the fur trade drastically reduced their numbers prior to the 20th century. Recent abundance counts suggest that this population continues to struggle to recover. A long-term, collaborative carcass recovery program in California has allowed the investigation of causes of mortality in sea otters for the last 35 years. Freshly deceased sea otters (n = 105) recovered between February 1998 and June 2001 underwent detailed necropsy at the California Department of Fish and Game's Marine Wildlife Veterinary Care and Research Center and the University of California at Davis. This effort has provided an exceptional opportunity to begin to understand the factors influencing survival in a threatened marine mammal species.
Primary and contributing causes of death were stringently standardized so that patterns in mortality could be identified. The distributions of common causes of death were evaluated among age and gender classes, and associations between primary and contributing causes of death were evaluated to determine if certain conditions predispose otters to a final cause of death. The temporal and geographic distributions of the common causes of mortality were evaluated to identify clusters of cause-specific mortality along the coastline.
The four leading causes of mortality identified during the study period were shark attack, encephalitis due to Toxoplasma gondii, cardiac disease, and acanthocephalan infection. The proportionate mortality attributed to encephalitis caused by the protozoal parasites T. gondii and Sarcocystis neurona has increased substantially in recent years compared to the mortality reported in otters examined from 1992 through 1995.1 Cardiac disease is a newly recognized cause of mortality in sea otters, and an association between T. gondii encephalitis and cardiac disease is currently being investigated. Otters with fatal shark bites were over three times more likely to have pre-existing T. gondii encephalitis, suggesting that encephalitis predisposes otters to attack by sharks. Mortality due to acanthocephalan infection was significantly more common in juvenile otters than adults. Geographic clusters of cause-specific mortality were detected for T. gondii encephalitis (in Estero Bay), acanthocephalan peritonitis (in southern Monterey Bay), and shark attack (from Santa Cruz to Point Ano Nuevo).
Infectious diseases, particularly parasitic diseases, were the predominant cause of mortality in sea otters from 1998 to 2001, with 45 percent of the otters examined diagnosed with an infectious disease as the primary cause of death. The prevalence of infectious diseases, particularly in juvenile and prime-aged adult otters, is not consistent with a healthy sea otter population destined for recovery. The identification of pathogens responsible for substantial morbidity and mortality in sea otters and the epidemiologic evaluation of the distribution of these pathogens is an important first step toward understanding the role of population health in the recovery of this species.
This work was made possible by the timely retrieval of beach cast sea otters by staff and volunteers at the California Department of Fish and Game, the United States Geological Survey, the Monterey Bay Aquarium, and The Marine Mammal Center. We also thank N. Kock who examined additional fresh sea otter carcasses. The University of California Marine Council Coastal Environmental Quality Initiative Program, The Marine Mammal Center, the Packard Foundation and the PKD Trust provided funds.
1. Thomas NJ, Cole RA. 1996. The risk of disease and threats to the wild population. Endangered Species Update 13(12):23-27.