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Open Access Publications from the University of California

Southern sea otters (Enhydra lutris nereis) and emergent disease in the near-shore marine ecosystem: Assessment of spatial trends in cause-specific mortality from 1998-2001 and evaluation of an aerial survey method for surveillance of trends in mortality


The southern sea otter population has grown at a notably lower average annual rate than other recovering sea otter populations and has been declining in abundance recently. Increased mortality, specifically due to infectious disease, is a likely cause for this depressed rate of population growth. Toxoplasmosis is currently the most common cause of death in California otters, and while exposure to this pathogen may be highly prevalent in wild animal populations, exposure is generally not associated with severe clinical disease in healthy, immunocompetent animals. Wastewater runoff carrying pathogens and contaminants into the coastal environment, and the bioaccumulation of these pathogens in shellfish prey, may be contributing to the occurrence of emerging diseases that are hindering the recovery of the southern sea otter population. Sea otters are a good biological parameter with which to measure the impacts of anthropogenic input into marine systems because both otters and their main prey live in geographically localized areas along the California coastline, which has been subject to extensive modification by human activity. Spatial and temporal patterns of important causes of mortality in southern sea otters will be investigated from 1998 through 2001 in order to identify geographic areas within the sea otter range with increased risk for otter mortality. Once important causes of mortality and spatial clusters of disease occurrence are recognized, the association of mortality in sea otters and anthropogenic influences in the near-shore environment will be considered. Accurate measures of sea otter abundance are also crucial to monitoring long-term trends in this indicator species. The aerial survey method that is currently used to assess the abundance of southern sea otters in 30% of their range will be evaluated under a range of pertinent viewing conditions in order to accurately estimate sea otter sighting probabilities and facilitate accurate interpretation of abundance data.

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