| Summary: | Abstract Southern sea otters (Enhydra lutris nereis) face numerous risks to population recovery from infectious disease, shark attacks, and biotoxins. Recently, the most significant contributor to fatal infectious disease in sea otters has been acanthocephalan peritonitis (AP) caused by Profilicollis altmani. Despite the importance of AP‐associated otter mortality, long‐term and seasonal risks for spillover of this parasite from its avian definitive hosts (seabirds and shorebirds) and crustacean intermediate hosts, Pacific mole crabs (Emerita analoga), are poorly resolved. We analyzed a 30‐year dataset of sea otter strandings (n = 4460 cases) within the extant southern sea otter range to assess the contributions of temporal, demographic, geographic, and environmental risk factors to the prevalence of AP in southern sea otters. We found the incidence of AP was driven by high prevalence in immature otters near or in Morro and Monterey Bays, primarily in the late spring. The seasonal fluctuations in AP prevalence aligned closely with the presence of Pacific mole crabs in the gastrointestinal tracts of stranded otters. For Monterey Bay otters, AP prevalence was also higher during El Niño years. Long‐term trend analyses suggest that seasonal, demographic, and geographic patterns of AP prevalence in southern sea otters have shifted. Southern sea otter population recovery efforts can be informed by long‐term monitoring that assesses the evolving risks of AP and other sources of mortality.
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