Sick Sea Otters and Potential Health Risks for Humans at the Land-Sea Interface

Southern sea otters are a threatened species that numbers fewer than 2,800 individuals, found only along the California coast. Although hunting has been prohibited for over 80 years, population recovery has remained sluggish and much of their historical range remains uncolonized. Birth rates are normal, but mortality is extremely high, with approximately 10% of the population found dead each year. Necropsies have revealed that a high proportion of California otters die following exposure to bacteria, fungi, parasites, chemical pollutants and biotoxins that may originate from the terrestrial environment. Examples include the pathogenic protozoa Toxoplasma gondii and Sarcocystis neurona, whose environmentally resistant oocysts or sporocysts are shed only in the feces of terrestrial animals (cats and opossums, respectively).  Other otter diseases with strong land-sea connections include poisoning by the cyanobacterial toxin microcystin, and infection by Coccidiodes imminitis, a fungus that causes “Valley Fever” in humans. For fecal bacteria like Salmonella and marine biotoxins like domoic acid, whose production is linked with coastal seawater nutrification, the land-sea connections are less well characterized. However, necropsy findings and animal stranding patterns suggest that otters living in coastal areas with significant exposure to surfacewater runoff are at higher risk.

Although factors affecting sea otter survival are complex, the direct and indirect impacts of terrestrial-origin pollution appear to be important contributors to poor Southern sea otter population recovery. These animals are especially sensitive environmental sentinels for coastal pollution due to their unique biology. They are nearshore feeders with high site fidelity. They often forage near rivers and bays that concentrate and retain polluted surfacewater runoff.  Sea otters commonly feed on filter-feeding marine and estuarine invertebrates that are efficient pollutant bioaccumulators. Finally, sea otters have an extremely high basal metabolic rate, requiring consumption of at least 25% of their body weight in invertebrate prey every day to maintain thermal equilibrium. This unique evolutionary strategy compounds the risks that otters encounter when living and feeding in polluted coastal environments. Equally concerning is that sea otters and humans consume many of the same marine foods, including crabs, clams and mussels. Thus, studies of the anthropogenic and terrestrial origins of southern sea otter mortality may also highlight important and under-recognized human health risks.