Common eiders (Somateria mollissima) are large diving seaducks commonly found along the coast throughout the northeast U.S. and Canada. These diving ducks may be regarded as sentinels of coastal environmental health, due to their migratory behavior and specialized diet of blue mussels (Mytilus edulis), a species which has long been utilized as an indicator of water quality. The common eider can be separated into four subspecies, two of which converge on New England coastal waters during the winter season.
In March and October 2006, mass mortality events accounted for the deaths of at least 800 eiders in the Cape Cod, Massachusetts region, numbers significantly in excess of the previous annual baseline mortality rate. Within 2006, other populations of common eiders suffered significant die-offs, including one of more than 3000 birds in Nunavut, Canada in July 2006. During this latter event, a primary pathogen was consistently identified, and the epidemic was determined to be a result of a highly contagious Pasteurella multocida infection leading to fatal fowl cholera.2 However, in contrast, no such infectious agent has been identified as the cause of the Cape Cod eider mortalities.
During the eider die-off in March 2006 off the Cape Cod coast, many birds had heavy, and ultimately fatal, acanthocephalan infections. The most apparent contributing factor to this was prey switching by the birds during a shortage of the preferred blue mussel prey, resulting in increased consumption of shore crabs, an intermediate host of the parasite.
We have yet to identify an ultimate cause of death during the fall event, when over 400 additional eiders were found dead around Wellfleet, MA. Common clinical findings in this mass mortality include emaciation, weakness, visceral hemorrhage, and a variety of systemic opportunistic bacterial and fungal pathogens. These secondary pathogens generally will only manifest as disease in immunocompromised or highly stressed avians, although no source of such an underlying stress has been established in our studies.
Eiders wintering off the New England coast that surviving these die-offs are showing some population level changes, evidenced by fewer than normal sightings by experienced bird surveyors, as well as a decline in the number of nesting female eiders.1 Are these mortality events affecting common eider indicative of a significant ecological health problem in the coastal environment of New England?
1. Allen B. 2007. "Fewer eiders in MA?" E-mail (personal communication) to author.
2. ProMED-mail. 2006. Avian Cholera--Canada (Nunavut). 25 Jul: 20060725.2051. http://www.promedmail.org. Accessed 20 February 2007.