Mass mortality of Common Eiders Somateria Mollissima in the Wadden Sea, winter 1999/2000: food related parasite outbreak?

Between November 1999 and May 2000, large numbers of dead and dying Eiders Somateria mollissima washed ashore in the Dutch and German Wadden Seas. Eiders roosting near the coast were reported to be in a very poor condition and most were unable to fly. Between December and May, well over 20 000 Eiders died in the Dutch Wadden Sea (NZG/NSO database) and just over 2000 were reported from German waters (B. Reineking pers. comm.). A long-term time series of beached bird surveys showed that the mortality was between 4 and 7x background levels (excluding mortality due to oil): the most serious non-oil related mortality outbreak in Common Eiders in these waters since the mid-1960s (Swennen 1972; Camphuysen 1997, 2000). Juvenile Eiders predominated early in the incident, more and more adults were found later in the epizootic. All beached Eiders were in an exceptionally poor condition (subcutaneous and deposited fat stores depleted, very poor condition breast muscle; C.J. Camphuysen & T. Kuiken unpubl. data) and the birds had lost at least 25-35% of normal body mass. The most probable cause of the emaciation is a lack of adequate food (starvation). The most significant pathologic finding besides the severe emaciation was multifocal enteritis caused by a parasite infection. Virtually all dissected Eiders were heavily infected with endoparasites, mainly the thorny-headed worm Profilicollis botulus (Acanthocephala) in the intestines, but including potentially pathogenic parasites such as the Nematod Amidostomum acutum in the gizzard (H. Cremers unpubl. data). Many acanthocephalids had penetrated the intestinal wall of the Eiders and protruded into the abdomen. The parasite infection is regarded as being the proximate cause of death. The parasite burden may be indicative of a shift of the Eiders towards secondary prey (Green Shore Crabs are the intermediate host of P. botulus), in immune status or some other factors.