Dead Seabirds Are Found on Beaches, Studied for Disease
By ROBERT A. CULBERT
Special to the Vineyard Gazette
A variety of seabirds have died and washed up on Vineyard beaches in the past few months. Many different people have noticed the above-average mortality rates, especially of common eiders. Scientists from the Seabird Ecological Assessment Network (SEANET), which is part of Tufts University's Cummings School of Veterinary Medicine, have discovered that the mortality is most likely due to starvation because of major parasite infestations, rather than avian flu or oil pollution.
In late February, people started noticing the dead birds washing up at Squibnocket Point in Chilmark. On March 2 I found 12 dead sea ducks on a four tenths of a mile stretch of beach at Squibnocket, which is more than I can remember finding in more than 25 years of watching sea ducks in the winter on Martha's Vineyard.
In the last two weeks of March, 17 volunteers surveyed a total of 24.1 miles of shoreline on 20 different beaches. We found 142 carcasses of 10 species. Most of them (118) were common eiders - including adult males, first winter males and females. Many other carcasses are either on beaches that were not surveyed or did not wash ashore at all. While this mortality is higher than normal, it may not be significant to the eider population since hundreds of thousands of them spend the winter near the Vineyard. Other species that we found dead included four red-breasted mergansers, three great black-backed gulls, two each of northern gannets and herring gulls, and one each of red-necked grebe, great cormorant, surf scoter, American black duck and razorbill. Six unidentifiable gulls and two unidentifiable scoters were also found. Because only a few of each species were found, their winter mortality is probably not unusual.
The highest densities of carcasses included 30 per mile at Squibnocket Beach, 20 per mile at Stonewall Beach, 10 to 15 per mile along the north shore and less than five carcasses per mile in Vineyard Haven harbor and Chappaquiddick beaches. Beaches with no carcasses included Joseph Sylvia State Beach, South Beach from the left fork to the opening of Edgartown Great Pond, Long Point Wildlife Refuge and Lucy Vincent Beach.
Mortality has also been reported from Cape Cod and Nantucket. We observed a combined rate of 5.9 carcasses per mile on Martha's Vineyard for the month of March. To put this rate of mortality into perspective, SEANET documented 716 carcasses of 62 species on 2,731 miles of beaches from New Jersey to Maine between 2002 and 2005, for an overall density of .3 carcasses per mile. Cape Cod beaches had 402 of the 716 carcasses found, with the most abundant species being great black-backed gulls (57), herring gulls (51), black ducks (31), and common eiders (30). The highest density of carcasses reported on Cape Cod was 33 carcasses per mile.
Rebecca Harris, the SEANET Coordinator at Tufts University, reports that five common eider carcasses have been necropsied. All five were emaciated and had major acanthocephalan (thorny-headed worm) parasite infestations. The acanthocephala is a phylum of parasitic worms characterized by spines on their heads, with the spines used to attach themselves to the host's gut lining. There are more than 1,100 species of acanthocephalan parasites, but one genus (Polymorphus) is known to parasitize seabirds, especially common eiders. Small crabs (including Asian crabs and mole crabs) and periwinkles are the most frequent hosts for the larvae cysts of these parasites. Eiders become parasitized when they eat these alternate foods, which they will do when their preferred blue mussels are not available. These parasites kill their hosts when ingested in large quantities.
There are hundreds of thousands of common eiders wintering in the coastal waters of the Cape and Islands. Most of these eiders are present in large rafts (flocks) near large beds of blue mussels. An eider can swallow a medium sized blue mussel whole, crush the shell in its digestive tract, consume the meat, and excrete the ground up shell. Major rafts of eiders near the Vineyard can be found off Squibnocket Point, Aquinnah, the north shore and off Chappaquiddick's East Beach, which corresponds to the observed distribution of eider carcasses.
But this year Vineyard birders, including Matt Pelikan, Gus Ben David, Susan Whiting, John Nelson, Allan Keith and myself, have also observed common eiders in our coastal salt ponds, which is unusual for these seaducks. These eiders are undoubtedly eating crabs rather than mussels, and thus may be more likely to come into contact with the acanthocephalan parasites. If these pond-feeding eiders were the only ones dying from these parasites, the distribution of carcasses would be different from what we observed.
Beachcombers are advised to leave the carcasses alone to allow the natural processes of sunlight, scavengers and decomposition do their work. Another alternative is to bury the carcasses either on the upper beach or to the landward side of the dunes, without disturbing the beach grass that stabilizes the beach. The burial hole should be at least two feet deep to reduce the chance that dogs, skunks or raccoons dig up the carcasses. Wear gloves, avoid touching the carcasses and use a shovel to carry them from the beach to the burial hole
Although we know the approximate cause of death for these eiders, we will not fully understand what ecological shifts have ultimately caused this winter die-off until we have a better understanding of the ecology of the common eiders, blue mussels and the distribution of the acanthocephalan parasites. Are the large old beds of blue mussels declining or shifting to new locations? Do blue mussels now host the parasites? Are the crabs that host the parasites now colonizing the large beds of blue mussels? Do the small crabs living in our coastal ponds host the parasites? Or, perhaps the unusually high level of eider mortality observed in March and April is an isolated event that will not be repeated in future winters. We do not yet know.
Robert A. Culbert is an environmental consultant who lives in Oak Bluffs.