Biologists monitor emerging waterfowl, raptor disease
Wildlife biologists and park rangers are continuing to monitor area reservoirs and lakes for signs of avian vacuolar myelinopathy (AVM), a disease that primarily affects waterfowl and raptors. Biologists are concerned with the emergence of AVM in S.C., but note an 8.5% increase in eagle nesting per year since surveys were first initiated 30 years ago.
S.C. Department of Natural Resources (DNR), Georgia Department of Natural Resources, Southeastern Cooperative Wildlife Disease Study and U.S. Army Corps of Engineers have been collaborating to monitor reservoirs that may support potentially toxic blue green algae, the suspect agent of AVM, which has been implicated as a cause in American coot and eagle mortalities. AVM is an often-fatal disorder that affects the central nervous system of waterfowl and raptors that consume the suspect toxic algae growing on submerged aquatic vegetation in some Southeastern reservoirs.
Research supports the working hypothesis that waterfowl such as American coots feeding on freshwater aquatic plants are susceptible to toxins found in algae growing on the leaves and stems. Once ingested, toxins cause cell and tissue damage primarily to the central nervous system and affected birds become uncoordinated and lose the ability to fly. This makes them vulnerable to raptors, such as eagles, that easily target affected birds. Eagles may then contract the disease from consuming affected prey.
AVM has been implicated in the death of over 100 eagles and suspected in the death of thousands of American coots in Southeastern reservoirs since the disease was first documented in Arkansas in 1994. In S.C., the disease was first observed in 1998 on Lake Thurmond, a 70,000-acre reservoir on Savannah, Broad and Little Rivers bordered by S.C. and G.A. and managed by the U.S. Army Corps of Engineers. According to Shirley Willard, a ranger with the Corps of Engineers who conducts fieldwork out of their J. Strom Thurmond U.S. Project Office, 46 eagles have been found dead at Lake Thurmond, and AVM has been implicated in their deaths. The losses also translate to the disappearance of 6 eagle nesting territories. According to DNR wildlife biologist Tom Murphy, "Eagle nesting below Highway 378 in our state has basically been extinguished, and we suspect this is a direct effect of this emerging wildlife disease." Only after fresh bird carcasses are submitted for necropsy and microscopic examination to the Southeastern Cooperative Wildlife Disease Study at the University of Georgia, or to other qualified wildlife health laboratories, is AVM implicated in a bird’s death.
Murphy, along with DNR’s Charlotte Hope, coordinates S.C.’s Midwinter Eagle Survey, which tracks the population numbers of eagles along standardized routes during the beginning of January. Despite concern for the emergence of AVM in S.C., they have documented an 8.5% increase in eagle nesting per year since the survey was first initiated 30 years ago. As further testimony to the eagle’s recovery, earlier in 2007 they were removed from the list of Threatened and Endangered Species, however they still remain protected under other state and federal statutes.
Biologists and rangers have closely monitored AVM disease events after the first occurrence was recorded on Defray Lake in Arkansas in 1994. Susan Wilde, a former algal ecologist with DNR, was instrumental in associating the relationship between the presence of the toxic blue green algae growing on aquatic vegetation found in freshwater reservoirs and brain lesions that formed in American coots after ingesting the vegetation. Wilde has monitored the disease along freshwater reservoirs as well as smaller farm and residential ponds in the Southeast. Further research also helped to determine that seasonality plays an important role in the occurrence of the disease. American coots and other waterfowl migrate to Southeastern reservoirs typically around October from northern areas, where aquatic vegetation is an integral component of their diet. Overwintering eagles stopover along some of the same Southeastern reservoirs where affected waterfowl may be present. In addition to this population of eagles, local nesting eagles are also susceptible to the disease once they prey upon affected waterfowl.
The collaborative efforts between SC DNR, Georgia DNR and U.S. Army Corps of Engineers involved in observation and research of AVM continue to work closely together to monitor this disease and develop steps for mitigation. As the bald eagle population continues to increase throughout the state, it is important to track the impacts of AVM and record disease events throughout freshwater reservoirs and other water bodies.
Because eagles nest in obscure areas, biologists and rangers alone cannot ensure that all affected birds will be found. Willard says, "The more eyes we have out there helping us to monitor and track this disease, the better." The public can help with the research and documentation of the disease if they encounter waterfowl or raptors exhibiting strange behaviors affecting movements or happen upon carcasses while in these areas where characteristics exist that may result in AVM-affected birds: submerged aquatic vegetation, presence of American coots and signs of eagle nests or eagle sightings.
The public is asked to call one of the following to report these types of observations among waterfowl or raptors:
DNR protects and manages South Carolina’s natural resources by making wise and balanced decisions for the benefit of the state’s natural resources and its people.