Observers who fanned out across the lake Jan. 8 counted two mature eagles and four immature birds, said Ken Boyd, a conservation biologist for the Army Corps of Engineers.
Eighteen eagles – four adults and 14 juveniles – were sighted in a similar census last year.
At Savannah River Site, where seven eagles were seen last year, observers flew by helicopter Jan. 11 and saw 14 eagles – four adults and 10 immature birds, said Tracy Grazia, the supervisory wildlife biologist at the U.S. Department of Agriculture’s Savannah River Forest Service office.
Both surveys were conducted as part of a broader national effort, the Midwinter Bald Eagle Survey, held each January to track and compare both national and regional trends.
Although the number of eagles seen at Thurmond Lake was disappointing, Boyd said cloudy, overcast skies and misting rain on the day of the census could have inhibited visibility. Also, one of the survey teams had mechanical trouble and might have covered less ground.
Last year’s count, by comparison, was held under clear skies offering maximum visibility.
Thurmond Lake is the site of one of the deadliest outbreaks of a condition known as avian vacuolar myelinopathy, which has killed dozens of eagles at the reservoir since the disorder was identified in the mid-1990s.
AVM is believed to be caused by unusual algae that grow on the dense mats of hydrilla that are popular feeding sites for migrating waterfowl and small aquatic birds called coots.
Because coots feed heavily on hydrilla and are often eaten by bald eagles, the small birds play a major role in AVM outbreaks.
One dead eagle, discovered in its treetop nest, has been documented at the lake this year, Boyd said.
Scientists are exploring what can be done to combat AVM. A possible approach is using grass carp to manage the submerged aquatic vegetation that is host for the algae.