It has been a decade since the bald eagle was taking its life in its own talons by choosing an Arkansas lake to winter on. The once endangered species was locally threatened again starting in 1994 and 1995, when 29 eagles at DeGray Lake died. Whatever was ailing them caused the birds to have difficulty flying and swimming — they were observed “wobbling like a sick dog” and doing backflips off their perches before death. Distressed biologists from state and federal agencies tried to figure out what was killing the birds. Tests of DeGray Lake for insecticides, arsenic, pesticides and other contaminants turned up nothing.
Coots — little black duck-like birds that eagles like to feast on — were also dying, and scientists knew there was a connection, but weren’t sure what it was. In the next eight years, Arkansas’s eagle death toll rose to 68; nationally, hundreds have died.
Necropsies of the birds discovered microscopic lesions in the eagle brains that explained their uncoordinated behavior, and eventually the disease had a name: avian vacuolar myelinopathy (AVM).
Nearly a decade later, a South Carolina researcher into algae believed she’d found the answer: a life form growing on invasive plants that occurred in Lake Strom Thurmond, on the border of Georgia and South Carolina; DeGray Lake and other lakes where eagles, coots and other birds were dying.
Now, researchers have determined that the cyanobacteria is a species previously not known to science, and hence were able to name it: Aetokthonos hydrillicola. Eagle killer that lives on hydrilla.
The eagle die-off on Lake Thurmond started in 1998, two years after the invasive plant hydrilla, an old-world plant brought into the United States by the aquarium industry because it thrives submerged in water and by other means, was discovered. When the number climbed to 17 in September 2001, Dr. Tom Murphy, an eagle researcher now retired from the South Carolina Department of Natural Resources, called on Susan Wilde, who had been studying cyanobacteria in the coastal ponds and waters of South Carolina. He said, “Let’s look in these places where eagles are dying and see if you can find a toxic algae,” she recounted.
A month later, Wilde was out on Thurmond with Murphy, where they observed a lone Canada goose sitting on the water, calling. It did not move as they approached. It was blind. AVM lesions had affected its optic lobe.
At the same time, Wilde was working with a Clemson University team studying hydrilla to see if it would cause AVM in mallards. Clemson sent her a batch, with the caveat from a researcher there that she was unlikely to find anything but diatoms on the leaves, “but I was naïve and decided to look anyway,” Wilde said. She decided to put a hydrilla leaf under a microscope, mounted on a glass slide in a drop of water. The bacteria is indistinguishable from the leaf, and you can’t scrape it off. “It’s stuck on really, really, really well. The only way to see it is to mount it and look on the underside of the leaf,” Wilde said. She shone an epifluorescent light on the sample and suddenly a leaf covered in glowing red filaments. “It was kind of dramatic,” Wilde said. The cyanobacteria’s pigment allows the cyanobacteria to photosynthesize in the low light that reaches the underside of the leaf.
“I said, oh, my gosh, there’s so much of this that coots are eating. … Coots love hydrilla. They will even dive to get it. If you’re looking for hydrilla beds, you just look for coot.”
The coots were eating hydrilla; the eagles were eating coots. “I felt pretty strongly this was a good possibility” for whatever was killing the eagles, Wilde said.
Wilde then got information on the Arkansas die-off and the Corps of Engineers shipped Wilde some hydrilla from DeGray. “The leaves had the same characteristic spots [under the microscope] on the leaf. I thought, oh, my gosh, this looks like the same species.”
That kicked Wilde into high gear, she said, and she began to gather samples of hydrilla from 10 other lakes where bird deaths from AVM had been documented, and as a control, hydrilla from lakes where no deaths had occurred. The cyanobacteria and the eagle deaths correlated.
Wilde published a paper on the “eagle killer that lives on hydrilla” last fall; the University of Georgia, where she is now on faculty, put out a news release in February.
So is Aetokthonos hydrillicola something that has been present in the mud just waiting for the right host — hydrilla (or Egeria, much like hydrilla and the invasive on DeGray in the mid-’90s before hydrilla edged it out) — to come along so it could multiply? Or is it the abundance of the hydrilla? What exactly is the toxin it produces, and how does that toxin kill the birds? Many questions remain unanswered.
While eagles are still dying on Lake Thurmond, DeGray Lake has had no deaths since 2002, and Arkansas has Game and Fish Commission biologists, the Corps of Engineers, grass carp and a fly larvae to thank. The carp, an introduced species, chomp down the submerged part of the hydrilla plant with tooth-like gills in the back of their throats. The larvae (Hydrillia pakistanae) burrow into the leaves at the top of the water column and kill them. Game and Fish nongame migratory species biologist Karen Rowe said the hydrilla is “considered eradicated” on DeGray, though there are still infestations on Lake Ouachita.
Rowe is happy that DeGray is no longer a threat to coots and eagles, but she’s moved on to other worries. “It’s really sad, but comical … when you deal with declining species you go from one problem to the next.” Right now, Rowe is working to persuade rice farmers to flood their fields after harvest to provide habitat for shorebirds, which have been declining as wet fields decline. She’s also worried about the decline of the King Rail because of the disappearance of emergent marshes. A survey in 2012 found only “three or four.” She’s trying to convince the Corps of Engineers to modify its wetland mitigation practices.