A bald eagle capturing prey. Photo: U.S. Fish and Wildlife Service
More than half of bald eagle nestlings in nests surveyed in Lake Superior’s Apostle Islands contained birds that exceeded acceptable levels of a contaminant long since banned from use, says a recent study.
One contaminant, DDE, is a breakdown component of the insecticide DDT. PCBs, substances once used to cool electrical equipment, were also found.
Banned in the 1970s, both are known to harm eagles by causing birth defects or their egg shells to thin.
The study, published in the Journal of Great Lakes Research, surveyed from 2006 to 2008 Lake Superior nests north of Wisconsin, including nests in the Apostle Islands National Lakeshore. Researchers also tested nests in two other national parks in the Great Lakes area — the St. Croix National Scenic Riverway and the Mississippi National River and Recreation Area.
Historically the Great Lakes region has had an issue with contaminants, said Bill Route, one of the study’s authors.
“So we felt that it was a vital statistic or vital thing that we ought to be monitoring,” he said. Route is an ecologist with the National Park Service’s Great Lakes Inventory and Monitoring Network.
Apostle Islands National Lakeshore is made up of 21 islands that are located from 1 to 25 miles north of Wisconsin. Islands range from about 3 to 10,000 acres in size.
Last census, researchers counted 15 occupied eagle nests and 6 nests with chicks in the lakeshore.
Researchers tested some of these nests and found higher levels of these contaminants.
“So what’s interesting there is that there appear to be hotspots,” Route said.
Scientists think these hotspots are areas where eagles prey on more contaminated food sources.
The study found nests with consistently high-contamination rates near bird colonies.
“On Lake Superior we certainly know of at least some sites where these eagles are feeding not only on fish but also on gulls,” said Dave Best, a fish and wildlife biologist with the U.S. Fish and Wildlife Service in East Lansing, Mich.
You’ll find these hotspots in all the Great Lakes, he said.
Scientists caution that these hotspots could have negative effects on these eagles. But what they are is unknown.
“We’re essentially clueless, in most cases, when we look at how sensitive a species is to a certain contaminant or a suite of contaminants. And, you know, even though we’ve worked a lot with bald eagles and some of the other fish-eating species we still don’t have definitive answers,” Best said.
But overall the level of contaminants of the nests tested in Lake Superior have declined and are under this threshold.
To measure this long-term trend, Route and his colleagues combined their two years of data with data from previous years.
From 1989 to 2008, despite the hotspots, on average DDE levels declined 3 percent and PCBs dropped 4 percent each year.
Data wasn’t available for St. Croix and Mississippi Rivers to determine long-term rates of decline.
View national parks where eagle nests were tested in a larger map
Route calculated that it took more than 20 years after these contaminants were banned to reach these safer levels.
“[DDE] seemed to have crossed that threshold in the late 1990s,” Route said.
“So it really did take a long time and similarly for PCBs,” he said.
Scientists attribute Lake Superior’s especially slow breakdown of contaminants to its frigid temperatures.
“DDE, you know the breakdown component of DDT, is also being found and it’s at higher levels than a lot of other locations. And, you know, there’s some logic to that in that Lake Superior is cold and things break down slowly,” said Julie Van Stappen, Branch Chief of Natural Resources of Apostle Islands National Lakeshore.
But compared to other Great Lakes, Lake Superior is actually the cleanest when it comes to contaminants like these.
“Essentially it is the cleanest of the lakes. It probably has rebounded as much as any of the lakes in terms of recovery of colonial water birds, fish-eating birds and raptors such as eagles,” Best said.
“Lake Superior is your best case scenario,” he said.
The study also shows that the rate of the contaminants’ decline has slowed.
Earlier research indicated that the rate of decline for DDE was about 5 percent and for PCBs about 6 percent annually.
Researchers haven’t figured out the reason why, there might be a point when the substances no longer purge out of a system.
Route is optimistic knowing that DDE and PCBs continue to decline, but thinks the issue of contaminants accumulating in the environment is a story without a conclusion.
“I think it’s a never-ending issue of, you know, keeping our eyes wide open to all of these contaminants that are on the Great Lakes,” Route said.
And many eyes now shift from these “old” contaminants to newer ones, like flame retardants and chemicals like PFCs.
“The fire retardants and all this other crud, really, is just the tip of the ice burg,” Best said.