Cobscook Eagles Key to Recovery
(Published in The Quoddy Tides on March 23, 2001)
Unlike most other places in the country, our national symbol, the bald eagle, is a common sight around Cobscook Bay. The sixteen nesting pairs and many immature eagles found here are a clear indication that this is a special place.
So special, in fact, that Cobscook Bay, with its abundant resources and productive fisheries, played a key role in restoring eagle populations in Maine and throughout the northeast United States. Bald eagle populations crashed in the 1960's and 70's as a result of the infusion of chemical pollutants into the environment after World War II. The pesticide DDT, in particular, had a direct impact on the reproductive success of eagles.
The Maine Department of Inland Fisheries & Wildlife began monitoring eagles in 1976 in anticipation of their listing as an endangered species following that crash. Charlie Todd, a wildlife biologist with MDIFW, was the first graduate student to study bald eagles through the University of Maine. One intriguing discovery of his initial research was that more nesting pairs were found in Cobscook Bay than anywhere else in Maine.
"Several nesting pairs had persisted unnoticed, and Cobscook eagles were producing eggs at triple the rate of other areas in Maine," reports Todd. "We found this stronghold in Downeast Maine, and thought if they could hold their own, we might have a chance at true recovery."
"Something noticeably different in Cobscook Bay was the prevalence of alewives as food, far above what we found in other areas of the Maine coast," says Todd. "Also, the great tides in the bay leave a larger area of tidal pools. Fish that become stranded are easy prey. As a result, it is not uncommon to find the remains of sculpin and other bottom fish in nests."
Nearly every eagle nest in the bay contained alewives during the nesting season, when eagles are particularly stressed. Full-grown eagles eat 1/2 lb of fish or meat a day. Eaglets grow to adult size within 11 weeks, requiring three to four times as much food. If 2-3 eaglets are in a nest, a lot of food is consumed throughout the nesting season. This is where the issue of productivity of a region comes into play.
"Although eagles are primarily fish eaters across their range, Cobscook Bay is the only place in coastal Maine where we find this to be true. In North Haven, Boothbay, Jonesport, and other coastal areas, the diet is dominated by gulls, cormorant, and eiders," Todd explains.
Although the recovery of eagles is something to be celebrated, there are still many threats to their longterm survival. In some areas where once abundant fisheries have been lost, eagle numbers remain low and feeding behavior has changed.
"In the Lower Kennebunk, for example, we used to have 15 pairs of eagles down river from Bath feeding on shad, herring, and other schooling fish. Now we only have two pairs," says Todd. "We've done something to the fisheries, and eagles turn to 'plan B,' but plan B comes with a price."
The cost is borne in eating animals higher in the food chain. Although DDT was banned in 1972, it still persists in the environment, along with other harmful chemicals. Predatory animals carry higher levels of these long-lasting contaminants. A cormorant has approximately 1,000 times the dosage than a foraging fish. As eagles in Maine rely more on waterbirds as their primary food, they consume a higher dose of contaminants.
"We still find DDT in every eaglet growing up in Maine. The chemicals become incorporated into the fatty reserves, and stay there for life," explains Todd. "As an eagle feeds on contaminated birds or fish, these chemicals build up in the eagle's body. Eventually the chemicals may reach a level that affects survival or reproductive success."
And, something is happening now with our non-breeding eagles. The immature eagles that are one, two, three or four years old, and a few non-breeders that are not paired, are not tied to any particular site. In healthy populations, nonbreeders typically congregate in large numbers on food bonanzas, such as spawning runs of fish or a large fish kill. They learn from the behavior of more experienced eagles. These congregations rarely occur in Maine now, except sometimes near salmon pens, where the learned behavior may be different from a more natural setting.
"As the eagle population rebounded, we would see these large clusters now if the fisheries were there to support it," Todd states.
The number one cause of death or injury to eagles is now due to collisions with utility lines close to water bodies. Some are electrocuted, but most die or are injured by the collision itself. A few local outages in the Cobscook area are related to eagle deaths. Because eagles are so particular about where they nest, changes in land use lead to new pressures on eagle habitat. Land speculation and development, aquaculture, and coastal recreation have added new pressures to eagles, even in Cobscook Bay.
Although concerns about DDT have lessened, levels of other contaminants such as PCB?s are stable or increasing in fish and some waterbirds. And mercury, which is deposited as a result of acid rain, has been increasing in sediments in Maine waterways. Mergansers found in important eagle wintering areas show high mercury levels in their body tissue. PCB's and mercury are also believed to contribute to poor reproductive success in eagles.
"Some people value what bald eagles embody as a symbol, others value their role as an indicator of environmental health. As a top-level predator living off several different levels of the food chain, eagles are in fact the most vulnerable part of the natural system. If the most vulnerable part of the system blinks out, we have to wonder what else is at risk," Todd says. "If we pay attention to these animals that serve as indicators of the health of our environment, we can judge how stable the system is for all of us, that system of which we are a part."
This column was prepared by Cheryl Daigle and Jim Dow. Cobscook Soundings was a monthly column produced by the Maine Chapter of The Nature Conservancy. Its purpose was to share what is known about the workings of the Cobscook Bay marine environment, so that all who make decisions about the use or care of the bay have the best available information.
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