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The
bald eagle (Haliaeetus leucocephalus) is currently listed as
a federally threatened species in Washington. In July 1999, the
U.S. Fish and Wildlife Service published a proposal to delist the bald
eagle under the Endangered Species Act. At the time the species
was listed, environmental contaminants were cited as the primary reason
for its decline. Beginning in the 1940's, dichloro diphenyl trichloroethane
(DDT) and other
organochlorine
pesticides became widely used as insecticides. In the late 1960's
and early 1970's, it was determined that dichlorophenyl-dichloroethylene
(DDE), the principal
breakdown product of DDT, accumulated in the fatty tissues of adult female
bald eagles and resulted in thin shells and reproductive failure (Wiemeyer
et al. 1972, 1984; Grier 1982). Due to the bioaccumulative
and persistent nature of DDT and the adverse reproductive effects elicited
by DDT, particularly on birds, its use was banned in the United States
in 1972.
Restrictions on organochlorine pesticides combined with concerted efforts
to protect and manage habitat and to stop persecution have resulted in
recovery of bald eagle populations throughout most of the contiguous United
States. Hood Canal is a deepwater fjord characterized by restricted
circulation and extreme water depths in western Washington. Although the
number of occupied bald eagles nests in Hood Canal have been increasing,
there was concern because their productivity remained significantly below
the Washington State's productivity average in most years and fluctuated
more dramatically than the Statewide values. In 1992, a contaminants
study was initiated to evaluate if contaminants were a possible cause
of the lower productivity.
The
primary study objectives were to: 1) evaluate productivity of the
Hood Canal territories, 2) determine if elevated concentrations of dioxins,
furans, polychlorinated
biphenlys (PCBs),
organochlorine pesticides, and metals
were present in bald eagle eggs collected from nests near Hood Canal,
and 3) evaluate whether the contaminant levels present could be significantly
contributing to the reproductive failures seen in the bald eagles from
Hood Canal. An additional objective was to evaluate if resident
bottom fish could be a source of contaminants.
Results:
Concentrations of PCBs and compounds with dioxin-like activity were elevated
at levels of concern in the eggs from the Hood Canal nests. Overall,
the PCB concentrations in the eggs collected early in the study were at
higher levels than those collected later. New nests continue to
be established in the Hood Canal area. As older birds with substantial
contaminant burdens are replaced by younger birds, we would expect to
see a decrease in the persistent environmental contaminants in eggs if
a constant or new source of the environmental contaminants available to
the eagles is not present. Evaluations of other environmental contaminants
that were not analyzed for in this study, but which could be potentially
adversely affecting the Hood Canal bald eagles, should also be considered
if a contaminants study is reinitiated. Evaluations of other factors,
such as disturbance and habitat alterations affecting either nesting territories
or prey, possibly should also be included if productivity is impacted.
Learn More by Reading the Full
Report:Mahaffy, M.S., Ament, K.M., McMillan, AK, and Tillit, DE, Environmental
Contaminants in Bald Eagles Nesting in Hood Canal, Environmental
Contaminants. USFWS, Olympia FWO. 2000.
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