Bald eagles living along Lake Superior raise fewer young than those nesting on inland lakes because less food is available for them, researchers say.
A study by University of Wisconsin-Madison and state Department of Natural Resources researchers found the problem to be environmental, rather than due to feeding on contaminated fish as some had thought might be the case.
William Karasov, a UW-Madison wildlife ecologist, said he conducted the study for two reasons:
- To determine how substances like the insecticide DDT, which has been banned since 1972, and the chemical pollutant PCBs, which have been the focus of environmental cleanup efforts for years, could still affect eagle populations.
- To measure whether eagle populations can serve as a "bio-sentinel," or indicator of overall environmental quality. He concluded that more study is needed to determine what looking at numbers of eagles says about the environment.
DDT is blamed for a steady drop in eagle numbers between 1940 and 1970 because the pesticide accumulates in the food chain and causes eggs to have thinner shells.
No eagles nesting on Lake Superior produced young from 1970 to 1976, but since then, numbers of eagles and other birds of prey have been on the rise. There were 108 occupied eagle territories in Wisconsin in 1993, compared with 645 last year, the report said.
In the case of eagles nesting along Lake Superior, Karasov's study found the birds' blood contained DDE, a breakdown product of DDT, and PCBs, but that wasn't believed to be causing them to raise fewer chicks.
Over the course of the eight-year study, researchers found that eagles on the big lake raised 23 percent fewer chicks than inland nesters.
But they found it was more a matter of how much food the birds caught than the contaminants in their blood.
Lake Superior is "a deep, cold, rather unproductive lake with few shoals where fish can spawn and eagle can forage successfully," Karasov said.
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