Epidemiology of Great Lakes bald eagles

Historical data are provided to support the hypothesis that organochlorine chemicals introduced into the Great Lakes ecosystem following World War II are the cause of reproductive loss among bald eagles (Haliaeetus leucocephalus) in the basin. This is supported with data on concurrent population fluxes of extrabasin North American bald eagle populations and the European white-tailed sea eagle (Haliaeetus albicillus) where the same chemicals were produced and released. Organochlorine chemicals appear as a unique stress on Great Lakes bald eagle populations when compared with stresses on successful populations of bald eagles continentwide. Shoreline birds bear significantly higher concentrations of these persistent toxics than inland birds. Association between contaminated prey and elevated concentrations of PCBs, DDT, and DDE in Great Lakes bald eagles are presented. A fledging ratio is used to support the hypothesis that maternal prezygotic exposure affects the viability of embryos and chicks. The ratio of the mean number of fledglings per successful territory to the mean number of fledglings per active territory, when the numerator is greater than 1.4, provides an index of exposure to contaminants by parental animals and affected offspring. When the ratio is greater than 2, parental exposure to organochlorine chemicals should be considered. The adverse effects of prezygotic exposure to the same contaminants in other animal species dependent upon Great Lakes fish, and extrabasin bald eagle populations dependent upon contaminated fish, provide consistency to the argument. The mechanism of action of the organochlorine chemicals further strengthens the causal argument indicting DDT, DDE, and PCBs. A strong association between DDT/DDE and bald eagle reproductive success is provided. However, the role of PCBs is not ruled out. Only data for total PCB concentrations in bald eagle tissue are available, and until specific PCB congeners are quantified there will be uncertainty concerning PCB's role in the Great Lakes bald eagle's lack of success.